I have severe problem with rough running, stalling and codes P0370 & P0251. I have replaced PMD (twice), IP (twice), ECM (twice) without fixing it. I first broke down on vacation two months ago.


The symptoms were first a drop in turbo pressure which then would max out and intermittant chugging/missing. I first replaced the boost pressure solenoid which helped even out the boost but still had missing.


It then started to severely miss and stall repeatedly. The codes were P0370, P0251, and P0606.


I then replaced the PMD with new longer remote cooler setup. Still no change. I took it to a mechanic and he first told me the ECM was bad, so I took it to GM and they had to replace it twice because the first one they installed was bad. Still no change.


The mechanic then checked again and only found codes P0251 & P0370 and told me the IP was bad due to a bad optical sensor. So I unhooked the optical sensor and drove back to Utah. Ran great other than long starting times and lack of power.


Once returned I installed a reman IP. It ran good for approximately 2 weeks then began again to stall and miss. The codes were P0370 and P0251. I tried another PMD with no change. I then installed another reman IP which still does not run, except with the new IP it will not run even if the optical sensor is unhooked, it will start but then it revs up and down and then misses and stalls. I haven't gotten the latest codes since the second IP was installed because I have to visit my local autozone for retrieval and it won't run that long.


I have tried checking the connection to the remote PMD and optical sensor for problems, I even replaced the short wire section that connects to the IP.


I love the vehicle but I am getting extremely frustrated with this. (Also I have replaced the fuel filter and checked the fuel pump).


My last guess is the crankshaft position sensor or the second ECM I have from GM may be faulty as well.


If I can't figure it out by myself I will have to tow to a diesel guy that timed the original one and see if he can figure it out. Please any help out there? Any instructions to run tests?

sounds like a loose connection to me, grab the wires going into you PCM while the truck is running and wiggle them, see what that does, then work your way up the harness, to the three plugs at the firewall and then around the IP, I think you'll find out that the truck will stall when you try it at the PCM....

I'll try that again but we did that back in SanDiego before we replaced the ECM and it had no effect. One of the problems with doing that right now is that it won't run long enough to check much, it will only run for about 10 seconds before stalling. I will try anything though. I'll let you know the results when I check after work today. Thanks

Dave,
Have you checked the liftpump/OPS operation to see if you're suppling fuel to the IP before and after start-up?

just a thought Edited by: quantum mechanic

hey dave i've been having the same trouble with my 96. it has been at the chevy dealer for three weeks now. first they said it was the injector pump, but then said they never said anything about the ip that it was the pcm and pulled the following codes 16,17,236,251,370,1641. the first PCM they installed was also bad. the second good. now they are saying left hand injectors by the exhaust are leaking at the return ports (design flaw- made out of plastic next to hot exhaust) anyhow the mechanic claims it will suck in air like a vacuum leak on your intake causing stalling hard starting and sputtering. anyhow we will find out tommorrow if there additional $$$$ from my pocket solve the problem. incidently he claim that he has never seen a stock mounted PMD fail. is this a GM brainwashing tactichttp://www.dieselplace.com/forum/smileys/Evil Smile.gif any thoughts on this quantum?

I had to pour water on the hot turboside exhaust, pull back the heat sheild and stick my hand to fix a return line in there to get home the other day.
you can tell when these lines are leaking by the steady drip of diesel fuel. personally, I pull over and look if I smell fuel in the cab.

As for the dealer, I would say that they will tell you anything they think you might believe. They couldn't make it if they didn't pray on mullets, and evryone's ignorant of something.


Edited by: quantum mechanic

Rusty75 it sounds like you are having a ball like me. I don't think that mechanic really knows the problem. If you were leaking through the injector it would not suck air but blow air at least under boost and that would have nothing to do with why it runs ok with the optical sensor unhooked and as quantum mechanic says you would see and smell the diesel. I don't see or smell either. I am sure that this is definitely an electrical problem I just don't know where. I also find it hard to beleive that it would act as if an intake manifold leak because with the exception of the map sensor you could take the intake manifold off and run the engine without a problem.


