Interesting Resolution to Engine Hesitation

[maverick2]

I bought a 94’ Tahoe w/ a 6.5 turbo diesel last summer. At the time of my test drive, I noticed a slight engine hesitation/stumble at times – the owner attributed it to the PMD and said it was a common problem. Made sense to me, as I had read about that (mostly on this forum) as I did a little background search prior to buying. I took the info at face value and bought the rig. The problem persisted, (it was very slight, not getting any worse or better) so I didn’t worry too much about fixing it right away, knowing I’d have more time to mess with it during the winter when my work slowed down. As time passed, I noticed something that made me think the hesitation was NOT associated with the PMD or other things (fuel solenoid,…) commonly associated with fishbite/hesitation/stumbles,… The problem was not erratic, but rather always seemed to occur on the same stretches of road, or the same type of driving conditions. As I payed more attention to it, I figured out what those conditions were, and I could replicate the situation whenever I wanted to. After pondering on this a while (I’m kind of a slow thinker sometimes) I convinced myself the problem was most likely associated with the programming/algorithm that controlled the engine, and figured I had a flaky chip. Knowing I wanted to upgrade chips anyway, I decided I’d try that before I swapped anything else out. The Tahoe had the original exhaust system (with the catalytic converter still on it) so I figured I’d make the exhaust a bit more free-flowing (larger pipe, free-flowing muffler, losing the cat in the process…) before I swapped chips. Did that work today (exhaust changes only – no chip changes) and found out I also lost my engine hesitation/stumble in the process… Ended up driving a 100-mile stretch of road that is notorious for making my engine do its hesitation gig (I drive this stretch 4-6 times a month) and didn’t get a single stumble. I then went out of my way to try and make it stumble using the same technique I had in the past, and I still couldn’t make it happen. Only thing I can think of is that the added backpressure of the cat and more-restrictive muffler had enough negative impact on one of the engine parameters that offers feedback to the chip, that it created a combination of signals that the chip couldn’t interpret correctly, so it would cut fuel off temporarily. Don’t know which engine parameter that would be and how exactly that process works, but thought it very interesting that the “engine hesitations” disappeared with only this particular change… Thoughts?

[Railroad Ron]

That is interesting. My '94 has had the same type of problem since I bought it last November. I thought mine was either the PMD or bad injectors, since the previous owner had the pump replaced, but not the injectors (195k miles). So I put injectors in, and also ended up replacing the PMD on a trip to Minnesota but it still has the hesitation. It also starts a bit hard, so I wondered about the lift pump. I checked that this morning and it seems to be ok. Maybe I'll take a look at the exhaust...:confused:

[maverick2]

For what it's worth, the condition under which my engine would "hesitate" is that moment when I'd transition from "coasting" to being "under power", usually when going down a twisting, mountain grade or following a river road. In both cases, I'd be on and off the throttle pretty regularly -- the moment I'd "feather" back onto the throttle, the engine would stumble. If I wanted it to stop the hesitation, I'd stab the throttle and problem would end. I'm guessing the added backpressure of a plugged cat and restrictive muffler had enough undessirable impact on boost pressures, that the boost sensor feedback was out of the ordinary when taking the other feedback parameters into consideration. Stabbing the throttle probably kicks in a throttle position sensor over-ride (or similar mechanism) and the engine resumes normal operation. I do know I was surprised when the engine "hesitancy" problem ended with a simple exhaust change.

[Railroad Ron]

That sounds EXACTLY like what mine does! Thanks for the help- I'll keep ya posted!

[TAG]

The truck in my sig does the exact same thing, always at a light movement of the throttle, usually after i get on the highway in the morning and get up to speed and let off the throttle. As i pick up the throttle again to maintain speed it will buck one time. The problem has persisted from the time the present inj pump was put on at 92,000 miles to now with 265,000 mi. I have gone through multiple pmds since then as well as performed all the modifications in sig & it has not changed one bit. I always figured id have to replace this pump at some point & maybe that would fix it, but the pump wont die. It also has started rough when cold since the very day this pump was installed by the dealer.

