On an earlier post we had determined that I have wiped an exhaust cam lobe. I am pulling the engine and transmission this week and want to do a compression and bleed down to further check the condition of the engine. My question is, will I get good reliable numbers if I do the testing with the engine on a stand? I have always thought that the engine should be warmed up before doing compression testing but it would sure be easier to do with the engine out of the car.

 

I don't see how you could safely do a compression test unless you mount the engine in a test stand. I don't see any reason you can't do a leak down test on an engine stand. It can be done on a cold engine. I think a warm engine might give more representative results in some cases and is generally preferred. My car has aftermarket forged pistons with big, cold tolerances. Warm compression reading results have less variance than cold reading on my car. It might not make a lot of difference with cast pistons or factory forged pistons. My car also has aluminum heads and I don't like pulling plugs on a hot engine for fear of stripping a spark plug hole.

 

You should do your compression test before pulling the engine, it will be much easier. You have the advantage of being able to warm it up if desired. Pull all 8 plugs out and run your test should not take too long if you have a helper to turn the key for you.

 

don't forget to wire the throttle at least 1/2 way open for good air flow..

 

I was trying not to run the engine with the bad cam if I didn't need to, but it may not hurt to run it at idle to warm it up. I will have the engine in a stand and thought it would be much easier to do the comp. test in the stand as apposed to trying to get to the plugs and plug holes around a hot exhaust manifold.

 

You can do the leak down test with the engine on the stand. It will give you more information about the condition of your engine than a compression test.

 

.....

 

Pulled the engine and transmission today. I will do the bleed down tomorrow or Thursday. I will post the results for your comments.

 

One word of caution - I suspect you will use a breakover bar and socket on the balancer bolt to rotate the engine to top dead center to test. Be sure to remove the breakover bar before airing up the cylinder. If it's a little off TDC the piston can stroke under the 100 psi and rotate the crank and launch the breakover bar - potentially into something nearby and valuable.

 

I did a compression test and leak down today. I did 2 compressions test on a cold engine, the first just standard test and the 2ND after adding oil the the cylinder. The results listed below.

Number 1 - 145# and 165#; leak down 90% - leaks past rings
Number 2 - 174# and 177#; leak down 35% - leaks past rings
Number 3 - 170# and 190#; leak down 40% - leaks past rings
Number 4 - 170# and 185#; leak down 35% - leaks past rings
Number 5 - 185# and 187#; leak down 20% - leaks past rings
Number 6 - 175# and 178#; leak down 45% - leaks past rings
Number 7 - 169# and 169#; leak down ?? can't keep piston at TDC
Number 8 - 173# and 174#; leak down 50% - leaks past rings

The engine has a wiped #8 exhaust cam lobe which is why I started working on the engine and decided to do some extra investigations.
The engine had been running good, good oil pressure and used under a quart in 3000 miles.
From what I see in the leak down I would have thought it would have used more oil.
I have done a lot of work on cars but have never had an engine problem that required any extensive work.
I would appreciate your thoughts on the problem and where I go from here.

 

How did you manage to crank the engine over a minimum of 3 rotations on a stand to do the compression check?

Something pretty strange here because the compression figures, if representative, don't indicate any significant problem. But your leak down results are downright pitiful.

 

You don't have a leak down issue if your oil consumption is less than one quart in 3,000 miles. I bet 60% of the engines run on this forum can't match that number. Any modern car or any of our old Vette engines using modern rings and cylinder finish could match or exceed that number. I think you are in great shape internally. If you do choose to rebuild make sure the machine shop uses modern methods for cylinder finish and rings or you might go backwards on your oil control.

 

Number 1 - leak down 90% - leaks past rings
Number 2 - leak down 35% - leaks past rings
Number 3 - leak down 40% - leaks past rings
Number 4 - leak down 35% - leaks past rings
Number 5 - leak down 20% - leaks past rings
Number 6 - leak down 45% - leaks past rings
Number 7 - leak down ?? can't keep piston at TDC
Number 8 - leak down 50% - leaks past rings

These are good leak down numbers??? Hows that? Only if the test was performed incorrectly - which then throws them all out.

 

How did you manage to crank the engine over a minimum of 3 rotations on a stand to do the compression check?

