Need advice - Leak-down great, Compression test not so much

[Henry Schmidt]

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?

[hcoles]

Originally Posted by dannobee

Introduce the water while the engine is running (fast idle). A little bit at a time. You might end up running a pint or two over a few minutes time. Just a little at a time. The more "new school" way is using GM Top Engine Cleaner as specified on the label and toward the end, let it back down to an idle and let it run rich enough until it dies. Then let it sit for a couple of hours and let the stuff work its magic.

With respect to leak down testing, check which cylinder the ignition rotor is pointing to and look at the crank pulley to set that cylinder to TDC. Use 100psi and the second gauge will read the percentage directly. There should be no big whoosh of air leaking anywhere.

GM Top Cleaner sounds like the safest way to go and I assume will not give the cat or the O2 sensor a bad time.
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.

[hcoles]

Originally Posted by Henry Schmidt

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?

Ah, okay I just saw this. I'm getting all my ducks lined up for selling the car. Many buyers want to see both tests. Note that the compression numbers were... I would call fairly close or consistent - this might imply the carbon from the plug areas fell in six places in the same manner. I want to have compression numbers that will not scare people off from buying the car or not scare them too far.
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.

[hcoles]

In case a picture of the plugs give any additional clues.

[Eagledriver]

Sounds to me like a normal good running engine. Nothing to fix.

[Neil Harvey]

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"""

[jimbauman]

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.

[safe]

Originally Posted by eagledriver

sounds to me like a normal good running engine. Nothing to fix.

+1

[brighton911]

Those plugs look consistent and pretty darn good. I concur with the "just drive it" group.

[jimbauman]

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.

[winders]

Originally Posted by hcoles

Update after better sealing of the Stomski attachment and using a better gauge valve.

Cylinder 1 - 135psi (had a bit of oil put in yesterday)
Cylinder 2 - 125psi
Cylinder 3 - 131psi
Cylinder 4 - 132psi (had a bit of oil put in yesteday)
Cylinder 5 - 122psi
Cylinder 6 - 129psi

Viewed 2 cylinders with borescope some carbon is noted at the bottom on the cylinder walls. Places where we looked on the cylinder still had cross hatching.

@dannobee - Places I read indicate the air rushing out is TDC on the firing stroke. I'm pretty sure of the cam timing - I used a dial indicator when I built the top end and followed the Wayne book. I put oil in two cylinders yesterday and the numbers came way up. Today the numbers on those cylinders are down a bit.

I'm thinking to get advise from Henry or ? on something to run in it to clean some amount of carbon and then retest to see what happens. Maybe leave well enough alone at this point.

How did you do this?

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.

[faapgar]

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

[safe]

Originally Posted by jimbauman

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.

The leakdown is fine, that says it all.

[hcoles]

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.

[manbridge 74]

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.

[hcoles]

Originally Posted by manbridge 74

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.

okay, good point - I didn't know that. I could have the compression test values and if asked by prospective buyers will provide, if not well they didn't ask.

[Walt Fricke]

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.

[hcoles]

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.

[AndrewCologne]

@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

[winders]

20% leak down is not acceptable for a 911 air-cooled engine…..