Checking Compression
Instructor
Joined: Jun 2015
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From: State College Pennsylvania
My intuition says that the temperature shouldn't matter. The cold will affect the AF ratio and the oil temperature, but the pistons should travel the same distance and the volume change (and thus the compression) should be the same.
But of course I would ask someone more knowledgable before I rely on my guess.
But of course I would ask someone more knowledgable before I rely on my guess.
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From: New Or-leens Loo-z-anna
Rookie guess.....The compression reading will be higher on a hot engine because the parts are fitting closer together because of expansion/the piston rings are coated with oil making them hold pressure much better.
From what I've read the readings should ALL be fairly close/no weird cylinder pressure 30 lbs. less than the other cylinders
From what I've read the readings should ALL be fairly close/no weird cylinder pressure 30 lbs. less than the other cylinders
Last edited by doorgunner; Feb 2, 2016 at 10:41 AM.
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From: State College Pennsylvania
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From: Mi
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Melting Slicks
Joined: May 2006
Posts: 2,364
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From: Centerville Louisiana
Most recommendations say warmed engine all plugs out throttle open. This would be almost impossible for me and a warm engine. Got a miss that shows up after the motor runs a bit pretty sure it's not plugs wires or ignition related.
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From: New Or-leens Loo-z-anna
Wear gloves.
Rookie question: Would an engine at operating temperature be more likely to reveal a problem like a leaking headgasket/etc. when checking the compression?
(Be gentle/I just wants to learn something
)
Rookie question: Would an engine at operating temperature be more likely to reveal a problem like a leaking headgasket/etc. when checking the compression?
(Be gentle/I just wants to learn something
)
Thread Starter
Melting Slicks
Joined: May 2006
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From: Centerville Louisiana
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From: Mi
Ahh now your at a point where diagnosis gets fun. I've seen headgaskets leak when warm and when cool almost zero leak. In that case yes, compression could be different warm, but not usually substantially. Usually is one of my favorite words. 
If its a coolant leak (from head gasket,cracked/warped head), then a different story altogether. Alot of times I'll leave the pressure tester on overnight, then scope into the suspect cylinder(s).
To the op, what engine/any mods? Have you tested spark when warm? Use a real spark tester that makes the ignition system work hard. I use a st-125. Is it a miss on one cylinder? Is it a miss at idle, under load or all the time?
Edit - sorry replied to wrong post, first part was for doorgunner
If its a coolant leak (from head gasket,cracked/warped head), then a different story altogether. Alot of times I'll leave the pressure tester on overnight, then scope into the suspect cylinder(s).To the op, what engine/any mods? Have you tested spark when warm? Use a real spark tester that makes the ignition system work hard. I use a st-125. Is it a miss on one cylinder? Is it a miss at idle, under load or all the time?
Edit - sorry replied to wrong post, first part was for doorgunner
Last edited by Gweesh; Feb 3, 2016 at 08:14 AM.
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Melting Slicks
Joined: May 2006
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From: Centerville Louisiana
Stock. Mostly at idle when it first starts with choke on its not really noticeable then when it idles down. I've pulled off wires can't really tell. So I'm looking at checking compression now. Also looking at vacuum lines can't find one size at my local auto part stores
Instructor
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From: Mi
Never a bad idea to check compression as its a good general gauge of health of engine. A cylinder leakdown test would be great too.
Can you try to load it hard when driving? By that I mean get near cruising speed - engine rpm low as possible, and get into throttle as much as possible without it downshifting. This can sometimes cause a ignition misfire to show itself. Sometimes you can see a bad ignition part by getting the engine compartment very dark and looking for arcing (especially if you spray with water). But the best way is a spark tester.
Have you checked distributor cap and rotor?
You mentioned vacuum lines. Do you have a vacuum leak then, or missing a vacuum line? If so that would need to be taken care of first.
Hard to point in the right direction without being there to see what type of misfire it is. Or if it is actually a misfire or a rough/poor idle.
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Melting Slicks
Joined: May 2006
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From: Centerville Louisiana
I would say more of rough idle. I did the easy stuff wires,cap,rotor pulled the covers watched lifters move, I'm trying to avoid doing the compression check cause I know where that's headed. I'm going to get back on it tomorrow
Instructor
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From: Mi
Ok. So compression check is a good idea, if nothing else gives you an idea of engine health. But since you say rough idle, couple questions. When did it start doing this? Any other symptoms? Any repairs or work done just prior to this problem starting? Did it sit for an extended period just prior to this problem?
Burning Brakes
Joined: Nov 2005
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From: Cartersville GA
How many miles on the engine? A compression test, hot or cold, you can tell if you have one or more cylinders with issues if there is a difference of more than 10%. A leak down test also would be a good idea. If the engine has high mileage, the distributor bearings could be worn, causing the rough idle. I had this experience before. Also, I don't know if it's a points car, but there are some cheap points out there, and they will cause a rough idle. Pay for the better points/capacitor unit, they are worth the extra $$. Check for vacuum leaks. If you have any carburetor ports plugged off, check the plugs as rubber will rot and crack (had this problem too). Make sure none of the vacuum hoses have any cracks for the same reason. Good luck, chasing down reasons for rough idling takes time and patience, hopefully it's not something that will require tearing into the engine.
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Melting Slicks
Joined: May 2006
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From: Centerville Louisiana
Car is new to me about 70k, hei. Stock appearing. Thanks for the suggestions I'm working on checking vacuum leak. Starts pretty easy no smoke. So turn the crank and see if rotor moves I guess.
Safety Car
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From: Lafayette Louisiana
Hi Croaker!
Hi Croaker!
If it were me, the first thing I would do is check the compression, jam the throttle plate WOT, and check to see if 10 to 15% within each cylinder. I agree with Doorgunner, hot would probably give you higher readings, but by the time you do all 8 cylinders, the temp changes and if you go back and do the first one again, you will likely get a different reading. More important is the variance between cylinders. So, IMHO, no point in checking all of the other things until you check compression. When I check the compression, I disconnect the ignition and try to count the rev's to try to get a consistent check.
Hope this helps and good luck! My 68 is still on jackstands, BTW, call me before you try to put a 69+ radiator in a 68, LOL's!
If it were me, the first thing I would do is check the compression, jam the throttle plate WOT, and check to see if 10 to 15% within each cylinder. I agree with Doorgunner, hot would probably give you higher readings, but by the time you do all 8 cylinders, the temp changes and if you go back and do the first one again, you will likely get a different reading. More important is the variance between cylinders. So, IMHO, no point in checking all of the other things until you check compression. When I check the compression, I disconnect the ignition and try to count the rev's to try to get a consistent check.
Hope this helps and good luck! My 68 is still on jackstands, BTW, call me before you try to put a 69+ radiator in a 68, LOL's!