My 94 k2500 has had a few issues since the reman install... I'll go over some just in case there is a pattern that some one might see.

It initially ran well and I put a few hundred miles on it then it seems to have slightly less power and fuel mileage went from 15 to 10. (I began to think turbo) Here in Vancouver WA we have emissions testing on the diesels too (Snap throttle test) anyway It failed, to much black smoke on the WOT snap.

I took it to a local shop on the WA state approved list to do some diagnostics. I told the mechanic that I have experianced this stumbling or miss in a few different ways, No particular pattern. Cold and warm. part throttle as well as wide open. I also felt there wasn't any boost happening. This being my first gm diesel I did not have anything to compare the power to (or lack of). He adjusted the fuel pump static timing, cleared the codes (dont know which) and he thought it was done. He also stated that he thought it had better than average power. I was not that happy to hear that.

The one modification I had done during the install was the ssdiesel HO h20 pump and duel thermostat. The mech thought also that this was running too cold and perhaps not getting up to good operating temp. (180 thermostats) He said the like 192-195 degrees and I was running 185. I put some carboard between the coolers and radiator and have since monitored the temps with a snap on mt2500 ( the Brick). Taking it back for testing it did not pass the test and since I had not spent enough $$$ for the waiver, I took it back and he had it for a few days... his diagnosis was to reman the ip and pmd... not wanting to spend $ where its not needed I went over some of the componets like the boost pressure circuit, baro circuit, wastegate controller using a snap on vantage... there is definitly some thing wrong because I either was not properly probing or in fact most of the componets did not have the 5 volt reference. I am sorry that I do not have the code that were comng up on hand, but I cleared the codes and checked things like vaccuume connections, ect, Volla WOT black smoke issue seems cured. However I still get a stumble or miss... BTW the mech said he tested the throttle pedal was reading norm... is this something that can be an intermittant problem...

This is what I think ... From all the info that I have gattered from this site I feel that it may be likely that if it is still the original or not replaced with the reman the ops may be faulty under load. I never get it to stumble more than once at a time but it now seems to be a pattern of happening when fully warm (most often) and a WOT accellerating. could that be enough load to interupt the lift pump and make it temporaraly act like its running out of fuel? Or do I have other issues with the pmd or ip?

I'm thinking replace the ops and lift pump (good maintenance anyway right)

any opinions or suggestions?

I'll check for codes tomorrow

Black Smoke is unburnt fuel. Did you check the vacuum pump ? Cracked vaccum lines, bad wastegate solenoid ? Check if the liftpump is running. Run a 12v wire direct to the liftpump. If it don't run, the liftpump is bad. If it runs, check the OPS, if it is sending 12V to the liftpump with ignition on or engine running. If not, the OPS is bad.

If the engine runs okay, when cold, but stalls when hot, i would suppose the PMD. Where is it. Still on the IP ? Relocate it with a cooler and longer harness.

Cu,
Sven

More info... Checked for codes and there were none:)... although I had the stumbling (kinda like running out of fuel) thing a couple times today... First time mostly warmed up, outside temp around 40 degrees, during a hard acceleration it started the stumble. when I let off it stops doing it, and if I keep the pedal floored it will only last about 15 to 25 seconds and its done like back to normal... today I tried the turn on the cruise control, which is a test to determine if its may be the pedal actuator I guess??? anyway it did like I had let off the pedal and went away. During the second time just a short while ago, outside temp maybe 45, I had ran up the freeway(75 mph) about 20 miles let it sit 20 to 30 minutes and on the ramp getting back to the freeway it started doing it again, but since I was not up to speed that I was previously it kept trying to accelerate and continued to stumble. Sooo that being said might this be a hesitation in the lift pump caused by load to the osp and during the time between the injection pump picking up the slack of the faltering lift pump and low pressure???

Tomorrow I will do the lift pump test then replace the ops and or the lift pump depending on the test.

Btw has any one ever tee'd the fuel line ahead of the lift pump and put in a fuel pressure gauge??? if so what pressures should i get either before or after the lift pump? Will the IP cause an affect on the line pressure?

KOEO will not produce pressure unless I have a addioional 12 v source???
KOER will show pressure from the IP a.w.a the lp???

Is there an ohms resistance or voltage test for the lift pump that will indicate its condition? I will also consult the Snap On Vantage, Will post results.

