6.5 Diesel Stutter and Defueling

[6.5_Diesel]

So I am an active member on Duramax forum and have posted this issue on that site with no suggestions. The truck in question is a 94 6.5 TD 2500 4x4 5 speed (4:10 gears) with an F VIN engine. The truck has 145,000 miles on it and I have installed a Heath Diesel PMD relocate about 6 years ago, a cold air intake, 4" straight pipe exhaust, new injectors at 108,000 and new glow plugs. I also installed a new lift pump about 6 years ago as well.

I have 2 issues with the truck that I have been trying to find an answer for:

1. When I am driving down the road at cruising speed it will occasionally stutter, almost like a quick miss in the motor. this is a split second event just enough to feel the stutter but no dash lights and the gauges don't move when it happens. It doesn't do it all the time only occasionally. I took a 350 mile trip with it not too long ago and I would say it did this about 6 times on the whole trip. Another thing to note is that when the truck is cold started (50F or below), it will smoke and misses like crazy for about 10 seconds. After that, it runs idles flawless and it starts easily every time after.

2. Issue number 2 is especially noticeable when pulling a trailer. What happens is when I hit a hill and I give it more pedal my boost will pick up but then it acts like it defuels then boost goes to almost zero then back up to 10 then back to zero and on and on. I have wired the wastegate shut thinking it was a vacuum issue, but it still does it which makes me believe it is defueling. I also found out that at 2500 rpm the pump will naturally defuel for some odd reason, but this boost surge happens at rpms below 2500 as well. I have read online that it could be a multitude of different things from AI temp sensor, boost sensor, PMD issue, and so on, but I was hoping someone could give me a really good starting point.

Any starting points would be greatly appreciated.

[arveetek]

Welcome!

1. Some folks call this "fishbite" and is a common occurrence. I have not found a cause or solution. Mine used to do it, but now that I think about it, it hasn't happened for quite some time now. It may have disappeared when I replaced my IP a couple of years ago. Your smoke and miss at cold start is probably not related, but is most likely due to a faulty glow plug or two, or perhaps slightly low compression in one or two cylinders.

2. Your ECM is most likely defueling due to a lack of boost control. The ECM is very sensitive to boost, and tries to control it through the vacuum-operated wastegate control system. Wiring the wastegate shut will make the problem worse, since the ECM can't control the boost, and will then go into Limp mode. There are a few ways around this issue. The first way is to make sure the entire boost control system is functioning properly: good vacuum from the vacuum pump, good wastegate solenoid valve, and no leaks in any of the vacuum lines. What I did with mine was ditch the vacuum pump, installed a Turbomaster manual wastegate control, and installed a chip into the ECM from Kennedy Diesel to keep it from going into Limp mode. Works awesome now.

Casey

[6.5_Diesel]

Welcome!

1. Some folks call this "fishbite" and is a common occurrence. I have not found a cause or solution. Mine used to do it, but now that I think about it, it hasn't happened for quite some time now. It may have disappeared when I replaced my IP a couple of years ago. Your smoke and miss at cold start is probably not related, but is most likely due to a faulty glow plug or two, or perhaps slightly low compression in one or two cylinders.

2. Your ECM is most likely defueling due to a lack of boost control. The ECM is very sensitive to boost, and tries to control it through the vacuum-operated wastegate control system. Wiring the wastegate shut will make the problem worse, since the ECM can't control the boost, and will then go into Limp mode. There are a few ways around this issue. The first way is to make sure the entire boost control system is functioning properly: good vacuum from the vacuum pump, good wastegate solenoid valve, and no leaks in any of the vacuum lines. What I did with mine was ditch the vacuum pump, installed a Turbomaster manual wastegate control, and installed a chip into the ECM from Kennedy Diesel to keep it from going into Limp mode. Works awesome now.

Casey

Thanks for the reply.

1. I am leaning toward the missing issue on start is due to low compression in a cylinder but have not verified this. I installed new glow plugs because of this issue and a new glowplug controller and tested each glowplug before installing them and they all worked but did not solve my issue on starts. I do have the original IP from 94 still in the truck, so maybe that could be some of the issue with the miss. It also is a bugger to start once it is below 15F which when I bought it with 72k on the clock it would start without being plugged in at -10F.

