Compression & Leakdown Results

Bauch · 10-01-2009, 11:49 AM

Just curious to get the forum's quick impressions on the following compression and leakdown results on a 1987 Carrera Coupe that I am looking at purchasing.

Thanks to all in advance.

1 - 3 @ 165
4 - 6 @ 155

Four cylinders were at 4%, one at 6% and the last at 8%. Until I have a final briefing from the technician who performed the inspection, that's all the info I have.

porschenut · 10-01-2009, 11:56 AM

Those numbers are fine. Most important with the leakdown numbers is determining not just the %, but where the leaks are - intake valve? exhaust valve? rings?

Peter Zimmermann · 10-01-2009, 12:27 PM

Be aware that there's a right way and a wrong way to do a leak down test. Unfortunately, many technicians don't know this, or care. To do it the right way, the engine must be turned to TDC of the cylinder to be tested, only then can the spark plug be removed. After testing the cylinder, turn the engine to the next cylinder, remove that plug, and test that cylinder. This is the only way to prevent combustion chamber debris from falling off the end of a spark plug and contaminating the valve seat of an open exhaust valve. Law of averages says that your technician took all the plugs out at once, which could explain the higher number for two of the cylinders.

Regarding the compression check, that's a pretty much useless test on a 911, unless the technician really understands how to do a compression check, in other words, how to watch the compression build during each revolution of the engine. That tells a better story than the final number.

Bauch · 10-01-2009, 01:12 PM

Thank you both.

I just confirmed that the tech did, in fact, remove all of the spark plugs at the same time. Thank you, Peter, for identifying that as a possible cause for the variance.

In your opinion, would the leakdown numbers likely be more consistent if the test were done as you described it should have been, or is it just as likely that they might be worse?

Eagledriver · 10-01-2009, 05:25 PM

The test would not be worse and could be better if the plugs were removed one at a time. These tests are showing that you don't have a broken ring and your valves aren't leaking. That is all you can ask for.

-Andy