[Hubert]

I think it is precise fuel metering issues when feathering just off idle or during light fuel rate load and variation of load softly. Maybe due to timing wear and overall IP wear and truck age etc and combination of all. Can't say for sure exactly. Now other stumble fishbite at heavier throttle is different but what you are describing is fuel metering IMO.

Now that you have added better exhaust and improved performance enough it has covered up the issue for a while. I'd say it will probably be back one day with more age.

I believe what happens is a build up of errors of "timing" the optic sensor error due to timign chain wear vs PMD current draw vs crank speed variation vs commanded fuel metering rate and the ECM hiccups somewhat rythmetically.

I'll post more later.

[w6bauer]

Wow! Last night on my way to work, about 15 miles into the trip I got the same stumble that is being discussed. Not very bad but definitely noticeable. Anyone else have any ideas on any other possibilities of the problem?
Reading the thread I am thinking that if a simple exhaust mod would take care of it that would be awesome, however, I think I might have to agree more about the timing and fuel delivery issue. I'm no certified mechanic, but I think it makes more sense.
220,000 miles on the clock, do you think I need to replace the timing chain and sprockets?
I had a 96 gasser burb with 270,000 miles when I ran into the same issue, but the stumble got much more noticeable over time and even turned into a jerk along with a clunk. Never figured out what it was, sold her first, poor guy didnt bother to test drive her before buying. :eek:
I do believe it is something in the motor. After work when I started her up she ran fine, cycled through the glows a couple time like normal. After idling for about two minutes there was a slight stumble, then again 20 seconds after that. That was it, no stumble on the way home at all.
What is this? I really dont want to replace tons of different parts until i figure it out, kinda strapped for cash, going to diesel school in 2 weeks.

Thanks in advance

Will

[joed]

Mine was a 5-spd manual, so the 'jerk' back on was fairly pronounced. Mine would typically do it when the engine was still warming up to operating temps, but would also do it when towing and stepping on the throttle after an upshift or downshift.

After trying a new remote mounted FSD and checking all grounds, etc., I came to the conclusion the actual fuel solenoid on the injection pump was sticking. The tell tale was the Fuel Solenoid closure times on a scanner were long - often over 2ms, I think the spec is 1.95ms or less. I would often get FSD related codes.

I just lived with it since it ran so well otherwise, plus I was out of warranty...

Hope that helps. Joe.

[Hubert]

Let me start with fishbite can be a decription of several different problems. Precise fuel metering with electronic controls that have age/wear and loose tight control can be one cause. Transmission, grounds, lift pump, bad fuel, shocks bushings, etc can cause it too. So don't replace parts on just this one post.

There is some uphoria with a new exhaust and the improved throttle respose. The improved throttle response is evidence enough of improved performance too. And I am proposing that there really isn't much difference in the amount of fuel delivery that can cause fishbite during light throttle/light load.

I have been thinking more recently that if you drive a truck to "get groceries" most of the time the actual stroke of the fuel metering solenoid is a short high frequency stroke that could tend to wear more heavily than a truck that gets flogged regularly. And with light useage the plunger may build up some gum too. I recently noticed if I really flog my truck it tends to drive better for a little while???? More fuel rate with additives could flush the IP better????

When the fuel rate is really low and the amount of fuel delivery required to change rpm is small and the timing chain is worn and the backlash of the optic sensor builds vs the crank speed. And the fuel solenoid is worn or bad contacts wiring etc the current varies some so that the ECM thinks it has to adjust the fuelrate and it can't precisely adjust fuelrate it can build a little rythmic pulsation through the drivetrain that is commonly sensed as fishbite.

[w6bauer]

i can rule out the trans and shock bushings as well as grounds. bad fuel is a maybe but not likely as i fill up at the station the big rigs use daily. i replaced the filter about 5 minutes ago, that could have been an issue. i am leaning more to the worn timing chain or fuel solenoid, possibly injectors. next thing i think i will replace is the injectors as they have never been done.

