ulrichd

My 78 came back with 150, 155, 160, 150, 160, 150. I've read that you want the numbers to be within 5%.

Ulrich

Bob Westmoreland

I wouldn't be concerned with those readings. They appear to be pretty good to me. A 78 SC probably has over 100,000 miles on the clock. There are several factors that can contribute to the readings such as engine temperature when the test is done. The condition of the test equipment and how fast the engine is turning over. You are good to go.

Dan Cobb

"What does a compression test tell you?"

Usually that you just spent too much on a car.

Seriously though, it is difficult to asses the positive aspects of compression test results. While good numbers might suggest everything is fine, they cannot be used as a 'clean bill of health' so-to-speak.
If the test results are poor, or inconsistent, they are indicative of possible problems, and a sure sign of issues.
It's best for the experts to review these test results IF the numbers are crazy, but likely only a tear-down will reveal the exact reason.
+++

Steve Weiner-Rennsport Systems

Ulrich:

Follow that test up with the far more useful Leakdown Test.

Now, THAT will tell you far more IF its done right,....

Those compression numbers look OK, but one cannot draw any conclusions whatsoever about them until a Leakdown is done.

BRB-83-911SC

Logical follow-up question: What does a leakdown test reveal? For example: 83 SC with 89,000 miles, good, consistant compression (around 150), leakdown under 4% on 5 of the cylinders, one at 20%. Oil consumption of 1 qt every ~1,200 miles, occasional puff of blue smoke on startup, no overrun smoke, no external oil leaks. This was performed before valve adjustment; valve ajustment has since been completed, but did not repeat leakdown test after. Engine runs great. Just curious as to what a poor leakdown number might suggest. Could a valve have been out of adjustment? I am inclined to have another leakdown test done. Thanks.

Steve Weiner-Rennsport Systems

Brian:

The real value of leakdown testing is its ability to pinpoint WHERE any cylinder leakage comes from. If the technician performing the test doesn't take note (and write down) of not only what the numbers are, but where it comes from, the test is utterly useless.


Leakage heard from the air cleaner indictates leaking intake valves

Leakage heard from the exhaust pipe(s) indicates leaking exhaust valves

Leakage heard from the oil tank filler tube indicates a ring/cylinder problem


One needs to take note of the location when the leakdown numbers are higher than average to see where the problem lies.

BRB-83-911SC

Thanks for the explainantion Steve - learning as I go! Could valve adjustment impact leakdown results? My initial thought is "no" since my understading is that valve adjustment affects lift and duration of a valve opening when contacting the cam lobe. But, could an overly tight valve not close completely? What are some of the more common root causes for leakdown failure in each of the 3 scenerios you mention? Valve springs? Burned valves? Worn seats? Piston ring alignment?

I'm not trying to diagnose - just trying to educate myself. Thanks!

Steve Weiner-Rennsport Systems

Hi Brian:

Make no apologies,.....LOL,...This is how we all learn new things,...

Loose or tight valves will affect a compression test since the engine is cranking.

Loose or tight valves will not affect a leakdown since the engine is stopped at TDC with the valves on the seats.

Bad leakdown numbers can be caused by:

1) Leaking, bent, burned, or carboned up valves and valve seats

2) Broken rings, worn top ring lands, piston damage, scored or worn cylinder walls

DGaunt

Of course, that is assumong that the valves are adjusted properly. Sometime not if someone new did the job. I would certainly check it on any cylinder that is way off the other ones.

Peter Zimmermann

Ah yes, and let's not forget one of Steve's early comments (which are all right on the money) - the leakdown test is accurate "if it's done right." A possibility always exists that the technician became distracted and removed the spark plug from the cylinder with the high number, before placing the engine on TDC for that cylinder. If that's the case, a small piece of crud could have fallen from the plug tip, landed on and stuck to the exhaust valve seat, which then prevented the valve from closing completely. If this happened, a quick drive around the block would dislodge the carbon particle, and a retest would show nothing wrong. Just for the heck of it have that one 20% cylinder retested, then you'll know for sure.
Pete