QM I have cracked the fuel petcock by the thermostat both before and after startup and fuel is flowing great. I ordered a new crankshaft position sensor and will install it this evening, I've got my fingers crossed.

I read a TSB that was about erratic signals from the CPS and optical giving people problems.
Erratic vs. no signal is totally different.

Very true. This definitely seems to be an erratic problem but one that is steadily getting worse or occuring more often. I figured that a CPS is cheap, only $40 and it couldn't hurt to replace it. Plus it is setting the codes but not going to full limp mode indicating that it is still trying to use the input from the sensors.

I replaced the CPS and checked the connection to the optical sensor and she started right up and I went fro a 30 min drive and it ran perfect. No check engine light or anything until just before I got back. I sputtered and stalled while I was accelerating. I then started to do stall repeatedly but I made it to home.


It kicked out codes P0251 (Injection Pump Cam System) and P0606 (PCM Internal Communication Interrupted). I have tried wiggling the PCM wires and the firewall wires and the the wires going into the IP and it runs so bad and for such a short time it is hard to tell if the stumbling and stalling has anything to do with my actions of wiggling the wires, although I tapped the side of the PCM and it seems to respond but who knows. After this morning stalling I took it again to get the codes read and know I have the same two but the addition of P0370 (Timing Reference High Resolution) The same three that I had when this started happening in San Diego.


I don't think it it the IP since there was little change between the two. I guess I will keep checking the wiring for bad connections but I think I may have another bad PCM. Any ideas?

well if the chevy dealer is right about mine then you probably do. i still haven't recieved mine back yet. they still needed to replace exhaust manifold gasket after repairing return lines. but my guess is that it is still have problems or the would have been done with it by now

have you checked the timing at all??? if the pump is too far advanced or retarded it will kick out an 0251 code(34 if you have a 94/95). We've had a couple of customers try to return pumps for warranty because they installed it and got an 0251 code, we hand it back to them and tell them to bring the truck to us so we can time it. by the way, our local chevy house was one of those customers. If you dont have access to a scanner then you can atleast get it pretty close with a flashlight. with the truck idling, if you stand just to the pass side of the front of the truck and look over the rad/fan shroud under the coolant crossover you should be able to see the foot of the stepper motor. if you have a 5521 pump with the black stepper motor and extra bracket mounted to it then you should see the stepper motor moving just above the limit of travel against the stop bracket. if the foot is all the way extended against the stop bracket then your pump is in full retard and the timing is too far advanced. if it looks like its a 1/8 inch or more above the stop, then your too far retarded and you need to advance the pump timing. It may be a little hard to tell because the stepper motor will naturally 'tach' up and down a little adjusting internal timing. It's still possible to tell if you have the older style stepper without the stop bracket, but its harder without having it for a point of reference. Edited by: Texas Diesel Guy

I haven't taken it in to have it timed yet. But i'm sure that it is within limits to run. For the past several days I have been tinkering with it and there have been several times that it ran great. On Sunday it was running great and in fact I started it four times and the SES light went out indicating it was not getting the bad signals for the 3 previous starts. But then this morning I took it for a drive and it again almost stalled and reset the SES light. It seems to first set codes P0251 & P0606 then if I keep running it it will set code P0370. I have tried wiggling and undoing every connection from the PCM up to the IP and I have never gotten a repeatable change in engine operation, so I haven't been able to find any bad connections or shorts. I can only guess that the PCM has an intermittent fault that is setting the 606 code. I'm leary of going into the Chevy dealer because they always want to blame it on the IP which I have replaced twice and I'm sure that is not the problem.

Did you read the other post where the guy had a problem like yours and removed his signal fliter off the optical connection.

No where is it at? I did replace the signal filter when I was in SanDiego when I replaced the PCM.

www.dieselplace.com/forum/forum_posts.asp?TID=11066&PN=1&TPN=2 (http://www.dieselplace.com/forum/forum_posts.asp?TID=11066&PN=1&TPN=2)
It may start before this page.
Edited by: quantum mechanic

TDG, I just checked the stepper motor for function and it moves alot. It seems to pull all the way up at times(I assume this is fully advanced) and seems to run smooth. But if I kill it and restart it then the stepper will sometimes extend about 1/2-3/4 inch and move around alot and the engine usually runs bad, stumbling and eventually stalling. I have tried repeated starts and restarts and the way it runs seems to vary with every restart. If it starts smooth it will usually stay running smooth, for awhile at least, but other times it will stumble and stall almost immediately, and other times it will rev to around 1000 rpm and slowly drop to 300 and repeat this until it stalls, usually about 4-5 cycles.


