spitzair

Hello everyone,
I posted this in the HELP forum already and it was suggested I post here also so here goes... My 92 W250 sometimes surges really bad at idle, acts as if it's going to die, and has little low end power, but once it gets up to about 1400 RPM it runs fine and has great power and accelleration. I'm going to change the fuel filters first thing tomorrow and see if that helps, otherwise what else could it be? Am I possibly looking at a new Injection pump or maybe governor springs? Any ideas what a new Injection pump would cost? Thanks,
Toby in Squamish, B.C.

stillsmokin

I think we need more details, does this happen on cold start? What color is the exhaust smoke when the symptoms occur? Anything in particular seem to set the problem off?

Bushy

HEY !!!.....High "Toby in Squamish"...... .....

You might try the thing you mentioned and change out the fuel filter if it's older or if you suspect bad fuel at some point...

You may also want to test your fuel pressure at the outlet of the lift pump.
You symptoms sorta suggest that when you have enough RPM on the engine the injection pump (which has it's own 'supply pump function) is able to catch up and run better...
The original/stock lift pump should supply about 8-10psi as a rough max and about 5psi when at full throttle. The manual says anything less than 3psi is no good and you need to replace the lift pump..
That threshold becomes more important as you increase your fueling and tweak for more and more performance. Personally I'd want to see at least 5-6psi on a moderately fueled/tweaked truck.
One thing we don't usually have to worry about is damage to the injection pump if the lift pump is weak or failing because the injection pump has, as I mentioned, it's own internal supply pump.

Finally, is your idle setting ok...????
It should really be set at about 800-850RPM when in "N" or "P" with no other load on the engine.
You want your idle set at about 750RPM when the trans is in "D" or what-ever and with the AC on HIGH...

bob....

spitzair

Well I went and changed the fuel filters, no change... I guess I'll try plumbing a pressure gauge into the system after the lift pump and see what happens. It sure does seem like it's not getting enough fuel at idle, as soon as you step on it even a little it clears right up and runs fine. It doesn't smoke whatsoever even when it's doing it's surging at idle... Bushy, My idle setting is fine, it hasn't been touched and before it started doing this it always idled great. It still idles good every now and again... Thanks for the help so far, I'll keep you guys posted as things progress.
Toby in Squamish, B.C.

Boatnik

The last time I ran into a Cummins that surged at idle I found an air leak in the fuel line. A simple check is to temporarily install a clear fuel line near the fuel delivery pump and look for air bubbles.

spitzair

Well I went and bought a new lift pump and put it on and in the process I cracked the inlet fitting... It didn't look like it leaked so I bled everything and she ran like a charm, no more miss at idle or anything, took 'er for a drive and everything seemed fine. This morning my dad wants to use the truck and it wouldn't start. So I bled everything again and it runs good, but the crack does leak... I put some JB weld over it hoping it will work, any ideas why it wouldn't? I don't wanna spend another $180 + tax on another pump... Thanks,
Toby in Squamish, B.C.

wannadiesel

If you have a crack it will suck air and you will continue to have problems.