14 replies to this topic

[vichang4]

I am trying to decide what needs to be done to this engine to get it back up to racing condition. Here are the leakdown and compression tests on the 1.6L spare engine I just removed from my car. The engine was running very well, but just down a little on power.

cylinder comp dry comp wet leakdown
1 135 140 10%
2 135 140 12%
3 135 139 10%
4 140 142 8%

During the leakdown test I could hear noise from the oil fill hole (valves leaking). No noise from exhaust or dipstick hole.

The compression numbers seem very low for an engine with only a 10% leakdown?? Would a fresh valve job bring these numbers up into the 190 range where this engine should be? Should I also be looking at rebuilding the bottom end. The engine only has 20 race days since a total rebuild.

[Jim Drago]

Did you do the leakdown on the engine warm or cold?


Compression numbers are not a very reliable test. Could be the gauge etc, they are are all close.


The air noise you hear through oil cap is rings, not valves.


If those numbers were cold, that engine is still in pretty decent shape more than likely.


Jim

[davew]

The compression numbers do look low for that leakdown. I would questions:

Battery strength, a weak battery turns the engine slower and you get lower numbers.
Compression guage accuracy, compare to another guage
Are you checking compression with all plugs removed? Engine turns faster with no plugs than with 3 plugs

Regarding leak down numbers;

There are 5 places for air to escape causing leakdown.
1- Intake valve. You will hear air from the intake. Easy to here with air crossover removed, open and close throttle to see if sound changes.

2- Exhaust valve. Air escapes out tail pipe. Due to length of exhaust the easiest way to check is by closing off tail pipe with your hand for a few seconds, then allow for air to escape and feel/listen for air escaping

3- piston/rings. You will hear air escaping from the oil filler cap. Again, put your hand over the hole and listen as you create an air leak. The dipstick tube is too restrictive to give any indication.

4- cooling sytem. Could be head gasket or cracked block/head. You will pressurize the radiator and can see bubbles and/or pressure in the radiator neck

5-Into the next cylinder. Usually this is a BIG leak, causing zero compression in both cylinders.

Your original post does not give enough info to accurately diagnose you engine.

In general 7% leakage will cause a power loss. 10% will definately cause lake of power. Fresh engines are 2% out the rings or better. Zero on the valves.

Hope this helps
dave

[davew]

And I agree with Jim on warm engine versus cold

Dave

[KentCarter]

And throttle open!

[vichang4]

Thanks for all the feedback.
The engine was hot.
Fresh optima battery.
Throttle was wide open.
All plugs removed.
Checked with two different compression gauges, one very old and one brand new, same results.
Also used 2 different leak down gauges, same results.
I did not do the open and close throttle test or the tailpipe test, but air was definitely coming out of the oil filler hole.

[davew]

Why an optima battery. It is significantly heavier than the oem battery, which makes it non compliant. And a legitimate rules infraction as you have added weight to the right rear which is a performance advantage.

The guage readings still seem weird. But you need to do the leakdown again to determine if you have any valve leakage.

I would recomend having someone familiar with Spec miatas assist with the test. Determining "how bad is bad" requires experience knowing what is normal/acceptable air flow out the oil cap.

dave

[Bruce Wilson]

I did pretty well last year with a 10-12% leakdown motor, but it was definitely time to freshen over the winter. Agree with re-testing by someone experienced, as the compression numbers don't correlate with the leakdown numbers.

-bw

[vichang4]

thanks for the info. I did not think the leakdown vs compression numbers made sense. I even checked the cam timing searching for something not right. Everything checks out except the compression numbers? I'll do the leakdown again looking for valve leakage.

[Protech Racing]

Faulty leak down #s. , + bad rings.

[vichang4]

OK, did a little more work.

Performed both tests again, with similar numbers. On the leak down test I pulled the front intake boot off to listen for valve leaks. With oil filler hole open, I could maybe hear a leak from the intake manifold. With the oil filler hole capped, intake valve leaks were obvious on all four cylinders. An exhaust valve leak was only detectable on 1 cylinder and only if I put my ear right up to the end of the exhaust pipe.

I am still facing the question, do I do a complete rebuild, or just the head. The compression vs leak down numbers do not make sense. I will take the engine to the local SM guru and see what he says.

[dstevens]

I like using a mechanics stethoscope to help determine where leaks originate. About 5 bucks.

http://www.harborfre...cope-41966.html

[KentCarter]

If you hear or feel air coming from the oil filler, it's rings. Of course, all 3 can be leaking (rings, exhaust and intake valves).

[Richard Pressman]

Since leaks can come from multiple places, would sealing all but one of the potential air leak paths give a more accurate assessment of the remaining "open" path? IE: close off everything but the intake, then everything but the oil fill hole, etc. Might help to decide what to fix and what to leave if budget is tight?

[Tom Sager]

OK, did a little more work.

Performed both tests again, with similar numbers. On the leak down test I pulled the front intake boot off to listen for valve leaks. With oil filler hole open, I could maybe hear a leak from the intake manifold. With the oil filler hole capped, intake valve leaks were obvious on all four cylinders. An exhaust valve leak was only detectable on 1 cylinder and only if I put my ear right up to the end of the exhaust pipe.

I am still facing the question, do I do a complete rebuild, or just the head. The compression vs leak down numbers do not make sense. I will take the engine to the local SM guru and see what he says.

Assuming a warm motor and properly done test, your numbers are not good IMO. With leakage at the valves and rings you should probably get the entire motor rebuilt. If you just do a head and still have more than desired ring leak when you're done, you probably won't be happy you cut a corner. If the motor is old and worn enough to have poor leakdown numbers, chances are there are other parts that need to be refreshed while you're at it. More expensive now but probably cheaper in the long run IMO if you plan to run the car for a few years.