I'm still leaning towards PCM fault because it keeps setting code 606.


QM do you know if there is a test for the optical sensor filter (I'm assuming that is the short connection,6", that goes between the wiring harness and the optical sensor with a black triangle of plastic in the middle)

An oscilloscope could detect the waveform before and after the filter.

with all that play in the stepper motor it seems like it's worn out internally, but I would like to hear TDG's response.Edited by: quantum mechanic

I would try advancing the pump and removing the filter first, just so you can rule those two things out. Sounds like if it runs smoother with the stepper motor retracted that far that your pump is too far retarded. What I suggest is advance it incrementally, until the stepper is locked fully extended at idle, then back it off just a touch so it barely moves. The stepper shouldnt move a lot at a steady engine speed, moving as much as you describe sounds like either air in the system or something else causing a problem for the stepper motor not to be able to achieve and maintain desired advance, such as an erratic crank or optic signal or simply being too far out of time and out of reach for the stepper. I've actually never seen one worn out, and only one or two that were inoperable. Edited by: Texas Diesel Guy

For some reason my last post isn't here. I don't think I was very clear about the stepper motor. It doesn't seem to have any play, it just extends and contracts alot only when the engine is running bad. When it starts smooth the stepper motor is basically still only varying if I give it more throttle. I will try removing the filter and adjusting the timing, maybe I'm wrong, but if the timing is too far out for the stepper motor to compensate wouldn't it run bad all the time rather than on different restarts?


Becuase this seems to be linked to each start I'm wondering if possibly the PCM or the ignition switch could have an electrical or mechanical malfunction that does not occur the same every time?


Also I have bought a year subscription to Alldata which has all the TSB for my vehicle and I can't find anything about the filter, but maybe I'm not looking for the right thing. I'm going to try removing it anyway though with these electrical gremlins sometimes it is something totally unrelated that is causing the problem.


So if anyone needs to find out a TSB for a 96 Suburban I can look for them.

Here is the Chevy tech sheet for code P0606:


http://www.alldatadiy/alldata/AFI~V3585309~C23452~R0~OB0~P2R0X~N/0/41746574/42420426/42420481/42420483/34853741/34857029/34857030/34857663/34850217/88213512/100055713/106008628


I have code 370 showing so I am going to see if Chevy will peacefully exchange my PCM for a new one to see if that fixes the problem.

http://www.dieselplace.com/forum/uploads/check_for_code_606.gif

you had a 606 and a 370, indicating the problem is not the PCM by that flowchart.


It's a computer controlled advance, all the computer knows is it sends power to the stepper motor to incrementally 'bump' the timing faster or slower, the computer does not recieve feedback on the position the stepper motor is in. if the computer repeatedly tries to advance or retard the timing of the pump and sees that it is not able to achieve the desired timing then you get a 251 code, which a lot of people misinterpret and blame the pump.Edited by: Texas Diesel Guy

your right I misread it.

have you tried maybe installing a clear line on the inlet of the pump to see if your getting air into the system? Seen a few of these trucks, more so the dual fuel tank ones, with bad tank pickups drawing air. wouldnt hurt, and might make it easier if you just tee'd in a gauge on the line while your there. Edited by: Texas Diesel Guy

No I haven't. I decided to give up and take it to a diesel mechanic here. Once he erased the three codes 251, 370 and 606, he restarted it and got a new code, 1216 with the prior 251 and 370, but no 606. I was reviewing the diagnostic flow chart for code 1216 below and there are several items that eventually point to item 10 (replace IP), but the odd thing is that if anything leads to item 10, once the replacement and timing is complete it instructs you to go to item 11 and replace the PCM as well.


Is it just me or are they just taking a shotgun approach and replacing everything?