I really appreciate everyones opinion and suggestions

you can hook up a fuel pressure gage to the T-valve output and open the T valve. It is usually located on one of the thermostat housing bolts and is used to drain the water off of the filter housing.

OBD-I system may have the optical sensor filter harness on it, and be giving some issues. May want to check it out.

I have had some stumbling recently as well.....traced back to loose battery connections and fuel gelling...

I have had some stumbling recently as well.....traced back to loose battery connections and fuel gelling...

Already took care of the grounds and run a fuel additive

On the Optic Filter harness, do you just get rid of it and run without it or just bypass it for testing? Sounds like you may get rid of it the way you said to check to see if it has one. What does it do? I have had a little fishbiting at say 50 MPH and 1500 RPM's lately. The last time I had this kind of problem replacing the Crank Pulley fixed it but I took it off and checked the balancer and everything looks good.

Just a thought on the fuel line pressure question from before... If I tee into the fuel line post lp and the lp is working correctly it should supply enough fuel to make line pressure right? but if the ip has to suck the fuel do to a bad lp wouldnt a gauge show vacuum?

I think i want to install a fuel pressure gauge into the cab to monitor... it could be useful to watch for fuel filter rectriction and lp failure
http://ecx.images-amazon.com/images/I/517F5WFAAJL._AA280_.jpg (http://www.amazon.com/gp/product/images/B0006V2BS2/ref=dp_image_0?ie=UTF8&n=15684181&s=automotive)
this type of unit reads both pressure and vacuum the problem is I would not want to rely on those type of connections for an incab install .

Is there an electronic type sending unit that I could tee that would give both pressure and vacuum which I could wire to a gauge pod?

any suggestions along this line are appreciated.

Remove it to experiment. It does serve a function on an OBD-I system. Worth a cheap try...

As said before, do diagnostic checklist... leave no stone un-turned

OBD-I system may have the optical sensor filter harness on it, and be giving some issues. May want to check it out.

Are there any threads that discuss further what and how to check out the osf.

I don't recall ever doing a write up on the optic sensor filter harness, but if you do a search, and check out heath diesel (site vendor) he has some info on his site... don't remember 'zactly what his write up was. It's the six pin connector on the top of the injection pump. The little do-hicky that connects between truck wiring harness and IP.

There's a picture of the optical sensor in the FAQ's

As said before, do diagnostic checklist... leave no stone un-turned
thanks gonna work on it

Are there any threads that discuss further what and how to check out the osf.

http://dieselplace.com/forum/showthread.php?t=206539&highlight=optical+sensor+filter+harness

As said before, do diagnostic checklist... leave no stone un-turned

23. Upon unscrewing the fuel cap (http://dieselplace.com/forum/showthread.php?t=42338), do you have a large vacuum formed in the tank? yes/no

I just opened the tank while the truck has sat over night and it did have a fair amount of vacuum

I will run it around a pick up a fuel cap and double check the vacuum in the tank... I see how that could be enough for the ocasional stumble which is what I am having now, nothing extream but could be the cause of the once a day or so stumble... combine it with the probibility the osp is perhaps old and who knows it the lp has ever been replaced... as I said before and that was to echo what I have read on this sight the ops and lp should be considered maintenance.

anyway an fuel cap is something cheap to try:)

Try it loose first, if it helps, then replace it... helps to make you sure that it is/was the problem in the troubleshooting maze.

still stumbling maybe even worse than before... still need to do lp test... any other ideas

will a bad waste gate actuator cause a stumble on acceleration??? and I am refering to the can and rod that move the wastegate... Does anyone have the range of numbers or spec range that I should be seeing on a snap on mt 2500???
IIRC mine seems to be 67% at idle and goes as low as 40 with no load acceleration. does not sound right to me,

I'll need to pull the intake plenum off to get the pump numberd off and check the otical sensor too, might take me a few to get around to it.

Thanks to all who posted

I might even bite the bullet and have the ip and pmd bench tested? Who knows, I did have one mechanic make the suggestion but just thought he was doing a hale mary throw some money at it approach.

Best test bed you are ever going to find is your vehicle itself.

Unfortunately, diesels are complicated. Complicated equipment requires you to work step by step. You have to complete test one before you move on, otherwise you will run in circles.