2. I am wondering if my whole vacuum system isn't starting to show signs of aging from what you are saying. I haven't checked the vacuum at the wastegate actuator, but I was always thinking of going to a turbomaster. The weird thing is that when I am empty and I get on it climbing a hill it will overboost initially (causing the truck to defuel) then it will build back up to 7PSI and hold steady until I hit 2500 RPM and then I hit the defueling point (this is with the wastegate wired shut). But when I get a trailer on the back it overboosts and then enters a vicious cycle of defueling and refueling to the point where I have to downshift because I cannot maintain power.

[arveetek]

Thanks for the reply.

But when I get a trailer on the back it overboosts and then enters a vicious cycle of defueling and refueling to the point where I have to downshift because I cannot maintain power.

When my '95 would defuel, it would stay in that mode until you turned the key off and restarted. I believe the 1994 models are a bit unique and have a little different programming compared to later models. If you have the original IP, that is definitely a big suspect.

Just be sure to realize that a Turbomaster by itself won't solve anything without changing the programming in the ECM to accept the new boost levels that it can't control. That's why I had to install the chip from Kennedy Diesel into my ECM.

Casey

[Aftermath]

6.5_Diesel
I had a similar problem where every once in a while my rig would stumble as if lost fire then immediately lit again. Over about a 6 month period, it happened more and more. Ended up being the PMD. The devil lives in all PMD'S by the way.

[6.5_Diesel]

When my '95 would defuel, it would stay in that mode until you turned the key off and restarted. I believe the 1994 models are a bit unique and have a little different programming compared to later models. If you have the original IP, that is definitely a big suspect.

Just be sure to realize that a Turbomaster by itself won't solve anything without changing the programming in the ECM to accept the new boost levels that it can't control. That's why I had to install the chip from Kennedy Diesel into my ECM.

Casey

If I only set the TurboMaster to allow 7-8 psi max, would I still have an issue with boost control? I could understand having an issue if I say set it at 10psi but if I stay within the computers limit of 7-8 psi would I be ok or would I still need a reprogram?

I think I may order a PMD and see if that helps the "fishbite" beings they are $130. Would you recommend getting the No.9 resistor? If that doesn't fix the issue I am guessing it may be a pump problem beings it is the original yet.

[arveetek]

If I only set the TurboMaster to allow 7-8 psi max, would I still have an issue with boost control? I could understand having an issue if I say set it at 10psi but if I stay within the computers limit of 7-8 psi would I be ok or would I still need a reprogram?

I think I may order a PMD and see if that helps the "fishbite" beings they are $130. Would you recommend getting the No.9 resistor? If that doesn't fix the issue I am guessing it may be a pump problem beings it is the original yet.

It will still probably defuel. It's not really the amount of boost, but the fact that the ECM expects to control the boost over a certain RPM, throttle input, speed, etc. Once those parameters are out of spec, and the ECM realizes it cannot control boost, it goes into limp mode. I tried setting my Turbomaster to low PSI, but every time I towed or got on the throttle, she would go right to limp mode.

Yes, I've got the # 9 resistor. I prefer and recommend the Dtech PMD:

http://www.dipacodtech.com/All-Products/GM-6-2L-6-5L_New/DT19209057

I've had real good luck with the Dtech.

Casey

[6.5_Diesel]

It will still probably defuel. It's not really the amount of boost, but the fact that the ECM expects to control the boost over a certain RPM, throttle input, speed, etc. Once those parameters are out of spec, and the ECM realizes it cannot control boost, it goes into limp mode. I tried setting my Turbomaster to low PSI, but every time I towed or got on the throttle, she would go right to limp mode.

Yes, I've got the # 9 resistor. I prefer and recommend the Dtech PMD:

http://www.dipacodtech.com/All-Products/GM-6-2L-6-5L_New/DT19209057

I've had real good luck with the Dtech.

Casey

Any good or bad reviews on the PMDs that SS Diesel sells?

[6.5_Diesel]

So a little update on the truck. I tested the AIT sensor and it ohms out correct. I also realized that when I plumbed in my boost gauge I put a tee coming out the top of the manifold (one side is the AIT sensor and the other is boost) and I am thinking this may be causing heat build up due to no air circulation over the resistor. Another thought is that my CAI is pulling in too hot of air under the hood, especially when towing, causing to high of intake temps. Anyone have insight as to if these items could cause defueling?

[a5150nut]

Is your CAI open to under hood heat? Or is it enclosed and getting out side air via the snorkel in the fender?

