TedH

I need to perform a leak-down test on my engine to see how the rings are seating. Question, how about a compression test too? Same tools required?

Please advise on procedure.

Tom454

Tools are not the same for the two tests.

The leak down test is more complicated, requires an external regulated compressed air source as a reference point for the leak down rate.

The instructions should come with the equipment.

dgruenke

Not sure how to perform, but I know that Summit racing sells the leak-down equipment. http://www.summitracing.com

Tom454

This might help...
http://www.autosite.com/garage/repairqa/ques057.asp

Back to Maintenance

Question:

What is a "leak down" test?

Answer:

A leak down or "cylinder leakage" test is similar to a compression test in that it tells you how well your engine's cylinders are sealing. But instead of measuring pressure, it measures pressure loss.
A leak down test requires the removal of all the spark plugs. The crankshaft is then turned so that each piston is at top dead center (both valves closed) when each cylinder is tested. Most people start with cylinder number one and follow the engine's firing order.

A threaded coupling attached to a leakage gauge is screwed into a spark plug hole. Compressed air (80 to 90 psi) is then fed into the cylinder.

An engine in great condition should generally show only 5 to 10% leakage. An engine that's still in pretty good condition may show up to 20% leakage. But more than 30% leakage indicates trouble.

The neat thing about a leakage test (as opposed to a compression test) is that it's faster and easier to figure out where the pressure is going. If you hear air coming out of the tailpipe, it indicates a leaky exhaust valve. Air coming out of the throttle body or carburetor would point to a leaky intake valve. Air coming out of the breather vent or PCV valve fitting would tell you the rings and/or cylinders are worn.

A leakage test can also be used in conjunction with a compression test to diagnose other kinds of problems.

A cylinder that has poor compression, but minimal leakage, usually has a valvetrain problem such as a worn cam lobe, broken valve spring, collapsed lifter, bent push rod, etc.

If all the cylinders have low compression, but show minimal leakage, the most likely cause is incorrect valve timing. The timing belt or chain may be off a notch or two.

If compression is good and leakage is minimal, but a cylinder is misfiring or shows up weak in a power balance test, it indicates a fuel delivery (bad injector) or ignition problem (fouled spark plug or bad plug wire).

MotorHead

The biggest problem I found with the test is trying to keep the piston at TDC with 100 PSI of air pushing it down, it still went down in gear with the parking brake on ( mind you the parking brake is not 100 % ) :lol:


[Modified by MotorHead, 7:09 PM 3/23/2004]

weendoggy

All that means is you're letting the engine go beyond TDC, or not far eough to TDC of that cylinder. It can be a PITA but you'll master it. Another easy way is to put the engine on #1 TDC and then remove the distributor cap and put the reluctor pointed to the HALL unit (you'll have to move the dist.) and then use the distributor as the reference to each cylinder. If you have points, put the cam lobe on the points follower.

TedH

Gonna be alot of work to test the cylinders... I know, I know, it has to be done...

ddecart

Thanks for posting that, Tom! Saved me the trouble of typing it all out!

A compression test is a less complete test than a leakdown, but valuable nontheless.

Ted, I have a leakdown gage if you have access to a compressor. You're welcome to borrow it if you'll pick up the shipping each way. I have a compression gage for that matter, too.

71coupe

I would always start with a quick comp test. If I found any low cyls, I would do a leakdown only on the bad cyls - time saver
:cheers:


[Modified by 71coupe, 11:05 PM 3/23/2004]

TedH

ddecart,

Let me know what shipping cost is to zipcode 33626. I would need both the compression tester and leak-down tester. I will be glad to pay for all of the shipping. Please first advise on the cost to me so that, if it proves nearly equal to the cost of the testers (approx $100), I would probably buy them instead.

Tom454

ddecart....

"Thanks for posting that, Tom! Saved me the trouble of typing it all out!"

yeh.... I started typing it all out too.... but then remembered I was on the internet.

Chortle chortle.
:jester

TedH

I'm going to pick up a compression tester first. It is the cheapest of the two at $25. If it indicates problems, I'll consider the leakdown test.

Thanks for the offer!

Vmo

Do both a dry and a wet test, this will tell you the condition of the rings. I wouldn't worry about a cylinder leakdown test unless the valve train was in question.

ddecart

Ok, let me know. A compression tester is a good item to have around the garage anyhow.

I shipped the leakdown tester to someone in Canada last year and it was only like $11. Not too bad.

TedH

Heck, I only know about one kind of compression test. What are 'dry' and 'wet' compression tests (showing my ignorance... I know)?

stpman

To do a wet test you put a couple oz. of oil in the cyl before testing. will tell you if there is any problems with the rings (if #'s go up you have ring problems)

Steve :steering:

TedH

Ah. I see. I perform the dry test on each cylinder, record the measurements and then perform wet test on each cyl and compare the readings.

Thx!

weendoggy

When/if you do a "wet" test, be sure to crank the engine after putting the oil in the cylinder before putting the gauge in to test. This will allow the oil to disperse better around the cylinder. A couple of squirts from an oil pump can should do just fine. However, you can get just as good results or better from a leak down test.

ddecart

You got it!
The figures will increase in the wet test simply because the oil is preventing some leakage past the rings. Where it will really tell a story is if you have one or more cylinders that are much lower than the others. If a squirt brings those up to snuff, it's definitely a ring sealing problem. If the dry numbers are within 5% of each other, you're doing great.

71coupe

Ted -make sure you get the screw in style comp tester, Sears sells a good one. One thing to keep in mind when you are doing the tests is the condition of the battery. If it's old/weak, your readings may be affected. If you are doing repeated testing, like dry/wet/etc, you will see a decline as the battery loses its charge.
Also, if you have a electronic ignition, if you unplug the secondary coil wire (the big one) - make sure you ground it with a jumper. This will avoid damaging the ignition system as the ignition tries to overcome the huge amount of resistance of being disconnected. You could also disconnect the small gauge primary wires that go into the coil, but it's more of a hassle tho (Don't ground out thoes wires) :nono:


:cheers: