Mechanics

CHEVROLET TRUCK COMPRESSION PROBLEM

1998 Chevrolet Truck • 150,000 miles

My ck1500 has a 5.0 and has been cutting out, kind of bucking on acceleration. After a few weeks of this it started to run real bad almost stalling at stops and gave a po300 / p0308 code. I changed the plugs, wires, cap and rotor, the code continued so I changed the fuel spider. The lower intake was also replaced for water starting to seep. This didnt fix it so I changed the coil and ignition modual. I misunderstood how to check fuel pressure and wound up replacing the fuel pump. Now Im back to the truck running but still bucking and power comes and goes, sometime it kind of kicks back just a little on the starter. The timming is within 2 degres. I looked at the lifters they are lifting good so I ruled out the cam. The ecm no longer gives codes. A parts guy told me the ecm may have problems sence Im not getting codes. Tomorrow Im going to get a compression check. It runs strong for a short while then it acts like its missfiring. I cant find any vaccum leaks and Im way out of Ideas.
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Raymonddiannamathews
February 10, 2013.



Compression test is a good start. Get it checked.

Get the back pressure checked for the cat converter as well. Simple test to verify the engine can breathe

Roy

ASEMaster6371
Feb 10, 2013.
I checked the compression, it was aroun 80 psi but I was using a push in gauge so it minght be higher. I had the cat checked. It was good. The truck started to run better all by itself but still cuts out some on high rpm or when you romp on it. My new fuel pump only puts out 55 psi when I turn the key, 48 psi when ideling and 57 psi when you flip the throttle. The pressure holds with the key off, but is this pressure still to low? Also I read where a guy had simular problems and a new knock sensor solved it. Still no codes.

Knock sensor would code.

Fuel pressure is low, should be 60-65 lbs.

What was the back pressure on the cat? How did you check it?

Roy

ASEMaster6371
Feb 11, 2013.
I had the muffler shop check it out, its a high flow cat installed 12 months ago. They put a hole in the exhast but I didnt see them use any kind of insterment. They said they would love to sell me another one but that one was good.

When should the fuel pressure at 60-65
ignition on without motor running?
Motor idling?
Motor accelerating?
Im not real clear on that.