[6.5_Diesel]

Is your CAI open to under hood heat? Or is it enclosed and getting out side air via the snorkel in the fender?

My CAI is not a box type and pulls all its air from under the hood. I am getting to the point where I am really digging into this because it is such a pain when pulling a trailer. I am getting my hands on a vacuum tester today to test the vacuum pump. I was told 15+ inches HG at the actuator is what I should have. I also ordered a new boost sensor and boost solenoid that should be here this week ($60 so I figured what the hell the ones on the truck are the originals).

If the pump tests good and I put a new boost sensor and boost solenoid in and the problem is still there, I am guessing it either has to be my CAI or how I moved the AIT sensor when I installed my boost gauge. When I installed my boost gauge I pulled out the AIT sensor and put a Tee fitting in its place and stuck the AIT sensor in one side and the boost gauge in the other. This may be causing a hot spot due to the brass fittings and the sensor not being directly in the intake air stream.

[DmaxMaverick]

First, pull any DTC's that may be present. A paper clip and the key is all you need for that. If it's defueling, code(s) are likely present and the SES lamp should be on, if only during the event.

I see a couple issues. Your "cold" air intake is not cold. It's pulling hot engine compartment air. Your IAT sensor is not sampling intake air. It is sampling engine compartment air temperature and radiant heat. The sensor must be IN the intake air to sample it. It's likely the sensor is being heat-soaked from the Tee, which is being heat-soaked by the engine compartment. Either of the issues could cause the problem you are having. Both together are nearly a certainty.

Restore your IAT sensor to original and relocate your boost gage fitting. Fix your "cold" air intake plumbing so it is actually pulling air not from the engine compartment, or restore it to original.

[6.5_Diesel]

First, pull any DTC's that may be present. A paper clip and the key is all you need for that. If it's defueling, code(s) are likely present and the SES lamp should be on, if only during the event.

I see a couple issues. Your "cold" air intake is not cold. It's pulling hot engine compartment air. Your IAT sensor is not sampling intake air. It is sampling engine compartment air temperature and radiant heat. The sensor must be IN the intake air to sample it. It's likely the sensor is being heat-soaked from the Tee, which is being heat-soaked by the engine compartment. Either of the issues could cause the problem you are having. Both together are nearly a certainty.

Restore your IAT sensor to original and relocate your boost gage fitting. Fix your "cold" air intake plumbing so it is actually pulling air not from the engine compartment, or restore it to original.

Thanks for the suggestions. I was looking at ordering a boost bolt to relocate my boost gauge and returning the AIT sensor back to original. I still do have the original air box for the truck, so I may just go back to original, but I was wanting to see if I could remedy the issue without it.

[DmaxMaverick]

I suggest addressing only one thing at a time, starting with the sensor. I suspect that will cure most of the problem. Once the issue is resolved, then do something to get less hot air into the intake.

[6.5_Diesel]

I suggest addressing only one thing at a time, starting with the sensor. I suspect that will cure most of the problem. Once the issue is resolved, then do something to get less hot air into the intake.

I am going to vacuum test tonight just to make sure my pump and solenoid are up to snuff. I just ordered a boost bolt for my boost gauge so that I don't have to drill and tap into the manifold (just more convenient), so when that gets here I will install it. I will hook the trailer up and see if moving the AIT sensor helps. If not I will move onto the air intake then onto the boost sensor and wastegate actuator.

[6.5_Diesel]

So I tested the vacuum pump last night and I got about 23" Hg from the line off of the pump and about 17" Hg at the wastegate actuator. One thing I did notice is as I was unplugging the hoses from the boost solenoid I was holding the solenoid in my had and it was vibrating like crazy and no it was not due to the engine because I removed the solenoid from the mount. Does anyone know if it is supposed to do that? It is almost like a valve internally is opening and closing very rapidly.

I also reinstalled the AIT sensor back into its original location and ditched the Tee fitting I had to add my boost gauge. I should have my boost bolt Friday, so I will put that in and take it for a drive.

[DmaxMaverick]

The unplugged, vibrating solenoid is normal. By disconnecting the vacuum lines, the ECM went into hyper-mode trying to modulate the vacuum, which it obviously couldn't. It likely set an according DTC. The SES lamp may have come on as well.

