Numbers from leakdown test
 

I keep fouling a plug with oil, and made pretty crappy numbers on the dyno so I first performed a compression test. The results were about 165 lbs across the board, then I purchased a leakdown tester and went to work.

The oil fouling cylinder had a leakdown of about 30 percent, while the rest were about 22 to 25 percent. The gauge reads this to be a low, acceptable leakdown rate. The moderate zone starts at 40 psi. The test was also done on a stone cold engine. I assume a warm engine would be slightly better.

Does anyone have experience with leakdown testing? Any idea if these numbers are actually acceptable?

Thanks,

Rob

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1988 Mustang GT
11.8 @ 123 (7/14/01)
11.5 @ 120 (9/14/01)
1992 BadAzz Wrangler
1993 Explorer

See my car at: www.mustangworks.com/cgi-bin/moi-display.cgi?179

On a cold engine, those numbers are fine. If you want to find out where the leakage is occuring, fill the cylinder with compressed air and listen at various points, like the tail pipe, the oil fill and the TB.

Take care
~Chris

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Retired Moderator
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I can hear hising from the crankcase, but I know that you will hear that on any motor. I found a post on the Corral about leakdown numbers and a couple guys had around 5 percent leakdown! Could an engine go from 25 to 5 just from warming up??

Warming it won't change the numbers that much. It sounds like to me that you're numbers are really high, especially for a high performance motor. I've got my motor set up pretty loose, with a small ring package, and I've been told anything above 9-10% is too much. I ran a warm compression check the other day and my motor was down about 20-30psi across the cylinders from when the motor was fresh. I didn't do a leakdown though to see where I am loosing the compression. My suspicions are the rings though. I really need to get out and buy myself a leakdown guage.

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Jeff Chambers
Trophy Stock #3
11.462 Seconds / 117.43 MPH
Chambers Racing Team

Try warming it up and do a leakdown again. There should be a noticable difference. Your compression is escaping by the rings, and a warmed up engine will have a tighter fitting piston/rings/cylinder. Try it and post the numbers.

Take care
~Chris

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Retired Moderator
MustangNet

My site: JimPorterRacing

RACECAR spelled backwards is RACECAR

HEY !!! Are you ASE Certified ??? If you are, ask me about iATN. The best tool you'll ever have, and it's free !!!

I agree with Chris. The pistons and rings will actually expand from the heat for a greater seal.


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For the performance fanatic
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My intake and valve covers are already removed, so I was going to do a cold leakdown test on my friends reliable untouched, strong running stock shortblock.

My car never made the power it should have, so I imagine my ring seal is not good.

I also have oil in my lower intake runners, my phenolic spacer is oily as well. I can actually see oil collecting inside where the intake port meets the head.

OK, I used my same leakdown tester on 4 different engines. Including a brand new Chevy Truck with a V-8. The numbers were all relatively the same. About 20-30 percent.

Now, my tester has 2 gauges on it. The first gauge is a 0-100 gauge that reads inlet pressure, and the second is the one you set at 0 to test. The 0-100 gauge reads about 35 psi, while the air compresser reads 125 psi. This was checked on 2 different air compressors. I tried a different tester that has only one gauge on it and the numbers were different.
All of the cylinders came up with 10-14 percent leakage. The test was again done cold.

I asked an engine builder about my numbers and he said they are probably fine. His 10 second Camaro is at 14 percent and runs exactly the same as it did with it's fresh 5 percent leakdown.

So after removing my intake I found my intake gaskets were leaking and pulling in oil from the lifter valley into the ports. I also removed all my valvesprings and the guides came up good.

At the track I ran 11.50 @ 120 mph with 3 melted spark plugs. Luckily all of the cylinders seem to be in decent shape.

Does this make any sense?????

Not quite sure what you want an answer to.

As far as your intake gaskets leaking internally, that also gave you a vacuum leak on the affected cylinders, which would cause them to lean out, and run hotter.

For future reference, there's a pretty easy way to determine if you have an internal intake gasket leak:

Fully close the throttle plates
Disconnect and plug any vacuum hoses attached directly to the intake
Disconnect the coil
Attach a vacuum gauge, and crank the engine. A reading of between 3" and 7" of vacuum is normal. A defective gasket will, in most cases, yield a zero vacuum reading.

Take care
~Chris

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Retired Moderator
MustangNet

My site: JimPorterRacing

RACECAR spelled backwards is RACECAR

HEY !!! Are you ASE Certified ??? If you are, ask me about iATN. The best tool you'll ever have, and it's free !!!