Troubleshooting rough running condition

[Mark Rinker]

It started under heavy acceleration, and within 30 miles had become a full on rough running episode. If it were a gasser, it would be a bad distributor cap. Random cylinders dropping out, then smooth running, then nearly a stall for lack of power.

Drove to the dealer, swapped fuel filters in the parking lot. If anything, it seemed to get immediately WORSE instead of better. (I think strongly this is a BIG clue, but not sure what the deduction or conclusion would be.) If the leak were before the filter, wouldn't the less restrictive new filter allow MORE air to be drawn in from a pinhole fuel line leak?
Took it to the shop, where I asked them to inspect it for air in the fuel. None apparent at the bleeder, he did the 'tube into the glass' test and no bubbles are present. Its hard even to start now, and will idle then die, run ragged, smooth out, die, etc. Basically, a diesel possessed.

Last year a section of rusted fuel line between the tank and the lift pump was replaced with rubber when this same scenario happened. The rough running was the same. However this time, no apparent leak can be seen or found. Maybe its a new one between the lift pump and the IP filter...filter to the IP?

Mechanic is jumping to FSD, optical sensor, oil pressure switch, and/or IP conclusions, but there are NO codes set to support this. Personally, I have never heard of a 'codeless injection pump failure'. (Only one was stored, LOW BOOST, I assume that is a result only of the ragged running and stalling.) We'll be swapping FSD's this morning - simple test. We WON'T be messing with the IP until all other stones are turned.

I am so confident that there IS air finding its way into the system, I am about to instruct the mechanic to replace the fuel lines from end to end before anything else is done.

Comments?

[moondoggie]

Good Day!

(I assume you're talking about truck #1 or #2.)

Try at your own risk - I've never done this myself, but would try it if I was in your situation.

Buy a cheap fuel tank cap.
Drill & tap it for an air hose fitting.
Install it on your fuel tank.
Empty your air compressor.
Connect your air compressor to the modified cap; turn on your air compressor; turn it off when it reaches 2 PSI.
Stand back & look for leaks - if air is getting in when the truck is running, it has to come out somewhere eventually.

If something pops doing this, don't bother calling me - 2 PSI should'nt hurt anything, the LP makes more pressure than that. One would hope it won't bulge the tank.

Actually, I'll never be in your situation - I don't need my truck for work, & I can always let it sit while I drive the other one (Sub or pickup). You probably don't have time to fool with this idea.

Let us know what you find out.

Blessings!

[gmctd]

The IP has an inlet fuel filter beneath the ESO solenoid - check that for clog-up after the rusted fuel line event last year.

Or, the ESO is fragged out

Also check the return line for stopped up.

Then there is the poly-carbonate screen filter in the tank, affectionately known as 'the sock'

Then there is the I'm all out of ideas you'll have to ask someone else about that................

[rfuntime]

Let's see.....running rough, low boost, acts like before but no air bubbles. Could it be the rubber hose has collapsed causing a lack of fuel. It's in a vacuum situation, especially if the sock is partially restricted. Paul

[Mark Rinker]

Yes, its #2, the one that got the '98 engine swap. It now has 16x.xxx miles after the swap at 132,xxx miles. It spends alot of time sitting around, waiting for the snow.

Thanks for all the ideas, I will print this thread and give it to my mechanic. He has ordered a new tank pickup assembly at this point, it was in pretty rough shape rust-wise, but no apparent leak.

If it is at the pickup, it could allow air to enter the system without leaking when the truck is off.

I like the idea of the filter at the IP, or the tank sock - since the truck sits alot. All great ideas. THANKS I let you guys know what the resolution is.

[gmctd]

Might also check for KILLER DIESEL FUNGI FROM OUTER SPACE - can quickly plug up the fuel supply, usually found in fuel tanks that sit interminably.............

[Mark Rinker]

Wouldn't the 'fungus amungus' have accumulated on the filter? The one that came out looked like new - no visible contamination.

[Mark Rinker]

Repair completed. Insisted that the mechanic replace the fuel lines before trying anything else.

Once he found air trapped at the filter consistantly, he started to believe me even though there was no apparent leak.

Replaced a rusty, but otherwise 'dry' pickup unit at the tank and reused most of the replacement rubber line (repaired hastily last winter due to rusted, leaking lines letting air in) and the truck is running well again. Filter sock in the tank looked good.

Shop (not GM dealer) found an aftermarket unit for $200, GM wanted nearly double for their OEM pickup.

[moondoggie]

Good Day!

"Replaced a rusty, but otherwise 'dry' pickup unit at the tank..." Does this mean he dropped the tank, this being the fuel pickup inside the tank? If so, it makes a good arguement for keeping the tank full, eh?

Thx for your update - documenting clear problem solutions helps us that use the "search" feature.

Blessings!
(signature in previous post)

[Mark Rinker]

Yes, the tank was dropped. Of course it was nearly FULL at the time of the problem, so had to be pumped out first. Everything appeared dry and normal along the entire fuel line to and from the engine. As for the rusty description, that was referring to the lines and exposed area outside the tank. The pickup/sock inside of the tank was not rusted, but keeping a full tank of fuel during short term storage can't be a bad idea, IMO.

MOONDOGGIE: Your idea of slightly pressurizing the tank would have likely uncovered the leak. If the rusty pickup hadn't solved the problem, we would have tried that next.

After purging at the filter, the truck would idle smooth for awhile, the run rougher and rougher until it died. After finding air trapped in the fuel filter each time, we knew it had to be between the lift pump and the tank, since a leak anywhere else would have been apparent since the fuel is pressurized after the lift pump.

Thanks for the input. Case closed. We loaded up the sander on this truck today, will post a picture of the rig with plow when it snows. Its a snow pushing maching with 1.5yds of sand in the bed!