View Full Version : '96 6.5L Suburban with poor throttle response
I am having a similar problem with my '96 6.5L. The throttle response will just suddenly go away at times. It starts and and idles fine, and even seems to run fine around 2000 rpm or greater, but trying to increase the rpm from idle, the engine rpm just drops out. IE if I get the engine to 1100 rpm and hold the throttle, the rmp drops right back to idle. I have pulled the batteries and cleaned all the terminals and cleaned the RH intake manifold ground connection but it is still the same. I even put on an old pmd and it was the same. Anything else to look for?
I even put on an old pmd and it was the same. Anything else to look for?
Was old PMD suspect?
I started experiencing what felt like a "lazy pedal" when I was going down a hill and gave it fuel. I felt and heard little to no change for a little throttle position change. I have always heard a subtle change in combustion sound when you can tell the engine is building power or compression braking and the break over between the two in either direction but this lazy pedal feel is different. Its like no throttle input at all and no real combustion. A couple of times I had to mash the pedal harder just to make sure it was working.
Anyway short story longer .... not too long after I experienced a few stalls. I have since installed a new DTech FSD and don't have a lazy pedal anymore or had a stall in over a month.
I know my IP is getting long in the tooth at ~ 170K miles and feel with age precise fuel metering is harder to maintain with aging IP, timing chain, fuel solenoid, and PMD/FSD. Consider getting a good spare FSD just in case.
Oh and I have always had a little hard of a time holding just off idle throttle with just a touch of throttle pedal. Some of which is normal. I think its the idle controls kicking in and the throttle position by electronic wire system.
Now if you try and hold higher rpm 2400 or more and get big swings of of climbing or falling rpm (greater than 100-200 rpm or more) then thats different and points to IP issue. Many have written it will swing wildly when IP goes bad. The throttle position sensor should throw a throttle position error code if it goes bad so don't think its that.
I think there is some funky electronic controls going on around 1700-2000 rpm with wastegate solenoid and throttle input when under light load for economy reasons so try not to get fooled by that particular rpm area.
The old pmd I installed as a test was one that had been giving me problems, but not identical to what I was recently experiencing. I knew it wasn't a great indicator of what is or isn't wrong, but I was thinking that if it were the pmd the chances of the exact same thing happening with two different ones (although one being faulty) would be low. Or at least I thought and hoped so...
The 96 in an OBD2 system and most electronic issues will set a code.
First be sure your fuel filter is clean.
Next open the water drain valve up front and be sure fuel comes out at idle.
If there is little to no fuel then you most likely have a lift pump issue.
If the lift pump is not running then you will get a fuel starvation issue.
The computer will likely not pick this up.
Make sure of these things before spending $$$$ chasing Ghosts.
A fuel starvation issue will manifest itself in many ways.
The fact that the engine starts and idles fine is a good indicator that things are not too far wrong.
A PMD thats toast will generally manifest as an intermitent stalling and or no start hot issue.
I lift pump issue is easily remedied
Give it a go and keep us posted
Robyn
Just the other day after I posted I tried to bump up idle to 1100 rpm and hold it with my foot. It only took just barely touching the pedal to jump to 1100 rpm and within 2 seconds +/- the idle dropped back some. I could readjust my foot and again bump up 1100 with a touch more foot and hold it steady but if I started from scratch each time it would fall back.
Explain how you are doing it again and how much problem you have. Try to bump up a second time and or up to 1600 rpm and see how much fluctuation you have.
I'll try again in the next day or so since I ran out of fuel additive and this might have some influence. I try and run a mix of Opti-Lube XPD and 2 stroke oil. And I can tell a difference in "idle control" with and without additive when launching in 2nd gear no fuel pedal input. I usually launch easy no input then "roll" into the go pedal.
By the way, I started this saying that I had a "similar problem..." I had searched the forums and was trying to add on to another thread, but it didn't happen for some reason.
This problem really has me stumped. I was thinking that it might be the TPS and there is this thread:
http://www.thedieselpageforums.com/tdpforum/showthread.php?t=33031&highlight=tps
I tried to do what is described by More Power in the third post, but what confuses me is the TPS. I am assuming that the TPS is mounted on the pedal assembly inside the vehicle. I have looked at NAPA online for a new TPS and the one they show has three pins. Mine has 9 pins. Also from the post, none of the wires leading to what I am assuming is the TPS match the description that is given in the TPS post and thread that I referenced above. Am I totally missing the boat here?
As for how I recreate the problem, I think that I described it pretty well already. The engine RPM does drop from 1100 rpm to idle. Around 1600 it seems to be less of an issue. Sorry I haven't added much here, but what I can add is that when my wife was driving it, and it first became a real issue, there was zero throttle response. She was out with her parents and a couple of the kids and coming down off an overpass when the throttle totally went away. It still idled but there was zero response. She pulled into a nearby parking lot and my father-in-law tried to to figure out what was wrong. There was even a guy nearby who was a heavy duty diesel mechanic and he said it was probably the TPS. My wife called a tow truck and when I showed up to pick them up, I started the suburban and it ran fine with what appeared to be normal throttle response. Still we had it towed home in case it happened again.
