Leakdown test
 

I just conducted a leakdown test to double check the results of a leakdown performed by a local shop. They obviously had something to lose if the results were bad, since they screwed up the cam timing and caused a valve/piston collision.

The leakdown tester is a Longacre dual gauge, which I just purchased. All cylinders showed nearly zero leakdown but there was definetly air leaking from the exhuast valves in all cylinders. Is it normal to hear air leaking from the exhaust valves or would this indicate a problem?

The car runs fine, no smoke, but the idle is occasionally erratic. I just want to eliminate any mechanical issues and move on to testing the ignition and injection systems.

Thanks.

Have you adjusted the valve lash before testing?

Cheers, George

Yes, the valves were adjusted.

It seems strange to me that I should hear air leaking, but the leakdown is nearly zero. I just want to make sure I don't have bad gauges, even though they're new and this air leaking is normal.

Thanks.

bump.

Ok, to make this thread more interesting, I'm thinking about installing a CHROME ignition lock bezel. Do you think this may be too rice? ;-)

Did you also do a compression test? What were the results. Did you do the leakdown test using high pressure like 100 psi? Sometimes you will end up hearing the air that will typically get past the rings at that pressure. Try redoing the leakdown test at 35 psi. Oh and leave the chrome at home.

Compression was good at 181 to 185 lbs. Yes, I used 100 psi for the leakdown. The air escaping from the exhaust valves started at about 70 psi. No air leaking below that pressure.

Now that I think of it, I could hear the escaping air while looking into the engine compartment and it was much louder listening from below the car. Would air past the rings do this? Is there any way to test for air past the rings versus the exhaust valves?

Oh and thanks for the advice on the ignition lock bezel. ;-)

If the air is leaking past the rings you should be able to hear/sense it through the crankcase breather.

JP:

I just did my own leakdown test so I know what you mean about wanting to be sure of the results.

As long as you are sure that you are using the tester correctly, e.g., you didn't kink the hose, it's not obstructed, then the leakdown result is the leakdown result. The engine should be as close to operating temp as you can get it (but don't burn yourself!)

Try the test again using 35 psi, that worked very well for me. The slightest change in pressure shows up as a very large percentage of the overall pressure, the lower the starting pressure.

Grady recommended 5 psi in this thread. . . .

http://forums.pelicanparts.com/showthread.php?s=&threadid=144650&highlight=leak+p si

Probably what you are hearing is the leakage past the rings.

John,

I'll try it again with 35 psi and see what happens. I don't think my gauges are accurate enough to get a good reading at only 5 psi.

Thanks.

John

JUst FYI, the leakdown on mine was done (IIRC) at 30 psi. At a 100 psi you WILL hear air leaking from somewhere.

Good luck,

Jerry M
'78 SC

Thanks, Jerry.

John,

My Sun Cylinder Leak gauge requires 70-200 PSI shop pressure but only applys very low pressre (say 5 PSI) to the cylinder. It has a single, differential pressure gauge that is direct reading in % leakage.

Best,
Grady

My Longacre device has two gauges; one measures the shop pressure and the other the actual leakdown. Both have 0 to 100 psi scales. I'm not quite sure how accurate it is but I would imagine that it's not that great at 100 psi, given that is the limit of the gauges.

My Tavia leakdown tester has two gauges. One gauge measures the regulated pressure being introduced to the cylinder being tested and the other measures the pressure maintained in the cylinder. I typically use 75psi regulated pressure with the engine as close to operating temp as I can reasonably work with. Here's a link to the instructions on Tavia's website (note the table they provide showing the percentage of leakage at 100 psi, 90 psi and 75 psi).

My Longacre is the same type of tester. It has the same type of instructions, with a table for readings when using under 100 psi. Do you hear air leaking from the engine when using yours at 75 psi?

Yes, I do. Every engine I'ver ever tested at 75 psi or greater has exhibited some amount of leakage which I can hear, although if the leakdown percentage is very low the sound can be a little faint. Tavia manufactures equipment for other resellers so maybe your Longacre one is made by them? The reason I mention this is that Tavia sells separately the premade hoses to fit different diameter plug holes, if you use your tester for other applications.

The reason for the difference in cylinder leak testers is cost. The current generation of testers with two gauges are dramatically less expensive than the old ones I have. Wayne’s sell them for not much $. The first one I bought was $650 in 1969! My second was about $750 in 1972. They still work perfectly with 30+ years of Porsche service.

The reason for the high cost is a precision 2-stage air regulator, a complex venturi system, and a true differential pressure gauge that measure the relative difference in pressures (P2 - P1). The advantage is that it is direct reading in % and has resolution of 0.2% in the range of 0.0% to 20.0%. I believe it is accurate to 1.0% and is certainly repeatable within 0.5%. When you are making important decisions based on the percent leakage measurements, you want to have confidence they are accurate and repeatable.


Here are some of the many threads.

Cylinder leak results:
http://forums.pelicanparts.com/911-engine-rebuilding-forum/144650-leakdown-results.html

Cylinder leak test:
http://forums.pelicanparts.com/porsche-911-technical-forum/159348-leakdown-tester.html

Leak down numbers:
http://forums.pelicanparts.com/porsche-911-technical-forum/151921-leak-down-numbers.html


Go search on more. There is lots of good info.

Best,
Grady

I think mine is made by Tavia, since it looks exactly the same and does offer seperate hoses for other applicatons. I did have to doctor up the hose, as it was too floppy and a real pain to thread into the sparkplug hole. A bunch of ductape on the lower 6 inches of the hose did the trick.