Stumbling when cold

[Iain]

My 1994 6.5 TD has about 230,000 km (about 143,000 miles) and when it is cold it will stumble on a couple of cylinders for about 1 second when accelarating from a stop. Seems to happen at about the 1200-1500 rpm range. If I rev it up a bit more it goes away. Once it is warmer, 85 degrees Celsius and higher there is no problem. It has had a new injection pump, rebuilt heads and new injectors. The truck is 50% highway and 50% in town use and never tows anything. I always use Stanadynes premium fuel conditioner on every fill. Fuel quality is good and the fuel filter was recently changed.
I had the compression checked a couple of years ago and the results were O.K. except for a couple of cylinders that were just above GM's minimum of 275 psi (I believe that is right) and were at 300 psi. The other 6 were around the 360 mark.
Is this problem due to the low compression on the 2 cylinders or what??

[piersbc]

Your problem describes my truck also. I also have a new Injenction Pump but thought maybe my problem was an injector (125,000 on them). Maybe someone can help both of us....

[moondoggie]

Good Day!

I'm no expert, but no one else has answered yet, so

[piersbc]

In my instance, this problem only happens on a cold engine. I have new lift pump new filter new OPS. Max load runs fine. Warm engine runs fine. This is a cold engine problem.

[Barry Nave]

125K is at the end of a good injector. Some have pushed longer though once they changed out to new ones,then they found out how good the truck can run. I would Opt. for new one.

300# two years ago! 360# on other six. Wounder what it would be now on those Two Cyl?
Once Eng. is warm Rings would seal better.
Ever notice much blow-by when oil fill cap is removed?
I'd Opt. for another Comp. check.
Sounds like you covered all the bases. Have you tried testing fuel PSI after the filter?

[eracers999]

Both suspects should be able to get hooked up to a good scanner, check in historic codes for a cyl balance fault, you way find that repeated attemps to bleed the injectors may cure the problem up.

Kent

[turbovair]

Two low cylinders may "miss" at low RPM(near idle) then begin to develop higher "relative compression" as engine RPM increases. As engine RPM begins to develop, less time is available for "bleed down" around the piston/rings etc and the effective cylinder compression will improve. The engine will smooth out in operation.This condition will be worse at low ambients and low operating temps. Run another compression test.If Your highest cylinder is 360, your lowest probably ought not to fall below about 288(20%diff)

[Iain]

The cylinder bleed theory makes sense. I notice that when I change to 15W-40 in the summer that the roughness is not as bad as when I run synthetic 5W-40 in the winter. I guess my next question would be if the bleed down continues to worsen (its been like this for a bit better than a year) how long will the engine go before I will need to rebuild it???

As to the new injectors and lift pump, the injectors only have 20k on them and the lift pump is O.k. as I have great power on the highway.

Thanks for everyones feedback!!

[BUZZ]

Have you checked for air entering the system when cold, maybe via loose old/ hard fuel supply line? I read you can install a piece of clear hose in the return fuel circuit and look for air bubles after cold start. Some of the others my chime in on this with more detail.
Buzz

[joed]

I, too, have been battling this for a while, but as of late I seemed to have cured it.

The devices that seemingly changed it: coolant temperature sensor and manifold pressure sensor when using a boost controller (fooler).

For a while now, it seemed my cold start timing was very (overly advanced). Using my car-code scanning software, I noticed when using the boost fooler the timing would tend to be advanced about 2 degrees more than without. I believe this is due to it altering (lowering) the barometric pressure reading, which in turn causes the PCM to call for more timing advance. (This is probably only true for 96 and newer trucks, as the older trucks have a separate sensor to read barom. press.) After removing the boost fooler, the truck ran smoother but didn't eliminate the surging completely.

I also noticed the voltage output from the coolant temp sensor (the one near the t-stats) seemed to jump up and down as the engine warmed up. This was a replacement sensor, so for the heck of it I put my old one back on. The truck now runs smoother than it has in a long time and I haven't noticed any surging during warm up. Overly advanced timing seems to be the factor on my truck.

Maybe some sort of concidence, but based on my experience it may be worth checking the sensors that affect timing.

Joe.

[turbovair]

If the engine doesnt have reasonably balanced compression on all 8,any money thrown at the engine otherwise will NOT be money well spent.Verify the integrity(compression) of the engine. If it checks out..OK, then look at the peripherals.If not,repair as necessary.