Compression test on an 85 Chevy 305

[tedly]

My truck dropped a cylinder and was sputtering and backfiring. Found a bolt on the rear passenger side of the intake that was loose, creating a nice vacuum leak. Just ran a compression test on the #8 cylinder and the best it managed was about 98psi. I haven't even bothered to check the others yet because of how low the reading was.

It's been a long time since I've done compression tests, but if I remember correctly 90psi is a dead cylinder, and normal range is between 120-160. I'm trying not to break out the BFH and start smashing things, but in the meantime could you guys help me confirm whether my memory is serving me correctly or not? I know the cylinders should be within about 15% of each other, and 98 is right in the border for both operational psi and the 15% rule of a healthy engine.

Should I even bother testing further or should I just go ahead and pull the head and have it checked for cracks?

Thanks for your input.

[silver_bullet]

the compression test costs you nothing and gives you a wealth of info... may tell the tale of a head repair/replacement, or a whole different engine...better to read the whole story than just the cover...

[anotheridiot]

its rare for a bolt to come loose from the intake and not suck in the gasket and tear it, but as far as compression, that 10% rule is what I go by. I dont think you would even get 98 with a cracked head. Bent valves usually will still pump to 60.

[Beagle]

I wouldn't be surprised if it burned a valve. Did it change any with a "wet" test?

[68scott385]

I never looked at specific numbers, unless they where very low. I always looked at the whole engine. As said above, lowest cylinder within 10% of the highest. Another thing I like to do is a leak-down test, tells where the compression is going. Before I had a leak-down tester I used the hose from the compression tester to pressurize the cylinder with the air compressor.

IIRC, B3M's boxer would have had ~100psi cranking pressure when new. I don't know why but those Subaru's always read lower than other motors I worked on at the time, and they were all stock.

[tedly]

Yeah, I guess i need to do the others. I'm just pissed and throwing a hissy fit. Already got the intake off it, I was just kinda hoping someone would say "That's nothing! 98 is just about perfect." Working in a gravel driveway in the snow blows goats, but I got no one to blame but myself. I should have taken care of this awhile ago.

Pretty sure I can scrounge up an oil squirter so I guess I'll try a wet test in the morning.

Maybe the urge to smash things will have subsided by then.

Can't tell if it sucked any of the gasket in or not. The dink that rebuilt it before I got it went nuts with the red sealant and the gasket got ripped up when I pulled the intake. I want to yank it and rebuild the whole thing, but that's probably just because all that red goop is making me twitch like a junkie without a fix.

Guess I'm playing in the snow again in the morning. Joyous rapture.

[Huskinhano]

What are the others reading? Throttle open all the way? I've had that skew readings quite bit.

[tedly]

What are the others reading? Throttle open all the way? I've had that skew readings quite bit.

Intake is off, and haven't done the others yet. I got pissed about the low reading and sulked the rest of the afternoon. I sulked inside in the warmth though, so I guess that's something.

Going to do the others in the morning. Found out if coolant got in the cylinder it could have washed out the rings. If that's the case they might re-seat after running. The wet test will tell if it's the rings or the valves I guess. That's assuming that the others test within the limits.

I'm just bitchy and pissed because I should have given more attention to the old girl before and now I gotta do it outside in the cold with a bum arm.

And yeah, I guess I would like a little cheese to go with that whine. Mild cheddar if you got it.

[Beagle]

I'll let you in on a secret. Donut holes fix damn near everything. Mad at the truck? Donut hole. Mad at traffic? Donut hole. Seriously magical stuff. Good luck man!

[68scott385]

Donut hole...donut hole...donut hole...belt hole...

[tedly]

So I got over my hissy fit and washed the sand out of my vagina this morning. Went back out and pulled the #4 plug, this is what I found.


Yeah, I knew the valve cover was leaking, that's one of the things I was going to fix. Man this is embarrassing to put this on here, but c'est la vie. Ok, fuel line dumping into a mason jar, dizzy is out so no coil spark, lets see what she's got. Cranked it over a few times and when I got back to the front of the truck the gauge read less than 80psi, and falling.

@#$*.

Maybe it's not tightened down enough. Nope, it's in there snug. Is the gauge on right or did it not seal because I didn't put it all the way on? Nope, it's connected right.

<sigh>

Alright, lets try it once more just because. *crank it over a few times again*

73 and falling. Wonderful. Well it's not next to the #8, so it's not the head gasket. Just for fun I'll pull the #2 and test it.



That's it. I give up. The engine's screwed.

This is going to be fun.

[TheSilverBuick]

Junkyard LS engine time.

[tedly]

If I could I would. That was one of the things I was toying with when I got it.

Given my non-existent budget at the moment, whatever sbc I can find in decent shape is going to be the most likely candidate. Swapping an engine in a gravel driveway during a Wisconsin winter is going to be fun, especially since my hoist is 4 hours away in a storage unit.

[Beagle]

I drove stock Vegas when I was a kid, so trust me when I say i've seen worse. Out of curiosity, why is the compression suddenly bothering you? Drive that bitch until it won't start on Ether, then worry about it?

junkyard 454. Gas is cheap right now.

[tedly]

It's bothering me because I dropped at least one cylinder and there wasn't exactly an abundance of power to begin with. Couldn't get above 50mph and I live way out in the middle of nowhere. It's about 10 miles from here to the closest gas station. If it gives up the ghost on the road, I don't even have cell reception in a lot of places and it gets really cold up here during the winter.