I called Bill Heath (Heath Diesel) and he had me check the ground connections (also described in the TPS thread above in post #2 by Robyn) I cleaned all the all the connections and the problem still did not go away. I have heard mixed opinions about the TPS, some say that it is rare for it to go bad some say that it is a frequent occurence.
In any case, the throttle totally went away once, and my wife was saying that before that occured the throttle response had changed and that there was a delay (a lag) in the response. I don't know what to do next.
DmaxMaverick
12-06-2008, 15:17
The TPS is unique to MFI fuel systems. Yours is EFI, and doesn't have one. Your pedal is the APP, and is the only connection from your foot to the PCM, or any other throttle circuit. If your Service Throttle Soon or SES lamp is not on (check for DTC's), the problem is not the APP. It is likely pump, ground, connector, voltage or PCM related. Eliminate them from least to most complicated. It would be helpful to check the PCM for stored codes, even if the SES/STS lamp is not on. Do this before removing battery power, as some of the related codes may reset with a power disconnect.
I just got back from Napa and I took the pedal with the APP attached. Found out what it was (the APPS), and a new one is $540. I don't want to head in that direction yet. I knew that my sub was an EFI, but every search on the web that I specified a '96 6.5 L TD still came up with the TPS as a replacement part and that was starting to confuse me.
I will try to pull the codes again but since I aready pulled the batteries any that were stored when my wife was last driving will be gone. I am still really stumped as I did clean all the ground connections once. I will try to go through all your suggestions again, but this is pretty frustrating. If it is not the grounds, and not the fuel system (ie lift pump, water in the lines, filter...) then I am assuming it will be the FSD again, or worse yet the IP. Still I only have about 30k on this IP so I hope it isn't.
DmaxMaverick
12-06-2008, 16:59
Curious....I'd like to know where the "other sources" feel a TPS should be attached. The TPS is located on a mechanical injection pump, linked to the throttle input shaft. Your pump, and all EFI pumps, have no such throttle input shaft. A 1996 MFI fuel system, used by the military (HMMWV), marine, and stationary applications, may have a TPS, though.
Eliminate the APP, if you feel the need. Use a Tech II or compatible scan tool to read real time throttle position, in operation. The APP is essentially a series of potentiometers, so it can also be read with an VOM, while disconnected. It has 9 connections, 3 for each independent circuit. Ground, input, and output. It operates at 5V, so be sure to not hit it with full battery voltage. I doubt this is your problem. The system is triple redundant. The 3 circuits operate independently, and the PCM throws a fit if just one fails. The throttle should still operate normally, even with only one circuit operating, although the STS/SES will be on, and may cause limp mode.
When checking your grounds, also verify the ground wire at the PMD harness is grounded to the pump, and not somewhere else. This has been known to cause running issues. The Optical Sensor Filter is also a known trouble causer, but should be accompanied with a DTC.
Still, check the PCM for DTC's. Not all of them trigger the SES, and they can be intermittent.
Thanks everyone for you help. I did a little more trouble shooting and here is more information.
FYI I bought the Sub in Dec 2002, it had 85,000 miles and had IP replaced once. In the summer of 2004 I started to have issues which turned out to be the FSD, but before I replaced the FSD with a remote one I replaced the lift pump back then. In the summer of 2005 at about 120k the IP went and replaced it (the IP had at least 35,000 miles on it), as well as all the injectors and the glow plugs. In the winter of 2005 my wife slid into a pole and damaged the remote FSD so I replaced it with a new FSD then. So it has been 3 years without any issues and I now have 150,000 miles on it. I am not sure if it is the FSD, the IP, or now maybe the lift pump (see below).
Anyway with the recent problems I didn't replace the APPS, but I did buy some contact cleaner and I sprayed the connections of the APPS before I reinstalled it. I also pulled the cable off my remote FSD and I sprayed the connections and plugged it back together. I also pulled the splice connection that extends the wiring for the FSD apart near the IP and I cleaned it and reconnected the wiring harness.
I then pulled fuel filter and I used a syringe to get all the remaining fuel and dirt out of the bottom of the filter housing. I then installed a new filter. One question here though, the center tube that fits in the center of the filter has a fine filter screen on it and it pulled off when I pulled out the old filter. I tired to get it back on but it didn't seem to want to stay as it came off again when I pulled the new filter off to fill the housing with fuel before I tried to start it. How vital is this filter screen? It seems to be a secondary filter and I left it off for now.
Anyway after much cranking and trying to bleed the air out of the system I got it to start. Oh, before I changed the filter I opend the water drain valve while the engine was idling and nothing came out (lift pump?).
Once running the throttle response seemed to be the same, ie while trying to hold the engine at 1100 RPM it would drop back to idle. It appeared to be fine above 1500 RPM.