First item on the list is check your lift pump. (Lift pressure while running) Wait till you narrow down to the IP before you cause more problems by taking it off. If you still haven't checked the optical sensor harness, that would the the second thing. That OBD-I filter causes problems.

One thing at a time there, Paco. IP's aren't fun to remove, trust us.

will a bad waste gate actuator cause a stumble on acceleration??? and I am refering to the can and rod that move the wastegate... Does anyone have the range of numbers or spec range that I should be seeing on a snap on mt 2500???
IIRC mine seems to be 67% at idle and goes as low as 40 with no load acceleration. does not sound right to me,

Possibly, do you have heavy black sustained smoke on acceleration, sounds like a fuel delivery issue, have you checked lift as recommended, and when was last fuel filter swap, old fuel in tank ???

You are putting cart before horse, before going much further as we can guess all day long, you need to copy/paste the diagnostic sheet and report efforts thus far.

Also read & study the FAQ topics for OPS/Lift pump , and turbo/wg loss also call folks at Heath Diesel they can answer your ??? real time and are sort of local (same state) Ellensburg, Wa.

Your MT2500 for wg is displaying command to wg solenoid not physical position of the wg, which means nothing if you have a bad solenoid or vac leak.

Thanks For The Input

In working through the diagnostic maze, you'll want to minimize how intrusive you are to the workings of the engine itself. You could create more issues that what you have now and create other variables. The purpose of the diagnostic checklist is to gather the information and publish it here on your thread and have us take a look-see at it. If something jumps out at us, we'll ask or point you in another direction of something else to try. Just think of the checklist as a fact finding mission. Sometimes you fix it before your done with list yourself. If you do, let us know, so that we all learn from each other.

Keep it simple... you'll kick yourself after doing too much work to find one little thing that's been boogering ya up the whole time. ;)

thanks knkreb

failed lift pump test. replaced the ops, still no lift pump, I will be working more on this.

Lift pump on order just in case.

Anyone have issues with Napa's op6677 ops or there oe replacement lift pump???

Thanks

Get that LP working, and watch her run like a champ. :)

(If the IP isn't abused too bad from no LP.)

Get that LP working, and watch her run like a champ. :)

(If the IP isn't abused too bad from no LP.)

I would think that the IP must be alright because it really does run out fairly well.

I wasn't able to work on this today, but will get at it soon.

Get that LP working, and watch her run like a champ. :)

(If the IP isn't abused too bad from no LP.)

I finally got to checking the lift pump.

first I opened the tee valve and no fuel came out. I reprimed the truck and got it running again.

I got it on the rack tonight, pulled the plug and used a jump box and leads to the connections and the pump was DOA. installed a new delphi l/p from napa, checked to make sure it was running then went up top and put a gauge to it. I am kinda shocked that its only running about 3.75 to 4 lbs pressure.

btw I replaced the ops a few days ago with a napa op6677sp ( grey unit)which never indicated proper pressure, zero at idle fully warm, Napa replaced it with the standard op6677 I installed it tonight also and all is normal now. I have no idea if and when it was replaced so for $22 and 10 to 15 minutes of time I have some piece of mind that should be ok for now

Hopefully this takes care of the stumbling issue and I can turn my attention to getting some more power out of her. Gauges first

Thanks to all who have posted

That should be about right on the lift pump PSI. They don't run much pressure. Their job is to get the fuel to the IP which provides the real pressure.

VERY informative post..... looking forward to the results....

Packratt is correct. Lift pumps only push around 3 PSI or so. Its purpose is to keep the intake of the injector pump flooded at all times. These trucks will run with a bad lift pump sometimes, and the injector pump will try to pull the fuel itself, usually wearing the IP out REALLY fast.

Haven't gone on any long drives as yet, but no stumbling so far. Not much of a change in over all performance, perhaps just slightly better acceleration.

Going back to the fuel cap replacement... I still have some vacuum in the tank... is that normal? Or should there not be any vacuum?

The other issue with the new cap is now I have a fuel stain below my fuel spout like the new cap does not seal correctly, kinda ironic to have both a leaking issue and a vacuum issue with the same item. I don't recall any spill form the previous cap.

Can someone address whether vacuum in the tank and if overflow of the fuel spout are normal? BTW it's much less vacuum then previously.