Thread: Zero compression in one cylinder

Tweet

2 year old crate-motor
350 Chevy
86 Blazer
6,000mi on it

At the end of a 200mi drive the engine started to miss.
Quite badly. Limp home 15 more miles.

Next day do a cold compression test.
All cylinders look reasonable with the exception of one that reads zero.

Easy to check for broken spring. Pull off the rocker cover and the valve springs look okay.

Could be a hole in the piston?
Could be broken rod?
Could be a failed rod cap?
Could be a burnt valve?

What else can it be?
What is your guess?
Why would a valve burn or a piston hole after 6,000miles,
during an otherwise, run-of-the-mill trip?

I see a good number of Chebbie valves "tuliped" or deformed from using crappy steel in the valves. The valve fails to seat after it deforms. You'll need to pull the heads to know.

Back the rockers all the way off on the affected cylinder. Using your compression adapter (with the schrader valve removed) hook it to a compressed air hose and see if it still leaks. If a valve receded in the seat or tuliped, the affected valve would stay open. Same thing if a seat fell out.

Either way, the compressed air will reveal the source and most likely cause of your 0 psi cylinder.

Damn! That was fast..
This sounds plausible since it was one of those Mexican cheapos.

The good news is they generally don't destroy the valve seats when they do this. Mostly a new set of valves a lap and I would replace the keepers, check the springs but with 6,000 miles they are probably ok.

Sparky; I'm not following your suggestion.. Are you saying just hook a compressed air line to that cylinder and run 80PSI into that cylinder with the rockers backed off. If it is a valve problem you'd have air blowing out the exhaust. If it's a hole in the piston air out of the oil galley?

cobra; Thanks for the "buck up" Makes sense too.

I think the guys are on to it with the valve thing. If the head came off the valve or if a piston was breaking up, you would have lots of noises. If the valve is not seating, all you will get is a loss of compression, which is what you are describing.

We get alot of Chevy heads (at our Marina) where the valves are recessing into the heads. Our one engine supplier said it is known Chevy was having a problem with their 350 heads. We are also seeing lots of valve heads breaking off and destroying the piston and cylinder walls in the process.

You may get away with just a new head and/or a valve job.


Don

Originally Posted by kcress

Sparky; I'm not following your suggestion.. Are you saying just hook a compressed air line to that cylinder and run 80PSI into that cylinder with the rockers backed off. If it is a valve problem you'd have air blowing out the exhaust. If it's a hole in the piston air out of the oil galley?

Yup, yup. And if it's an intake valve (doubtful, cause it'd REALLY run like crap) it'll blow up the intake.

Thanks guys!

I'll let you know what we find.

A standard air hose fitting will screw right in a Chevy plug hole, no adapter needed. Not really the right thread, but close enough for this purpose. Just don't tighten it too much.

yup, its called a cylinder leakage test, other places to check would be radiator for bubbles(head gasket) and if you had 2 cylinders at zero then you would check the neighboring cylinder for leakage from the other one(busted cylinder wall)

Originally Posted by kcress

2 year old crate-motor
350 Chevy
86 Blazer
6,000mi on it

At the end of a 200mi drive the engine started to miss.
Quite badly. Limp home 15 more miles.

Next day do a cold compression test.
All cylinders look reasonable with the exception of one that reads zero.

Easy to check for broken spring. Pull off the rocker cover and the valve springs look okay.

Could be a hole in the piston?
Could be broken rod?
Could be a failed rod cap?
Could be a burnt valve?

What else can it be?
What is your guess?
Why would a valve burn or a piston hole after 6,000miles,
during an otherwise, run-of-the-mill trip?

my 455 toronado that's in my '76 GMC motor home has the same problem, but it's # 5 and 7 . no flicker on my tester, period. 'took the head to NAPA, the machinist dissasembled that head 3 times in disbelief, he said the head was new. giving me the same test criteria to try at home that you fellas are talking about, no leakage, no holes in new pistons, actually able to see fresh hone marks on this fresh long block dealer installation. still, not even a twitch from my brand new guage. for fits and giggles, i held a piece of cardboard over the piston and starter rotated the engine. it blew my hand off forcefully. after 6 years of sitting it remains a testament to my lack of mechanical skill

On the Chevy, I'd guess an exhaust valve, too. The motor home sounds like the head gasket blew between the two cylinders.

OK, here's what I know. I, too thought head gasket. I bought the whole gasket set (ya never know) then I pulled the head, revealing a brand new, intact head gasket. With the rockers backed off, no flicker on my guage. damn. I bought a new guage 'Cause that's just not possible. Hmmm it's possible. Inspecting the cyl. walls for what should be major damage, revealed those fine little lines created by the machinist's hone, but no damage. I didn't verify whether there was fire at the plug, but without compression it's not gonna fire anyway. I had a freshly rebuilt dist. which I put in, it was'nt the problem. The only illogical answer appears to be related to appearances of my deceased parents in the driver and passenger seats as if they were going down the hiway.(numerous witnesses) I'm a nonbeliever. Due to the passage of time, I hope someone comes back with another, more plausible solution.