Something pretty strange here because the compression figures, if representative, don't indicate any significant problem. But your leak down results are downright pitiful.

I pulled the engine and transmission together then did the compression test with the unit on the hoist and just hooked up the batt. and turned the engine.
The used a leak down tester, took pistons to TDC, set pressure to 100# and then pressurized the cyl. and read the leak down. As you can see #1 would only hold 10%. What I'm getting doesn't seem to me to be indicative of how the engine was performing.

 

I mentioned this earlier today from your other post - "I'm just curious about "leaks past rings" are you sure its not leaking at the valves? Most of your wet and dry compression numbers look pretty good with the exception of 1 & 3 and in the cylinders that have very little difference between wet & dry, your leak down numbers are high. Trust me I'm not an expert but I would think that your compression numbers whould show a larger variance if it were the rings."
Bob

 

I have the intake manifold off, I can't feel or hear any air from the intake valves or out the exhaust manifold, everything seems to be coming from the crankcase.
I don't see how the engine could run as well as it did with this amount of leak. Maybe I should go through the process again tomorrow and I could also try another gauge.

 

OK - that was pretty brave but I guess you get kudos since it worked out fine.

My vote would be that something was done wrong with the leak down test or there is a fault in the unit. Are you sure you were on TDC on the compression stroke for each cylinder. Of course if you were on the exhaust cycle you would probably get 0%. So, I don't know. To the extent a compression test is useful, it doesn't indicate any issues that would represent a problem with the engine.

 

Yea, the numbers don't match at all, it's impossible to have those leakdown numbers with those compression numbers.

I've done 100's of leakdowns and never seen numbers like that. I don't even bother with compression checks anymore.

 

I will re-run the leak down and see if I get the same results. This was my procedure, let me know if it's correct.
Started with #1 at TDC by the timing mark and also can see the piston at TDC through the plug hole. The intake manifold is off so I can also see both valves are closed. Set the pressure to the tester to 100# connect to the cylinder and read the pressure that the cylinder holds.
Continue cylinder by cylinder by firing order.

 

As the engine is running Piston rings rotate, if the ring end gaps line up in a stack you get huge leak down numbers because it makes a straight path for the air to escape but compression is not affected as much. If the end gaps are oposing you get to read real numbers and judge how the rings are sealing. Rings always rotate when the engine is running a function of a good cross hatch in the cylinder wall, unless a cylinder is out of round when they will rotate to the large spot of the cylinder and hold there. With Torque plate honning you never see out of round cylinders a good reason to torque plate hone. Out of round cylinders are pulled out of round by the head bolts when you torque the heads. Torque plate honing eliminates this distortion, the Torque plate simulates a head being torqued to the block while you hone. Rings that don't rotate leave a very distinct pattern on the ring face and it tells the story for you if you read them on tear down.
For this reason you can run an engine, test leak down and get a terrible reading in a cylinder, run it a little more and get a great reading. Some assume they just messed up a test when in reality it is just luck of the draw on ring positions.
Spinning the engine with the starter I have no idea if you would get ring rotation but I suspect not.
Piston rings are lubricated with splash from the connecting rod bearings, if you are not running the engine the normal oil supply is not there and will affect your cranking test of compression and your leak down. Rings also grow with the heat of combustion helping seal your cylinder, cold rings will offer pretty good compression test and less than ideal leak down.
Heat the rings up and leak down gets better as the gaps are narrowed. Perfectly sized top rings will end up with very close to zero gap at wide open throttle.
This is the top ring affecting the leak down test.
In the Sealed Power Piston Ring Labs in the 90's Leak Down tests were never used to test rings. They tested cylinder pressure, a properly seated ring with a good cross hatch in the cylinder would make the best cylinder pressure at RPM. Z Gap Piston Rings made much better leak down tests but did not match on the cylinder pressure test. My friends in Ring Engineering told me they did not give a Sh** about leakdown they only watched pressure. According to them good leak down never won a race. They conceded that Alcohol engine owners always wanted good leak down tests to keep alcohol out of the engine oil. Another pearl from the old school, the seal of the piston ring to the piston ring land, how flat it actually lays on the piston ring land is a key ingrediant of cylinder pressure. Running old pistons with worn out ring lands can cost a lot of ring seal and cause blow by. Hope that adds a new twist and does not bore you.