[6.5_Diesel]

The unplugged, vibrating solenoid is normal. By disconnecting the vacuum lines, the ECM went into hyper-mode trying to modulate the vacuum, which it obviously couldn't. It likely set an according DTC. The SES lamp may have come on as well.

The solenoid was doing the vibrating while plugged in idling too, but when I pulled vacuum at the actuator it was steady at about 17" Hg. Is it supposed to vibrate when hooked up and at idle? I would think if there was an issue I would have seen it on the vacuum gauge at the actuator.

My boost bolt and new boost solenoid and boost sensor will be here today, so I am going to get my boost gauge plumbed in and take it for a spin.

[JohnC]

Don't know if this has anything to do with your problem, but, the two ports on the solenoid are not interchangeable. If you connect it up backwards it will not work correctly.

One port has a restriction, visible when looking in to it. Unfortunately, I forget which way it needs to be connected. I recall, (I think), that the idle vacuum on the wastegate actuator should be about 11 inches. (I'm thinking the port with the restriction goes to the pump, but could be wrong; sold my '95 12 years ago...)

[DmaxMaverick]

The solenoid was doing the vibrating while plugged in idling too, but when I pulled vacuum at the actuator it was steady at about 17" Hg. Is it supposed to vibrate when hooked up and at idle? I would think if there was an issue I would have seen it on the vacuum gauge at the actuator.

My boost bolt and new boost solenoid and boost sensor will be here today, so I am going to get my boost gauge plumbed in and take it for a spin.
The vibration is normal. ECM boost control is not on/off, but is modulated and will allow wastegate opening at a gradual rate. 17" is a little high, but within the margin of error. 15" is ideal at sea level at any "full closed" RPM, with 11" the minimum. Less than ideal variations could be a matter of a well seasoned solenoid, or aftermarket programming adjustments. If you have an aftermarket power program, it wouldn't be unusual to have increased wastegate actuator vacuum, as a means to compensate for increased exhaust back pressure against the wastegate.

[6.5_Diesel]

The vibration is normal. ECM boost control is not on/off, but is modulated and will allow wastegate opening at a gradual rate. 17" is a little high, but within the margin of error. 15" is ideal at sea level at any "full closed" RPM, with 11" the minimum. Less than ideal variations could be a matter of a well seasoned solenoid, or aftermarket programming adjustments. If you have an aftermarket power program, it wouldn't be unusual to have increased wastegate actuator vacuum, as a means to compensate for increased exhaust back pressure against the wastegate.


Ok, I will hook the trailer up tonight and take it for a spin after I get my boost bolt in so I can monitor boost again. I relocated my AIT back to stock as mentioned. As far as the boost solenoid goes, to my knowledge it is the original from new. I do not have a chip or anything to increase power. Just a PMD relocate, straight pipe exhaust and a not so cold air intake.

[6.5_Diesel]

Well, I installed my boost bolt last night and did a check with the new boost solenoid to see how it compared to the one on the truck. Both resulted in a vacuum reading at the actuator of about 19" Hg. So I left the stock one in place and took it for a drive with my 850 lb 4 wheeler in the box and a 3400 lb trailer on the back. It would still build boost as I was accelerating until it hit about 10 psi then would drop down to about 8 psi, which it appeared to hold. It appears to be working, but with the load I hooked up I did not get the engine temp up to about 200-210 like when I am pulling hard, so I will have to wait till next weekend when I take the camper on a trip to see if it actually works when under heavy towing.

[6.5_Diesel]

So I took the truck on a trip this weekend pulling the camper. I still get either a defueling symptom or an inability to control boost. It seems to happen when I drop below 60mph but as soon as I get to about 50mph it straightens out again (when in 5th gear). When it acts up I tried downshifting into 4th and that would not cure the problem it would continue to defuel/surge the boost.

I did have something else happen as well. I pulled the camper for 2.5 hours and then parked the truck for the weekend. When I went to start the truck after 3 days to hook the camper up, the truck would crank and crank but not start. It would not even try to start. No smoke out the tailpipe or anything. I was getting fuel to the fuel filter, but it seemed like nothing was going to the engine. I then went and unplugged my relocated PMD and plugged in the old one on the side of the pump, but nothing. Then I plugged in my relocated PMD and it started. Then about 10 miles down the road I had a huge miss in the motor, so I am putting money on my PMD is about to fail.

My last question is could a failing PMD cause my other symptoms of the defueling/boost surging? I would think that it is a definite possibility.