[hankb]

I have been a member of the page for a few years now, but have made no posts, till now.

Out of desperation...here goes.

I have a 94 Blazer that I retrofitted a 94 6.5T out of a light 3/4 PU. Changed the entire drivetrain, wiring and dash, over 90,000 miles ago. All stock, with Cat. OEM muffler. Dual thermostat housing and WP. Never runs hot. Never driven very hard,(heavy trailering). 373 gears.

About 70,000 miles ago I noticed a "Fishbite" ("hesitation" is not dramatic enough). This was as if someone had turned off the engine for a millie second and turned it back on. As if the engine came to a complete stop. It really hammers the drive train. It would do it once or three times in a 500 mile trip. I have more or less lived with it because it was something that was so intermient and unreproducable. Lately it has gotten really bad. It has gotten progessively worse. It will do it about once a minute, allthougth not as sever.

I fear the drive train/ transmission has suffered.

I have tried removing the Cat, no change (This seemed like a good idea because both the wife and I could both hear and feel the Pop /bang, in the floor in the location of the Cat). I am using a stock dodge cummins muffler. The muffler is next to be replaced. It would also sound as if it was backfireing in the turbo/ filter box area. It has never done it more than one POP at a time, never one after the other.

I do all my own maintenance. Never been to the dealer.

Possible causes:
...Water in the fuel, It would be more continous.
...Air in fuel, same.
...Fuel solinoid, very possible.
...Fuel metering, Hmmm, The hesitation that some describe, just at throttle transition, seems like a issue with the throttle position sensor. I have not experienced this problem at any time other than at highway speed at light to medium thottle. If I mash the throttle...no problem, good power, and not just for a second or two but long hills where it would have plenty of time for a problem to show itself. That means fuel delivery is basicly OK. Precision fuel delivery may be another story.
Fuel Pump, (not lift pump), never replaced by me. Wouldn't it act up more often?
...PDM, I am on my third. Remote now with a phone book sized heat sink. It has had this problem thru all replacements.
...A loose conection, no, It would act up at any time.

I think there are a bunch of 6.5 owners that are dealing with this. I would be greatful for more input.

Thanks, Hank

[More Power]

I have been a member of the page for a few years now, but have made no posts, till now.

Out of desperation...here goes.

I have a 94 Blazer that I retrofitted a 94 6.5T out of a light 3/4 PU. Changed the entire drivetrain, wiring and dash, over 90,000 miles ago. All stock, with Cat. OEM muffler. Dual thermostat housing and WP. Never runs hot. Never driven very hard,(heavy trailering). 373 gears.

About 70,000 miles ago I noticed a "Fishbite" ("hesitation" is not dramatic enough). This was as if someone had turned off the engine for a millie second and turned it back on. As if the engine came to a complete stop. It really hammers the drive train. It would do it once or three times in a 500 mile trip. I have more or less lived with it because it was something that was so intermient and unreproducable. Lately it has gotten really bad. It has gotten progessively worse. It will do it about once a minute, allthougth not as sever.

I fear the drive train/ transmission has suffered.

I have tried removing the Cat, no change (This seemed like a good idea because both the wife and I could both hear and feel the Pop /bang, in the floor in the location of the Cat). I am using a stock dodge cummins muffler. The muffler is next to be replaced. It would also sound as if it was backfireing in the turbo/ filter box area. It has never done it more than one POP at a time, never one after the other.

I do all my own maintenance. Never been to the dealer.

