Let my first post here be productive.

My just purchased Cummins had been giving me fits. It would surge on and off like you were turning the key on and off. Sometimes so bad you drop down to 25mph. No codes. Everyone claimed the VP44 was dying. True to the fact, one day I came to a stop on the way home.

VP was replaced, no start. After much troubleshooting, I put my finger on the end of the fuel hose and felt little fuel pressure. Replaced filters and it ran fine for two days. Bucked a couple times. Next day it surged the whole way into work. Thought fuel was gelling or two stroke oil was plugging filters. Added Diesel 911 and Powerservice. Checked wiring, and it went away.

Two weeks ago it came back. Did it BAD this morning. Pulled over, checked wiring, wiggled stuff, opened the water drain with the engine off and had a hard vacuum!

Now everyone knows there is a tank vent, but I thought "It is hard to get to and check, let me "suck test" the filler cap. NOTHING. Tighter than a frog's butt and that is watertight.

In true Bomber fashion, I went and took the cap from a 2000-something Dodge Dakota. Suck test showed it worked, but not what I think would be comfortable. Remember, gearotor fuel pumps like to push fuel, not suck.

Took the cap apart. Cut a full coil off the big spring with a pair of diagonal cutters(Which controls the fuel tank PRESSURE. Don't need a gajillion PSI here.) Heated top coil a little and bent it down to resemble the original configuration.

Then took three coils off the small spring. This one controls fuel tank VACUUM.

Reassembled parts. Now when "suck testing" it allows a very very slight vacuum. Anymore than very slight results in the sound of an injured baritone sick duck's mating call and a whole lotta air through it.

Guess what, problem is no more!

 

thats amazing that you were able to pinpoint your troubles to a bad fuel cap. nice going!!

 

Awesome, I wouldn't have thought about that. easy fix to maybe just go ahead and do.

 

Awesome! I will remember this one!

 

Ok, this truck is possessed.

Since this fix, it has cropped up again. Found a pinched fuel line to the Airdog, wasn't it. I have, however, found something that stops it in its tracks when it is surging/bucking. If I hold the ignition key to the start position (Starter won't engage if the engine is running) it immediately goes away. If you let go, it returns.

Think it may be the PCM or auto shutdown relay? I'm stumped. Everytime I think I got it, it comes back.

 

Have you checked the tps they will cause the symptoms you describe and are frequently intermittent due to dust, temp, and electrical noise. Might be worth a look you also might want to try cleaning your manifold air temperature sending unit.