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Clutch1819

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Seem to have a problem I can't figure out. My truck is a 98 Sierra. 5.7. If I'm going 55mph uphill and the trans downshifts and stays in that gear for a few, it will start missing. Feels like it's only 1 cylinder. Also does this if I pass someone. Pretty much anytime the engine is under heavy load. I've replaced the fuel filter, spider assembly, distributor, wires, plugs, and the coil. Anybody have an idea what could be causing this? I also tested fuel pressure and it's in spec.
 

Schurkey

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Connect a scan tool, record the data stream when it's misfiring.

Study the recording to see what's out-of-spec.

HOW are you testing fuel pressure? With the engine at full load, as it misfires?

Distributor cap failures get a lot of press on the '96-up Vortec engines.
 

Clutch1819

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The fuel pump is staying steady at 60 psi. The skip has now turned into something worse. At this point it looks like I'm about to tear the top end off and do some checking. It will barely run now. Only code its throwing is random/multiple cylinder misfires. It sounds like a very bad internal vacuum leak. Sounds like the intake sucks very hard on one cylinder. Gonna check compression and go from there I guess.
 

Schurkey

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It will barely run now. Only code its throwing is random/multiple cylinder misfires.
Codes can be helpful. Sometimes, they're even as useful as the data stream.

What is the data stream showing?


It sounds like a very bad internal vacuum leak. Sounds like the intake sucks very hard on one cylinder. Gonna check compression and go from there I guess.
Consider the "Three Sisters of engine diagnosis": Cranking compression test, Cylinder leakdown test, and cylinder-balance test.

Low compression, high leakage, or weak/dead cylinders can each point to various engine problems, from burnt valves, defective piston rings, incorrect cam timing, etc.

OTOH, good compression, low leakage but weak/dead cylinders point to fuel or ignition problems.
 

Clutch1819

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Well it looks like an internal problem. Done a compression test earlier and here's the results,
1- 110
2- 125
3- 95
4- 130
5- 0
6- 125
7- 135
8- 110

Looks like it's time to put the new engine in. Lasted 313k miles so I'm not complaining to much. Guess when I take it out I'm gonna tear down and see what happen.
 

Schurkey

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A leakdown test could tell you WHY you have crappy compression on those cylinders.

Do some more diagnosis BEFORE you take it out and apart.
 

Supercharged111

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Could have a lifter that went full retard and won't allow valve closure on the problem cylinders. 313k is a lot of time for a lot of gunk to build up and cause weird problems.
 
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