andy karas

I plan to do a compression check on a 1981 US model 928 (4.5L). In preparation, I have read the information on Greg Nichols site, done a search of the 928 archives, and checked non-porsche sites on the internet as well as the little instruction sheet that came with my compression tester. From that effort, I've come up with the following steps and some questions. I would appreciate any advice available.

Compression check steps:

1. Engine at normal operating temperature.
2. Disconnect spark plug wires.
3. Remove spark plugs.
4. Remove fuel pump fuse.
5. Disable ignition system.
6. Insert compression tester in first spark plug hole.
7. With wide open throttle, crank engine through four revolutions (about ten seconds).
8. Record reading on compression tester.
9. Repeat steps 6 through 8 for each cylinder.
10. Compare results, looking for no more than 10 percent varience between high and low cylinder readings.

First, is this a correct and complete sequence of steps?

Second, regarding step 5: how, in as much excruciating detail as you can provide, do you do that?

Thank you for any help provided.

Erik - Denmark

Andy,
It all sound correct and reasonable, except the hot engine, that's no fun to remove the spark plugs from a hot 928 engine!
I hope the tester you got, is the type with a flexible hose, because the most standard testers cannot reach to the spark plug holes.
Regarding item 5, just remove the power wire from the ignition coil - terminal 15
Good luck with your project
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WallyP

A couple of comments:
It is much better to do the test with two people or a remote starter switch. You need to watch the needle during the test. The pattern of pressure increases is important. You should see a large jump on the first stroke, a smaller jump on the second, a much smaller on the third, a very tiny increase on the fourth stroke, and no increse on the fifth and subsequent strokes.
If the cylinder skips a stroke - that is, if there is there is no increase on the first, second, third or fourth stroke, but there is on the next stroke, you have sticking intake valves. In this case, you need to change the oil more often and use a better oil.

It is possible that the battery will run down during the test. Do NOT use a high-amperage charger during the test, but you can use jumper cables to a fully charged battery in order to maintain a constant cranking speed.

It is normal for the compression readings to decrease slightly as you go thru the test, as the engine oil gets scraped off the cylinder walls. That is, the first cylinder has much more oil on the cylinder walls, thus a better ring seal. After you have turned the engine over seventy times during the test, there isn't much oil left on the walls of the last cylinder.

A properly maintained, undamaged 928 engine should have very good compression - engine wear on a 928 is amazingly small.

andy karas

First, thank you, Eric and Wally, for your advice. It was just what I needed to do the job correctly. My son, Philip, and I did the compression test today on our "project". When I bought it, the PO told me that he thought one of the cylinders was bad. He had driven it once when he first bought it (just long enough to get it onto a car hauler) and then parked it in a field for two years. He then sold it to me, never having found the time to work on it himself. I have had it running since I bought it and, while the car has a number of issues to be dealt with, have been impressed by the strength, smoothness, and responsiveness of the engine. I decided to do the compression test to determine if the PO was correct or my impression while driving the car (that there was nothing of significance wrong) was correct.

Results of the compression test were:

Driver side, front to rear: 162/160/160/165

Passenger side, front to rear: 165/155/155/155

Needless to say, I was happy with these results. Thanks again.

G Man

Hey Andy. Good info. I too have an 81' that I am needing to do a compression test on and was not sure of the exact procedure. It is good to hear from fellow NV residents. I Have seen only two other 928's here in Carson City. What is the 928 population like in Vegas? Later.

andy karas

Jim

I'm relatively new to the 928 community. So far have met Tony Harkin and e-mailed with Dave McCready. Expect that sometime in not too distant future, Tony, who has made contact with most 928'ers in area, will try to have a get together. If you come down this way, look me up. Email is andrew-karas@earthlink.net.

Andy