I took it out for a drive and went about 20 miles on the highway (at 65 mph) and I didn't have any issues, the throttle response through my neighborhood was fine and out on the highway it was fine. On the off ramp heading home I had to stop and then there seemed to be a lag in the throttle. I had stepped on it after I had stopped and it didn't respond right away and in that instant before it did go I was wondering what was happening. But like I said it was only an instant and then it took off. Through the neighbohood and back home I didn't have any further issues. After about 20 minutes I had to run an errand and so I took it out for a quick 15 minute trip. Again there were no issues.
So I still have the same issue while trying to hold the engine at low RPM (other than idle), and I had one issue where the engine didn't respond like I thought it should. None of the indicator lights on the dash showed any problems.
I will pull the intake cover off to access the IP and check the ground there, and I will check the other grounds and connections again. I will get out my code reader and check for any codes. Probably won't happend until next weekend, but I we will use the sub to run around town when we can.
DmaxMaverick
12-06-2008, 21:58
........Anyway after much cranking and trying to bleed the air out of the system I got it to start. Oh, before I changed the filter I opend the water drain valve while the engine was idling and nothing came out (lift pump?)........
OK. Stop right there. Now, you got something. The LP should flow fuel very well with the engine idling. Don't do ANY other diagnostics until it does. If nothing else, it may keep you from chasing a wild goose if it works like it should. With the engine idling, it should be able to fill a pint bottle in less than a minute, and hold 4-10 PSI (just plug a gage into the drain hose). All the while, the engine should not die. A healthy IP will run the engine, although poorly in some respects, with a failed LP.
I agree. Still I took it out this morning and I was about 2 miles from home when the throttle (pedal) went away on it. The "Service Throttle Soon" indicator light came on. I pulled off to the side and the engine was still idling. I shut it off, then I started it and the throttle came back. I turned around and went home.
Is it possible that both the lift pump and the APPS are shot, or is the lift pump causing these other symptoms? ie if it is starved for fuel will that cause the service throttle soon light to come on?
Is it possible that both the lift pump and the APPS are shot, or is the lift pump causing these other symptoms? ie if it is starved for fuel will that cause the service throttle soon light to come on?
Bad coincidence maybe.
I wouldn't think that the fuel supply would have a direct effect on the APP. Fuel starvation could on the other hand cause stress and problems with the fuel solenoid and IP over time. And it can have an immediate affect on fuel delivery to injectors.
As far as I know the PCM controls idle and governed redline WOT exclusively no APP input needed or used. PCM takes commands from APP and then commands the FSD for all in between fuel metering rates unless in limp mode or SES light etc. And I don't think there is any feedback mechanism directly.
DmaxMaverick
12-07-2008, 19:44
If this is the first STS lamp you've seen, your R/R may have something to do with it. I suggest unplugging it, check the pins, and use some dielectric grease on them. You may have a poor connection. Like said before, if anything is amiss with the APP, the STS lamp will come on. I think it is a wiring issue between the APP and PCM. If it were the APP, the STS would be on long before the throttle acted up. (remember, triple redundancy, and it isn't likely all 3 fail at once)
Fueling (or the lack of) will not effect APP operation. It is only an input device.
The STS is the first that I have personally seen. When my wife called and we had it towed home, she told me that the "some light" was on, but I she wasn't clear and I thought that she meant the check engine light. I now think that she meant that the STS light was on. It might have come on at other times and I not know about it since my wife is the primary driver of it. We also hadn't been using it too much this past year with the price of fuel.
DmaxMaverick
12-08-2008, 23:01
If you have/had throttle problems, and the STS was on, there will be stored DTC's, and they will indicate a throttle problem. If this is the case, then the APP needs attention. They are expensive, but there is a "renewal" process you can try. Not always successful, but better than $400 for a new APP if it works.
My suggestion would be to change the Oil Pressure Switch. Every time that I have run into a simular problem it has been the OPS.
N9Phil
Fix the issues one at a time.
Get the fuel delivery issue fixed first.
Oil pressure switch and lift pump and any other related goodies.
Once you have good fuel delivery then go on to the problem of the STS light coming on.
Get a scan and see what codes it tossing and this will possibly lead you further.
Keep us posted
Robyn
I am just online checking out the recent posts and getting ready to take another look at it. There is a lot going on today so hopefully I can squeeze in some time for the sub. I'll let you know what I find out. Thanks again.
I replaced the lift pump on Saturday and I took it out on the highway and put about 30 miles on it and tried to run it under different conditions and I accelerated hard at times. I never had any throttle lag like I did the last time I took it out on the highway and it seems to run better. Having done that my wife is now comfortable driving it again, and I told her that if the throttle does go away again that all she needs to do is to shut it off and restart it and she should be good to at least get home.
So, hopefully the problem was only the lift pump, or now with it getting regular use again the problem will pop up again and and I can start narrowing it down again. I'll keep you posted.
I have had all the same issues on our rig what a pain. I have a APPS pot new from the manufacture if you would like to test with it but I think your problem lies else where.
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