[Robyn]

The PMD is always on the hit list.

The PMD is the big weak link in the injection system on these engines.

What brand of PMD are you using ???

[a5150nut]

And how good is your extension wire for your remote PMD?
Maybe clean connector pugs with electrical contact cleaner and spray a little di electric grease in them.

[6.5_Diesel]

The PMD is always on the hit list.

The PMD is the big weak link in the injection system on these engines.

What brand of PMD are you using ???

I have the relocate kit from Heath Diesel. I cannot remember what brand PMD it came with as it was like 6+ years ago I did the install. I have a new one on the way from Leroy Diesel along with the #9 resistor.

[6.5_Diesel]

I have the relocate kit from Heath Diesel. I cannot remember what brand PMD it came with as it was like 6+ years ago I did the install. I have a new one on the way from Leroy Diesel along with the #9 resistor.

Well I took the truck on another trip with the camper and it happened again.....a no start. I found out the no start is from my harness going to the relocated PMD. To make it back home I go lucky that the pig tail off of the pump was just long enough to reach the PMD by setting it on the intake. The original PMD is a Dtech with a No. 7 resistor. I have ordered a new cable and will be mounting my new PMD with a No. 9 resistor onto the heatsink. I will be keeping the Dtech that was on the truck for a spare.

On the trip down camping, I still had the defueling/boost surge issue when I got between 60 and 55 mph (in 5th gear), so when I was at the camp ground I threw in the new boost solenoid I had to see if on the way home it would still do it. It still does the defueling/surging when I hit just the right speed ranges in both 4th and 5th gear. I have a new boost sensor that I am going to try, but after that I am out of ideas on what it could be.

Also, I noticed an oil leak coming down the passenger rear side of the bell housing. When I look up top, I can also see some oil in the valley under the intake. Any ideas on what could be leaking? Could it be the oil pressure sending unit?

[JohnC]

I seem to recall issues with surging when I was experimenting with a #9 resistor, about 18 years ago... IIRC, mine was calibrated for #4.

[6.5_Diesel]

I seem to recall issues with surging when I was experimenting with a #9 resistor, about 18 years ago... IIRC, mine was calibrated for #4.

I was wondering if the resistor change could mess up the air to fuel algorithms and how the ECM responds causing the issue. I have read that the computer can be very picky when it comes to maintaining proper air to fuel. What is the stock resistor for a 3/4 ton F VIN engine? Is it still a #4? I remember reading somewhere what it was but I cannot remember.

[JohnC]

The resistor is to calibrate the individual injection pump to the software. (Don't know how to better explain it). So, the resistor goes with a particular pump, not a vehicle. IIRC, 5 is the target setting, so if you're not sure what you should have, 5 is a good guess.

[More Power]

I was wondering if the resistor change could mess up the air to fuel algorithms and how the ECM responds causing the issue. I have read that the computer can be very picky when it comes to maintaining proper air to fuel. What is the stock resistor for a 3/4 ton F VIN engine? Is it still a #4? I remember reading somewhere what it was but I cannot remember.

There is no such thing as a "fuel to air ratio" when talking about a diesel. A diesel should always have an excess of air, with fuel-rate determining engine speed/power/emissions. A turbocharger just allows more fuel to be injected, to make more power.

Most fuel injection pumps use a #5 resistor. The resistor is used to calibrate the fuel-rate of the injection pump. Due to mechanical tolerances, some pumps need a little more help than others, which explains the need for a calibrating resistor. Can't go wrong with a #5.

[6.5_Diesel]

There is no such thing as a "fuel to air ratio" when talking about a diesel. A diesel should always have an excess of air, with fuel-rate determining engine speed/power/emissions. A turbocharger just allows more fuel to be injected, to make more power.

Most fuel injection pumps use a #5 resistor. The resistor is used to calibrate the fuel-rate of the injection pump. Due to mechanical tolerances, some pumps need a little more help than others, which explains the need for a calibrating resistor. Can't go wrong with a #5.

Interesting. I was always under the assumption that during the combustion process you look at your stoichiometric ratio of air to fuel and you always wanted to stay slightly above that ratio in order to ensure completeness of combustion. If you dive below this point you will not consume all of the fuel resulting in an inefficiency or incomplete burn. I was under the assumption that all vehicles even diesels had to monitor this and used air (boost) and fuel curves in order to ensure the engine didn't enter a fuel rich or fuel deficient stage.