The mechanic is waiting from a phone call from the local Chevy dealer regarding the codes, so I'll keep you informed


http://www.dieselplace.com/forum/uploads/AFB_check_code_1216.gif


http://www.dieselplace.com/forum/uploads/check_code_12162.gif

air would explain all your pump codes, stepper motor operation and driving complaints, I would hope for that. I wouldn't expect anything enlightening from the dealership.

well i finally recieced my truck back from the chevy dealership 4 weeks and $610.00 later they have exhausted their ability to work on the truck, but it is still stalling. drove it home and it ran great, but i left it idling in the drive and it started studdering and running rough. unplugged lift pump and took ohm readings (really high 2.4k)then found that after leaving it unplugged ohm reading would drop and could plug it back in and it would resume opperation, smoothing out studder (i had 13v supplied at all times so i know it's not the ops) i think i've got an intermittent operating lift pump. going to replace it and see what happens.

Mine is in the shop right now, first took it to a diesel shop, they believe I have a bad PCM. It is now at the dealer and I they are trying to confirm the bad PCM before they will replace it. If it is the PCM it is covered under warranty for both labor and parts (just installed two months ago). It actually drove ok to the dealer, just chugging a little at different times but no stalling. I should know by the end of the day of they find the PCM bad.


TDG I attached a clear tube to top of filter and let the pump run and did not have any air come through, but I only pumped about two pints through it. If they can't find a problem at dealer I will try a little more in depth testing of the fuel. I haven't tested the actual pressure, but it will shoot about 8 inches in the air with the filter cracked open and engine off. Not the most accurate test but it seems to be good.

I was just scanning over all of these posts, and wondered: did anyone check the engine grounds? The firewall ground to the passenger head can give you fits so I've read. I've seen computers really mess up with low voltage too causing all sorts of weird things. Let us know what the shop says.

I agree. Check all of the ground connections before spending any more money. Bad grounds have been known to cause these symptoms. If that doesn't fix it, at least you've ruled it out.

Good suggestion but I did check all the grounds in fact I even installed a new ground strap from the engine to firewall same gauge as battery cables. I checked both ground straps from PCM that attach to intake manifold on passenger side and the one further back on the passenger head.


Unfortunately I just spoke with the dealer and now they cannot get any of the codes to reproduce and they said it is running excellent. Who could have guessed. It repeated every time at the other diesel shop, ran rough all the way to the dealer and now the dealer says it runs perfect after clearing the codes.

New info today.


The dealer did get some codes to set and it stalled on them last night. In fact he got several new codes that had never shown before, including one for the wastegate solenoid which was also replaced, two months ago.


The mechanic called me and he stated that he beleived the box I had attached to the firewall (PMD) was causing all the problems and that this aftermarket part was at fault or the wiring harness going out to it. I explained to him that the PMD is a stock part and had simply been moved off the injection pump. At first he did not believe me and insisted that they did not have these stock. But after some more discussion he remembered that the PMD is stock and then said he thought the problem was the extended wiring going out to it. I told him to simply check for continuity on the harness and he said "oh yeah". He wants to change it back to stock and remount it on the IP so it will work correctly.


I also understand that a faulty PMD will very rarely throw any codes let alone 5-8 different ones. So his assumption that the PMD is bad doesn't instill me with a whole lot of confidence in his knowledge.


He still believes that I have a wiring problem but not a PCM problem, he said he has never seen a PCM go bad on a 6.5 ever. The fact that the number of codes is increasing and they are often not releted, seems to me to indicate a PCM problem.


He then stated that he could, "throw a PCM at it" but that he didn't think it would work and that there is a wiring short somewhere else possibly going to the PMD.


I then told him that I had reviewed the diagnostic flow charts for the codes I knew about and they all had specific tests for certain wiring connections and if all were well each one boiled down to replacing the IP or the PCM (he didn't even seem to be aware of this, I don't think he has even looked at the flow charts, instead he had been looking at the wiring schematics).


At the end of our conversation he stated that he would throw the PCM at it but had no faith that it would fix the problem.


I am going to go over and physically be there when he replaces the computer (if they will let me in the shop area). I am not very confident in this mechanic's abilities, so I think I should be there.


