gary pierce

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

MikeGyver

iTrader: (5)

Look under the right-side valve cover first. Maybe you broke a spring.

viciousknid

iTrader: (8)

excellent place to start. If the valve is stuck open then you have no compression.

DR132

i'd start here too, if you don't find anything, i'd pull the heads and get them checked for cracks, warping etc and go from there. might get lucky and just need a head gasket or a valve job. hows the pressure in the other cylinders? mainly number 6, but the numbers from the others would be helpful in determining how much time and money the engine deserves

gary pierce

Thanks for the advice, I hope its just a spring. I'm going to do a leak down test this afternoon when I get home. This would be a good time to change over to the muti injector setup.

Atomic

iTrader: (42)

When I had no compression in a cylinder I had a hole in a piston...

dmelvin

iTrader: (8)

When I saw this thread with your name on it on the front page, I thought you blew another motor, I was nervous for you.


As far as no compression. Do what the others said, be sure you don't have a spring broken, if everything looks normal under the valve covers, pull the head and make sure the piston is in one piece.

1Bear

iTrader: (1)

Beings it is an old school small-block, I would put air to the cylinder in question and listen for air coming out of the throttle-body and/or the tail-pipe. Most likely it would be exiting the tail-pipe as it is fairly common for one or more of the exhaust valves to burn on the paired together cylinders. They tend to run a lot hotter than the rest. Due to this, I wouldn’t rule out a valve-spring either, as the springs on the exhaust valves tend to break as well from running hotter. Piston damage is not quite as common on the older motors and if you do determine it is a valve, at least you get her back going reasonably quick.

gary pierce

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.

1Bear

iTrader: (1)

I figured as much, it is probably missing a good chunk out the valve. Due to the age and mileage, your right in going ahead a pulling both heads in my opinion. At this point, besides a new valve and a valve job, I’m sure the guides are worn, not to mention the seals and springs. You will probably find it cheaper and faster to just pick up a new set of stock Vortec heads rather than rebuilding your current units.