Possible causes:
...Water in the fuel, It would be more continous.
...Air in fuel, same.
...Fuel solinoid, very possible.
...Fuel metering, Hmmm, The hesitation that some describe, just at throttle transition, seems like a issue with the throttle position sensor. I have not experienced this problem at any time other than at highway speed at light to medium thottle. If I mash the throttle...no problem, good power, and not just for a second or two but long hills where it would have plenty of time for a problem to show itself. That means fuel delivery is basicly OK. Precision fuel delivery may be another story.
Fuel Pump, (not lift pump), never replaced by me. Wouldn't it act up more often?
...PDM, I am on my third. Remote now with a phone book sized heat sink. It has had this problem thru all replacements.
...A loose conection, no, It would act up at any time.

I think there are a bunch of 6.5 owners that are dealing with this. I would be greatful for more input.

Thanks, Hank

Have a look at this member tech page (http://www.thedieselpage.com/members/features/dtc36.htm) for a description that may cover some of these fish-bite issues.

Jim

[hankb]

Thanks for the info Jim...and working on Sunday...

Well I removed the CAT...no better.

The only difference is, it is doing this continuous now, getting worse by the minute. I am certain I will be dead in the water within the hour. I have resigned myself to replacing components or wiring till it is fixed.

I was sitting at the stop light today, foot on the brake and it did its shark bite thing...IT LURCHED FORWARD!!! I didn't have my foot on the go peddle...it was on the whooo peddle. It didn't die or miss a beat, it wanted to...go...? There has also been a surge when it is put in reverse(Real gear ringing type).

I have been thinking this may be a PMD issue. The current unit is from Rockauto, a reman unit. Hmmmmmm. @ $$$$$250.00

I got an Email from Northern Auto parts today. "diesel parts, on sale"
PMD's!!! Dorman products has just released a brand new PMD. It the write up it mentioned..."surge in reverse". My heart missed a COUPLE OF BEATS.
It is on its way. @ $$$$$253.00

Then I read this blog this evening and see that STANADYNE has a new "Bullet Proof" PMD. @ $$$249.00 At a 2 year warranty, seems like they can do better than that for "Bullet Proof". Didn't most of the old ones last that long? I think they just gave it a wax job.

Here is how I see this PMD issue...They have designed this fuel pump with too heavy a duty cycle for ANY electronic component to handle. It is just a gross mismatch. You wouldn't use a Metro radiator in your Duramax!
CATs' and Fords' (Navistar)use Hydraulics to accomplish the same task.

Someday they will invent a high speed, reliable solenoid and control electronics (read FSD/PMD) for intake and exhaust valves, It will replace cam shafts for unheard of infinite adjustability for power and mileage. (BMW does this now using other methods, but still uses a camshaft).

Jeeeese...no wonder my hair is getting grayer.

...Mercedes diesel in a new Jeep Rubicon... ya.

Hank

[simon]

The other day a buddy of mine dropped by with a 94 (5 sp std)with symtoms similair to above posts.No codes found either present or historyc
IP was replaced twice,once 10 yrs ago,last one 1 yr ago. there was occasionly fish bites under load and when accelerating trough the yrs.slow steady increase in rpm was smooth, but stomping on it would cause fishbytes troughout the range.The cause could not be found.last IP install also caused loss of power. 5521 IP.
I happened to have a brand new PCM+ eprom off a 95 with 5 speed on the shelf.After swapping the PCM,incl Eprom,and did a relearn TDC off set,all his probs disapeared completely.Power is back and fish byting is gone as well.

So, dont rule out the ECM as probbable cause.

[Robyn]

I am suspect that the removal of the Cat caused a slight change in the manifold pressure at that special little spot in the power curve.

What was probably happening is a little glitch in the data being sent to the ECM from the MAP sensor.

Possibly, the IP is worn just enough that there is some fuel issues right at that spot and the change in pressures and flow rates through the engine changed things just enough that the gremlin can no longer exist there.

These things can drive one crazy.

The old DB2 pumps just never had the issues we see with the DS4 series system.

With the engine breathing sooooo much more freely many things can happen.

Now here is another possibility.

GM states that the valves on these critters are supposed to be set at a recess depth of between .034" and .048" into the heads.
There is a caution about this. Too deep a setting can and will cause starting issues as well as driveability issues.