Any comments from anyone on whats happening, please give me any suggestions, I would love to find I simply had a bad PMD or something else simple?

I'm having similiar problems and it seems electrical or like theIP is going.
I had two small fuel leaks and I'm hopeful it will be air in the IP and that's it.

Well the new computer seemed to change the codes being produced, now back to only producing codes 251 and 370. I built a new extended harness to reach remote PMD just in case Chevy mechanic was on to something about a bad harness but no change in running.


I am going to go back through the wiring again. Does anyone know where all the grounds are located in case I have missed one? I know about the two attached from harness to passenger side intake manifold and I have replaced engine to firewall strap with battery cable sized ground. Anymore? Also on the connections should I apply dielectric grease or leave dry?

There's one from the passenger side frame rail to somewhere on the block, I can't remember where at the moment. If you pull the splash guard out of the right side wheel well, you'll see it just to the rear of the opening.

Found it and it has a good connection.


I was poking around the engine this morning and last night and funny things happened.


I started it and it was running great SES was off and closed drivers door and it began to stumble and eventually stall, setting the SES light. It started again and was running good and I shut the door again and it did the same thing. It only did it a few times and got to where it would not run for more than 10-20 secs before stalling. I tried wiggling wires on drivers side but no response.


This morning I started it and the SES light was out and it ran great, tried the door shutting again with no change. Then I went to passenger side and pushed on the wiring harness near the firewall and it did it again, it stumbled and stalled almost immediately, setting the SES. I repeated the start, wiggle & stall several more times, then the engine seemed to set a fault and the idle increased to 800 and idles firm.


Also every time the engine would begin stumbling the A/C light on the dash flickers and the compressor would begin to click on/off very rapidly, even though the fan control was set to off.


With what the A/C is doing it looks like I may have a short somewhere in the wiring harness, possibly where the harness is wedged between the A/C condensor and the turbo. I am guesing that the A/C wire is shorting to one of the wires going into the PCM causing some spikes and corrupted signal.


I haven't checked the wiring schematics yet, I was hoping for an easier answer. I might just look for a junkyard wiring harness and just replace the whole thing. Anyone know of one? 96 Suburban K2500 "F" VIN

did you check the PCM Connections right behind the glove box? I"m telling you they are so light and frail, we had a 6.5 van do the exact same thing, pump and injectors in her she ran beautiful, shut her down put the dog house back in, back to running like a bag of sh*t. and took a while the first time to realize what the problem was, we even changed the sub harness on the engine because we kept getting in fact the same optic sensor codes as you, ended up fixing the contacts with a seal pick, guy's been gone almost a year now.

a couple of weeks ago I used a paper clip to bend the connections in a little for the PCM but it didn't seem to change. I would like to actually remove each wire individually and make sure they are well connected but I can not get them out of the harness, even after removing the plastic lock. Do you know how?


According to my wiring schematic the connection at the PCM that relate to code 251 and 370 are A2(orange) and A4(brn/wht) and I cannot tell if they have a good connection or not.


TDG on the one you worked on did you have any problems with the A/C like I described above?

I figured out how to get each individual pin out and last night I took each one out and tightened them and found three pins were partially broken (one of the tension sides was craqcked and broken. I tightened the other three sides and think thye are all making a good connection.


I test drove it and it ran great again up until the A/C switch started flickering again (with a/c off) and then it bucked and set the SES and went in to bakcup fuel mode.


Does anyone have a schematic of the A/C wiring so I can track down all the A/C wiring and locate wher it seems to be shorting to the IP wiring?

Thanks for all the help. I found the problem.


There were three wires in the harness shorting on each other (A/C engagement wire going to PCM, the main IP wire and a ground wire). On the engine side of the firewall the harness travels between the turbo housing and the accumulator and after taking the harness partially off I found that three wires had exposed wire and when they became hot they would sag slightly and touch the A/C line and short across each other. I think that is why it would run ok most the time when it was cool and then start running bad after a short time.


Anyway I fixed the shorted wires rerouted the harness so it won't happen again and I'm a little wiser. Now to get the IC project.