5.7 Vortec No Compression Cylinder 5

[kikip71]

My 1999 Tahoe began to idle roughly while driving down the highway as I slowed down and pulled over it got worse then stalled. Had it towed home and began checking everything. I got a P0303 & P0305 from scan. I replaced the cap, rotor, plugs and wires I noticed a crack in the distributor so I replaced that as well. Scanning a 2nd time got just the P0305. There are no leaking fluids. When started it sounds like a Harley. Performed a compression test on Cylinder 5 there is no compression. Should I just replace the heads?

[00Silverado4x4]

I'm not a GM Tech but do have a question in return. Is it just #5 cylinder that has no compression or the entire bank? If just the one you could check the rings and/or valve seats. I wouldn't just throw new heads on until I check some others things first.

[GMstrong]

If the rings are bad doing the heads wouldn't accomplish any thing.The rings make the compression.

[wavery]

Most common problem for loss of compression is a blown head gasket or intake or exhaust valve not sealing. Could be a bent or burnt valve or it could just be a valve that's hanging open for some reason.

 

Pull off the valve cover and watch the rocker arms on #5 to see if they are moving. Check the movement with a dial indicator and compare to a good cylinder.

 

You wouldn't replace the head unless it was cracked or badly warped.

 

@ 153,000 it may be worthwhile to pull of both heads and have the valves done. When the heads are off, check the cylinder walla with a micrometer. If they are out of spec, rebuild the entire engine.

 

One thing about doing a valve job on an engine with 150K on it is that you should expect the engine to start burning a little oil. Not a big deal.... just expect it. If you're burning oil now, this would be a good time to throw in a new set of rings and rod bearings.

[kikip71]

I was told that GM does not recommend the heads be machined just replaced. Is that true? If so why?