Not exactly sure about this other than deep set valves (Wear from time or from poor machine shop practice) will cause a drop in compression ratio.

Cold start issues I can see but the driveabilty issues are unclear.

Wear over time can do wierd things for sure.
Also a loose timing chain could contribute by allowing the timing to wander around some to the point that the ECM gets a little cranky while its trying to correct things and the result is the little stumble

Also some of the programing that was used on these things just plain sucked.

A poor injection advance curve (start of injection) could also be a contributing factor.

Again the change you made obviously altered things enough that the condition can't happen.


BEST

Robyn

[JohnC]

Hank:

If it's surging at "idle" then most likely the PMD or the fuel solenoid is acting up. If you can get a scanner on it, watch solenoid closure time.

A "reman" PMD????

[hankb]

I have installed a new(reman) ecm...but had to use same eprom, (I really wished to have replaced it, and I will). No Change.

Reman PMD?, thats what I said. You want to see the box it came in. "CARDONE" brand, Rockauto. Copied this from rockauto site: A-1 CARDONE Part # 772836 http://www.rockauto.com/catalog/images/MoreInfo.gif {[Injector Driver Module] Reman.}
w/OEM #12562836

FYI: Starts easily, good power, good mileage, 15 in town, no smoke(little bit when you stomp on it)205,000miles, no funny noises, changed injectors once, never been opened up...knock on wood, Don't drive it hard.

PMD will be here by Friday. Does anyone know who makes the Doorman product?

I have a take off IP with a Fuel shut off solenoid, going to swap that. The solenoid.

To top all this off...The hood flew open when the wife was on her way to work this AM...
Damage report: just the hood, its FU..beyond repair ...I had been working on it and left the hood ajar. :eek:damit!

About the harness extentions used to remote mount the PMD/FSD.
The wire in used in these harnesses looks to be monel. COPPER and NICKEL alloy. Extremely corosion resistant.
I have fabricated my own using every low resistance tecnique that exists.

Question for the electronics engineers out there: Can't the heat generating componant of the PMD be isolated from the circut portion? The transistors/ Mofset located on the heat sink side have to be the problem. For example ECM's are very reliable. There are no high heat devices like the PMD have. They only have to drive injectors(gas).
I held my PMD/ heatsink assembly once, it was too hot to hold. Don't get me wrong. I have a bigger and badder heat sink than anybody...:D

Hank

[hankb]

I have parked it.

The FSD was not the problem...I was sooo sure.

FYI, The Doorman, A1 Cardone units are Dtech units. I do not at this point feel the FSD was bad. I had 180 Thermostats in it...and went back to the 195 deg. and changed the fuel filter. This made it worse. I do have a code 36. Have had.
....Jim, I think you are right.

I tried to take it on a trip last week, but it didn't make it out of town.
Took another rig, it was a long night!

When I got back, It would start for a second or two then die. I tried this for a few minutes and it finally came to life. It sounds terrible. I have since tried to move it in the yard...same.

Do I give up, part it out, or fix it. The IP is next. The rig is just wore out.
The engine ran good... 28 days of the month.

For Sale, 94 Blazer, Silverado, sick.5 turbo diesel, 4L80 trans, 220,000 miles, FU hood, small dent in drivers side(forgot there was a trailer back there), OE wheels, wore out tires, could use a good presure washing...outside too. Bright Red. $1000.00 AS is. If you can find the gremlins and shoot them in the head, it could go for another 100,000...yes it ...could...

Down for the count, Hank

[85-m1028]

Mine had a fishbite once while idling in the drive way, no codes, no surging, stalled on me after a short trip to the fuel station. I ordered a new PMD however.....

I noticed a reoccuring theme here and it relates to the timing chain wear and tear. I have almost 300,000 miles and I'm pretty sure the timing chain is original..

I did have the keyway shear off on my injection pump flange and this is also related to harmonic vibrations from a worn timing chain.

Should the computer throw some sort of code for a timing issue????