A friend of mine's truck misses at idle, and up to about 1500 rpm, it shudders, when reving it up and holding it at 1500, with no load!

testing: loosened up the inj line at the 4th cilinder.. nuthing changed, sounded the same! so its clear that its that one, that makes the truck miss! BUT... the injector sprays! (anyway, there is fuel at the line, when loosening it!)

NOW: the guy told me one of his pistons is all messed up... (burnt) therefor no compression! can that be?

or is it the headgasket?
or the lift pump?

PS there is fuel at the injector!
..another thing, the truck starts hard in the morning, he already replaced the fuel lines!

 

He knows this but wonders why it doesn't run properly? Verify with a compression test. Then rebuild.

 

The only thing you've verified is that the delivery valve is opening and sending some fuel to the injector, but unless you actually saw the injector fire it's anybody's guess. If it was me, I'd have #4 injector pop tested, or at least swap it with another cylinder and see if the problem follows it. If after swapping you still have a problem with #4, then it "could" be a bad piston, but it could be other things too. Delivery valve not sending enough fuel to the injector to get it to "pop", could be a bad head gasket, dirt in the line from your first fuel line swap that plugged the injector, etc.

 

After confirming low compression on that cylinder, you can do a leakdown test to narrow down the problem. With both valves closed on that cylinder, inject compressed air into the cylinder through the injector hole. (the hose with proper fittings can be purchased). If the cylinder does not hold the air pressure, listen for where the air goes. If you hear it in the crankcase (through the oil fill tube), chances are that it's a damaged piston. Listen at the air intake and tailpipe to confirm positive valve sealing. Remove the rad cap and look/listen at the radiator.

 

OK!

rare1: he does not know for sure if its a bad piston...
where do i get a cummins compression tester?
and what psi does it need to have?


he somehow bent the line, to verify that the injector fires, and it does, AND he has swapped injectors a few times, with some extra ones that he has...

but, he said, it seemed not to be a lot of spraying!



i told him to test the fuel pressure! As we had here a truck a while ago that missed at idle, at the first cilinder... the same happened: there was fuel injection, but it missed, and that prob got fixed with another lift pump! ... weired

 

I would not think that its a bad piston, as the first thing in A bad sounding cummins.

Here it wasnt either!!! I asked him about it the other day, and he told me his last injector was cracked...

ok: he swaped already 2 injectors, cuz he cracked 3!

he thinks its of the way i modded his pump, (a little more, then most around here do)

 

ok from my own experiences with this cause mine did about the same.i had a bad washer on one of my injectors that was causing the miss. replaced that and it was fine. although it did have air in the cylinder so i put 3 bottles of i think it was called seafoam or something like that and went down the road built 50 lbs of boost let it come back down then again to get the air out. i did this and it took 3 bottles to get the air out but its fine now so maybe that could be ur prob