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In general, compression tests on 911 engines is inaccurate.
When you remove the the spark plugs, carbon gets dislodged and settles on the exhaust valve. If there is no detectable leakage from the leak-down test, the compression test is almost meaningless. What are you trying to accomplish? |
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I don't have an issue with using 100 psi, I used 80 psi and can convert to % or do over then next time I test after GM TEC. One issue with using 100 is the needles are near the "end of readings" on the gauge and that's not the best accuracy. Yes I used the rotor pointing method and watched the crank pulley mark to align with the mark on the fan housing. Thanks very much for the comments and helping me along. Stay tuned... I'm going over to Jerry Woods tomorrow and ask what RA they might have targeted. |
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The leak-down only dropped 1 or maybe 2 psi from 80 for all 6 cylinders. I heard no air moving. I appreciate all the comments and help. As noted below (I guess now above) I'm going to try GM and then maybe a water intro. at fast idle. I might just put in some GM and then drive up HWY 280 in 3rd gear and load the rings on and off at a high engine speed. Then do a compression test again to see if there are any changes. Thanks again. |
In case a picture of the plugs give any additional clues.
http://forums.pelicanparts.com/uploa...1632095859.jpg |
Sounds to me like a normal good running engine. Nothing to fix.
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I would never reply to this sort of post, but this is gone past reality.
You have even said, the oil consumption is good and the car runs fine. So, what are you going to do, pull the engine apart to do what? Don't bother Jerry with what RA finish he used? After 40K miles you can bet its not the same. Get on with what is important in life. Your mission is commendable for the new owner but...... have you ever heard the saying, " Just like an old lady with a new table""" |
While your compression numbers might not rise to the comfort level of some prospective buyers, what seems more important is the consistency across the board. Take the car out for a hard drive as planned, maybe squirt in the GM juice first, and see what you've got. If the numbers are still consistent, I don't see a problem.
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Those plugs look consistent and pretty darn good. I concur with the "just drive it" group.
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So to everyone saying "there's nothing wrong, just drive it," you'd be comfortable spending $70k+ on a 3.2 Carrera with "low" compression numbers? He did say he was selling the car and didn't want those numbers to scare folks away.
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Here is what you should do: 1. The engine needs to at operating temperature. 2. The coil should not fire and plug wires should be off the spark plugs. 3. The throttle should be fully open. Now, remove one plug and screw in hose and attach compression tester. Turn the engine over for 5 to 8 complete revolutions until the value on the gauge stabilizes. Write down the value. Repeat for all cylinders. |
the test
Those compression readings are fine.Leakdown is good.Where is the problem?Different compression testers give different values.Snap-On tester reads 10% less than a Mac-Tool tester.It is just a number.The only number that does matter is the leakdown.That covers everything.Chill out.Fred
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I reassembled things yesterday and drove it. Ran fine.
Good point on testing with a hot engine. I know to do that and will do it on the retest. Google could not find any writing that said to take out one plug at a time. On engines with head gaskets and water it was mentioned to take out all plugs so that a blown gasket would not affect readings. In any case it only affects the cranking speed. We tried open and closed throttle, no change but we used open throttle. |
Seems odd. Maybe you don’t have full throttle at the throttle body.
Porsche specifies only leak down test is valid for determining engine condition. |
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My compression numbers, at 5,000 feet, have always been low to what others have found. I leaked my stock US 82 3.0 down, and got ~0 (the two gauges don't quite line up with no pressure on them, but when I hear no noise at all I can buy 0) on all but one cylinder, which was at 2%. Since I have a pop off valve, I could get my listening hose right into the #2 intake, which is where the faint noise was. Unlike most times I have done a leak down, I heard no sound in the oil tank (from rings) - usually there is a little there. Exhausts were ~0. I call that good.
The compression numbers were in the low 100s. I put a wrench into the pop off valve, to simulate WOT. I was surprised to note just about no difference in the compression readings. Car ran fine racing. |
Walt, thanks for the info.
Same as what I found re. open throttle or not. Maybe air is getting past the ICV. I was thinking more on what Winders said. One plug out at a time might reduce the chance of carbon getting stuck in the exhaust valves or keep more heat in. Right now I solved the problem of the AC stopped working so I'm putting things back together. |
@hcoles
giving 135 PSI to a combustion chamber for a leak down test is far too much for a precise test result. By this most test results are between 2% and 5% which looks good on the first view. Most quality leak down test Gauge/Units like from Hazet do internally limit the input pressure to approx. 4 Bar where by this a wider reading bandwith of the actuall pressure loss is possible. Give em about 58 PSI and re check the leak down value. As shown below everything down to 20% is acceptable as long as all cyclinders are almost matching https://d2q8g8w0j97itz.cloudfront.ne...808/4795-1.jpg |
20% leak down is not acceptable for a 911 air-cooled engine…..
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