lost compression???

[smokinshogun]

OK.....so its a 95 atx and its been running rough for a while now. Figured it was the because the knock sensor had been broken off while replacing it which forced me to leave the connector unhooked. Turns out it wasnt because afte replacing it I ran a compression test and got 180+ from all five cylinders except 3 which was ZERo. The guage didnt move at all even after checking the connections. So just wondering whats typical to fail on a sho and show these signs? Im guessing valve seals??? Also the spark plug on that cylinder was the only one that showed rust/oil/wear? Thanks and im gonna do a leakdown soon.

 

[SHOtime2511]

what do you mean the plug showed wear? contact with somthing?
valve stem seal wouldent cause a compression loss.

but 0 compression thats very strange!!!!

 

[smokinshogun]

Ya I thought it was weird too but it fits exactly. Plugs was rusted around base and had some oil looking residue coming from between metal and plastic?.The car has no codes and the cylinder balance test showed number 3 to be the culprit. Its been misfiring and the whole engine shakes at idle. So im thinkng, broken valve, like an exhaust valve burnt due to the cats overheating. or the Block cracked, head gasket blew, valve not seating right, valve seal got blown out of place, piston rings got fried, or hole in piston. right? Burnt exhaust valves would damage the whole engine along with the head gasket and cracked block. Some damage would have occure to the other ones I think. The problem did come around the time the cats started getting red hot? So something melting or a valve breaking frm heat doesnt seem to far fetched to me....But I just wanted to know if anyone has experienced this before...good compression on all cylinders but one completely dead.

 

[smokinshogun]

how is the valve stem SEAL not gonna leak???? Also i do drive the car past 4000 rpms quite often....like every time i drive AT LEAST. but if the spring stuck it be closed? If it was a spring that broke it be open right? could it be possible its something easy like a broken exhaust valve.....


A couple more questions.....so I do not want to remove cams for risk of damaging them...so I can replace a broken valve without removing it right? Also I could replace a piston with engine in the car and crank connected...

 

[SHOtime2511]

these engine are maid to rev,plenty of people revv them to 8000rpm everyday.
a valve stem seal can leak,but it wont do anything but leak oil.
that seal does not see compression.
if your reading 0 compression then reguardless of what the problem turns out to be you will at least have to remove the head.once you do that you should be able to tell what the problem is.
in order to remove a valve the head must be removed.

 

[smokinshogun]

so cams can stay in? would it be possible to spin the valves over to check for valve seating with the head off.

 

[sdpatt]

You should be able to check the intake valves seating by just removing the intake manifold. With the intake camshaft at the heel section on cylinder #3, the valves should be fully seated. Any fluid (intake cleaner?)applied on the two intake valves should stay visible on the top of the valves. With the intakes fully opened by the cams, the exhausts should be closed. If you can apply air flow with a large diameter hose to one of the intake ports, while plugging the other, you could check for blockage of flow into the cylinder. If there is flow, it would be either the exhaust valves leaking or a big hole in the piston. Found any metal debris in the oil pan?

 

[St Louis SHO]

Well, from what I read, you can rule out an intake valve. It'd be popping through the intake, I'd guess on a burnt piston with 0 psi, possibly an exhaust valve, but you'd still have a little compression. Definitley a leakdown test will help, but looks like you'll be at least pulling the head for sure.

 

[smokinshogun]

well the intake is pretty far down......and I can barely hear the car rumbling with my head in the car. It does seem to have lost 1/6 of the power. But a weird thing happened today. It started acting normal, like no shakng of the engine. Usually I see it shaken like shit evrytime I look at it while idling. But today it stopped doing it for a while. Then started again and when I got home it seems fine once more. Im gonna do another compression check on that cylinder because it felt like it had the power back. Im even pretty sure while in nuetral the engine senses this knock and is constantly rasing/lowering timing because of it. Or my idle needs reset again.

I just dont want to pull the head and not be able to find the problem because its a sticky spring or something. Like if the ring clearence was excessive and I dont know because I dont WANT to tearinto the bottom end. But a hole in the piston that resealed itself??? That would be visible with a good cleaning no matter what I think. Same with the way the valve and the way it seats into the cylinder. With some cleaning and measuring I should find I hope whats wrong. And since the head gasket is gonna be replaced rules that out. If I dont find anything obvious and just clean it up and put it back together and come to find out it still lacks compression will the head gasket be good for another tear down. And rebuild of head and block.

Can I just put in about 50 psi of compressed air when the valves are closed with the quick disconnect part from the compression test kit. Just because Im pretty sure all that air is gonna come out somewhere so I dont nned to regulate the air and measure the decrease.

 

[somedude_001]

I'd guess on a burnt piston with 0 psi, possibly an exhaust valve, but you'd still have a little compression.

my saturn (i know its not a SHO engine by any stretch but the valvetrain is the same design) broke a exhaust valve. I don't know what caused it or how it happened but there was a big chunk of the valve missing. 10 psi of compression showed on a gauge.

if your knock sensor was disconnected if could in theory run with too much timing and increase your EGT's too much and burn a piston or valve but on a NA car i have a tough time seeing that happen. also I thing the computer would go into limp mode with a disconnected sensor.

typicaly no you don't ever reuse head gaskets but if 2 come in a kit and you are only using one then you may have a spare if they are symetrical but i don't know if they are.

 

[93rev2sev]

Disconnect the Distributorless ignition module. Remove spark plug.
Cover the spark plug hole with your hand while cranking the engine, it might be a tough reach in the back but with someone on the key you should be able to tell if the piston is making ANY air move at all. If you can cover that spark plug hole with your hand and don't feel puffs of air rush past, you have mechanical failure that requires at LEAST head removal. (edit: given the fact that it runs, I assume the timing belt is intact)