Learning something new every day.

[DmaxMaverick]

Interesting. I was always under the assumption that during the combustion process you look at your stoichiometric ratio of air to fuel and you always wanted to stay slightly above that ratio in order to ensure completeness of combustion. If you dive below this point you will not consume all of the fuel resulting in an inefficiency or incomplete burn. I was under the assumption that all vehicles even diesels had to monitor this and used air (boost) and fuel curves in order to ensure the engine didn't enter a fuel rich or fuel deficient stage.

Learning something new every day.
This is essentially true for all internal combustion engines. However, only recently (2001 M/Y for GM/Duramax) is air by volume measured and fuel adjusted accordingly to more efficiently combust Diesel fuel. Unlike gasoline and other volatile fuel engines, a Diesel engine simply cannot have "too much" O2 (it simply passes on what it doesn't use). Too little air/O2 for the (attempted) combustion of fuel results in black smoke (old school).

In regards to the resistor, it's a nothingburger. Minimum to maximum extremes won't yield any appreciable "power" or efficiency. Perhaps something that can be realized with a laboratory dyno, but even then, gains/losses will still be within the margin of error. You'll never feel any real-world difference on the butt-dyno or MPG's. Use a resistor that doesn't cause the PCM to complain, and life is good. Problem is, IP's are rarely calibrated such as they were before original delivery on a new vehicle. Once a pump is rebuilt, the original calibration spec goes out the window, and we're then shooting from the hip. Very few distributors (if any, anymore) include the correct/calibrated resistor with a new or reman IP. If you're using a #9 and it works, stick with it. Same goes for a #4.

[arveetek]

On the trip down camping, I still had the defueling/boost surge issue when I got between 60 and 55 mph (in 5th gear), so when I was at the camp ground I threw in the new boost solenoid I had to see if on the way home it would still do it. It still does the defueling/surging when I hit just the right speed ranges in both 4th and 5th gear. I have a new boost sensor that I am going to try, but after that I am out of ideas on what it could be.


I'll mention again what I did to my '95: I removed the vacuum pump and associated plumbing, installed a Turbo Master wastegate control, and Kennedy Diesel's chip in my ECM. I've never had a defueling problem since, and I can drive by the EGT gauge now. These upgrades made a tremendous difference in the towing capability of my rig.

Casey

[JohnC]

Putting a #9 resistor on a pump that should have (say) a #4 tells the PCM that the pump delivers less fuel for a given command than it really does, so, when the PCM commands a specific rate the pump actually delivers slightly more. This is the basis of the "hot rodding" idea, but, as Dmax said, it's barely a measurable amount. What it does do, however, is make the PCM's commanded fuel rate transitions more dramatic than they should be, hence (I believe) the surging issues.

It is my recollection (could be wrong) that the resistor isn't actually part of any control circuitry. Rather, the PCM reads the value of the resistor to determine which fuel map it should use for a given pump. That value is stored in the PCM's memory and stays there until you perform a TDC offset learn, even if you change the resistor or pump. The proof is, you can remove the resistor, without any effect on performance, and a month later the PCM will still report the same resistor value.

[DmaxMaverick]

Putting a #9 resistor on a pump that should have (say) a #4 tells the PCM that the pump delivers less fuel for a given command than it really does, so, when the PCM commands a specific rate the pump actually delivers slightly more. This is the basis of the "hot rodding" idea, but, as Dmax said, it's barely a measurable amount. What it does do, however, is make the PCM's commanded fuel rate transitions more dramatic than they should be, hence (I believe) the surging issues.

It is my recollection (could be wrong) that the resistor isn't actually part of any control circuitry. Rather, the PCM reads the value of the resistor to determine which fuel map it should use for a given pump. That value is stored in the PCM's memory and stays there until you perform a TDC offset learn, even if you change the resistor or pump. The proof is, you can remove the resistor, without any effect on performance, and a month later the PCM will still report the same resistor value.

The resistor value remains in memory and isn't checked again until an IP diagnostic condition exists, or a specific number of starting/warming cycles. If you drive your truck once a day, week, or infrequent, it may be months/years before the resistor value is updated. In this case, it doesn't matter which resistor you have installed. It will map fueling according to the last value read until it reads it again (and will set a DTC at that time if something is amiss).