99 Tahoe 5.7 started running rough and had it scanned and showed #4 missfire so I did a comp check, and have 0 in #4, and 150 in #2. I squirted oil in #4 and still had 0. Now what do I do, pull the motor or just heads. 175k miles and in good condition otherwise. Thanks for advice.
Gary

 

Leakdown test will tell you more about where the pressure is going...out the intake valve, out the exhaust valve, or past what's left of the piston.

 

have you tried removing the pushrods and double checking? at least that way you know the valves will be closed and eliminate that variable before pulling a motor. you can also do a leakdown test and determine where it's losing, that will pinpoint, as well.

 

If all else fails...

http://cgi.ebay.com/ebaymotors/12530...Q5fAccessories

 

I guess I need to do a leak down test after I find out how to do it. That crate motor sure would be nice.

 

bent valve.

.

 

Absolutely zero compression....? I agree, pull the valve cover first before pulling the head. Kinda rare on those motors, they are pretty bullet proof.

 

I hope its just a spring, I'm doing a leak down test tonight before I pull anything. Thanks for the advice

 

Is it backfiring out of the intake or exhaust?

Don't pull the motor just yet.

 

Valve sprints are junk in those motors and they burn valves very easy. I have had several in my shop like that. Or like the othe poster said bent valve I have seen that too during a long hard pull up hill having junk springs

 

Just finished airing up #4 and it came out of the tail pipe like there was no valve, but on top it looks closed. At least the piston might be ok, but the head has to come off. I guess I might as well do both heads since the comp in all cyls was 140 to 150. Thanks again for the help.

 

That sure is odd.

My 99 GMC Sierra Z71 has the Vin R 5.7l and i bought the truck with 82K on it and it is now up to 240K on the clock. Never had any of those problems. I tow a 32ft 5400lb camper with it too. Truck has been great but it is getting a little tired.

Tranny died at 120k which is typical on the 4l60e in a truck
Fuel Pump died at 110K and is still running on the ac delco i bought/installed as a replacement for it.
Reared had the drivers side axle bearing whining for a couple years i finally got tired of hearing it on the highway and completely rebuilt rearend including the differential clutches.
And of course the dreaded intake gasket leak. Make sure you use the problem solver felpro gaskets which have the steel plate sandwiched in the gasket.
Any questions feel free to PM me. Ive owned my truck since Jan 2004.
Next year it is getting a 5.3l bored to a 5.7l....more hp than the crate motor vin r 5.7l ($2300) and you wont have to mess with replacing the $60 Ignition rotor and cap every year either.
5.3l are dirt cheap and easy to come by! Stock up! I got 2.

 

If all the cylinders were close to each other I don't know if I would pull both heads off. Consistency is more important than high numbers, unless they are really low.

Did you take both rockers off or just roller it over till both valves were closed when you did the leak down test?

I know you said it looked like it was close from the top. Did you turn the engine over by hand with the valve cover off to make sure everything is moving?

It could be a burnt/bent valve but there also could be a piece of carbon between the valve and the seat. If it's carbon that can be taken care of.

Personally I would do some more testing before I took it apart. Start simple first and go from there.

A good example of this is after my H/C/I swap my oil pressure started to slowly go down at hot idle and wouldn't go past 40psi@WOT. I replaced my OPSU and that was it. I almost tore the front of the motor off and put a new oil pump in it but thankfully I didn't.

Best of luck and keep us updated.

 

I pulled the heads and found that the exhaust valve in #4 barely hit the piston and left a scratch. I'm taking them to a shop Monday to check out and re build. The cylinders look good and can still see the cross hatching and no wear ridge, but there is black oil coating under the intake on the block and heads. They are the good heads and hope there are no cracks.

 

OK so #4 exh hit the piston....question is...Why? Rocker too tight on that one?

 

i hate to tell you this but looks like you are running an LT1, not a LS based motor, you sure that is a 99 Tahoe? cause they didnt put the LT1 in any car after 1997.

 

Sorry if I'm on the wrong forum, but I don't know what LT or LS differences are. I thought LS motors had aluminum heads, 5.3, 6.0

 

For your specific problem there's not much difference, compression is compression. But that head is definitely not an LS piece and it appears to be an iron Vortec model.

The oil in that chamber points more to a ring/piston failure from what I can see, unless the valve has failed such that oil is getting past the valve guide in a major way.

 

Im fairly sure 1999 was a crossover year where you could get the older vortec 5.7Ls or the new LS 5.3L

 

That's a sbc vortec motor.