Compression Test Proceedure
 

Compression tests are usually done on a warmed up engine. But I am worried about unscrewing the plugs in a hot engine. Don't want to mess up the plug threads in the heads.

What is the best (safest) way to do a compression test on our engines?

Please advise and share any problems you have had doing this test.

Thanks again,

Ron D

I think you answered part off the question already. Warm not Hot.
If you are worried about puling the plugs hot then pull them cold and use a SMALL amount of antiseize on the plug threads. Pop them back in, warm the motor up and pop em’ back out.

Warm the motor up and pull all the spark plug wires off. Then take an air blowgun and blast out around all 4 spark plugs before removing all 4. (I have found all kinds of crud around plugs and don’t want anything getting sucked into the motor while cranking) Some folks disable the fuel pump and remove the fuel from the carbs to cut down on gas washing the oil off the P&Cs.

Disable the coil and prop the carbs wide open. Set up the tester in a plug hole and crank to max pressure readings. I like to count the # of compression cycles that it takes to get the max readings, too many and there might be some “issues”. Ether use a remote start switch or have a friend crank the motor. This is the short answer as there are many tricks and methods that can be used with a compression tester to help read a motor.

Did this last weekend. IMHO if you don't burn your arm or skin a knuckle, you're not doing it right!;)

Ron, did you ever pick up a screw in compression tester? I think you'll suffer alot of frustration with your old press in place one.

Remember, there's a wire on the left side of the engine compartment to crank the engine so you can do it solo.

be sure to prop the throttle open so you dont wash the cylinders with too much fuel... 4 pumps, 1 for each cylinder is enuf to strip the oil.

Good Luck... hope your readings are better than mine!

Best,
Morrie

Green 912 & Morrie:
Thanks for the tips. Should make the test easier and safer.

Yes I did make up a screw in compression tester using and old spark plug, hose with disconnects and a gage. Hope it works, looks cool. Put an O-Ring on the spark plug for a good seal.

I believe the wire you mean is the loose unattached yellow one near the voltage regulator. I just need to connect my jumper switch to it and a 12 volt supply to crank the engine. The PO instslled a hot start relay on the starter motor, hope he didn't disconnect the yellow wire from the starter solenoid. If so I'll have to crank the engine with the ignition key.

Plan to do the test Saturday.

The PO said the numbers were good but we will see.

Thanks again,
Ron D

Ron,

To avoid scumming up the plug threads I installed these on my 69. For the insert tools and 6 inserts was about $95.00.

http://www.timesert.com/sprksert.htm

I also installed a hot start relay and have no problems using my remote starter switch from the engine compartment. That starter switch resides in my glove box as I used to use it to overcome the hot start until I installed the relay.

It is much easier to remote start it than pushing it!

Craig

Craig:
Thanks for the idea! Hope I don't bugger up any threads.

I thought I read somewhere that our engines were originally made with Heli-Colil type inserts in the spark plug holes. Is this true?

Ron

Ron,

I did replace standard helicoil's with these time-serts, the first time I pulled the spark plugs 2 of them came out with the helicoils stuck to them. I could not get the helicoils off the plugs without destroying them and had to clean them and re-use the same plugs until I got the helicoils replaced. I would not try doing this with the engine installed, I had a Porsche mechanic do it for me.

Craig

Craig:

Yikes!!! Maybe this is why we should use some anti-sieze on the threads. I have heard pros & cons on this subject. You are proof that it may have been prevented your problem.

Thanks,
Ron

No cons to using small amounts of antisieze on early cars! The issue about antiseize is in regards to later cars, grounding and that modern spark plugs normally have a release coating on them to start with. Lube em’ up, always pull them cold or only warm, use common sense or a torque wrench to put them in and you will likely never have a plug problem.

.

Compression Test Results
 

I ended up buying a CT with the flexable hose. It worked OK but not as eaisy to screw in the spark plug holes as I had thought.

The results were:
No. 1 150 psi
No. 2 120 psi
No. 3 145 psi
No. 4 160 psi

Why would one cylinder be so low? Maybe the rings are gummed up. The car is not run very much and only for short distances. Would a high speed drive for a few miles help to loosen up the rings or is there something I should be adding to the oil to free up the rings? I use Castrol 20W-50.

The valves were adjusted before the engine was warmed up.

Thanks for your help.

Ron D

Hi Ron,

Sound like you have one cylinder with issues. As you can see from my numbers, I have one where yours is and one worse. Misery loves company.

With the one cylinder reading that low, you have problems.....

Morrie's #s:
#1: 120
#2: 145
#3: 100
#4: 150

:(

Anyhow, there's a simple procedure you can do to help determine what the problem is.....

First, I suggest you recheck your valve clearances. A bad setting here will kill your compression and possibly damage the engine.

After doing that, recheck your numbers. If you still have the same issues, your engine probably needs work.

One method reported to help diagnose the cause of low compression is dumping a tsp of 30W oil into the cylinder through the plug hole and re-running the test. If your numbers rise significantly, then rings are probably bad. If they stay pretty much the same, its probably valves. If the numbers rise significantly, rings are likely to blame.

It was suggested to me to replace 1 quart of the oil with a quart of Marvel Mystery oil. If the engine was rebuilt not too many miles ago, there's a slim chance this may help reseat rings. Adding some to the gas could help clean up the valves. It may be vodoo to some but I have yet to hear of anyone reporting the stuff to do harm, so its a low/ no cost shot in the dark.

Good luck Ron!!!
SmileWavy
Morrie

Plug Condition
 

Morrie:
Thanks for the suggestions. Strange that the #2 spark plug looked the best with a light tan color all then rest looked like the mixture was too rich and had the black soot color. Go figure.

Ron D

Ron,

Funny you should say that. I had EXACTLY the same experience. My low comp cylinders look good and the "good" ones were sooty.

I leaned the "good" ones just a little. Does anyone have some insight into this??

Morrie
67 912

Running Compression Test
 

Has anyone ever done a "Running Compression Test"?

See http://www.misterfixit.com/dyncompr.htm

Sorry don't know how to create a link here.

Thanks,
Ron

Ron & Morrie:
I'll throw my .02 in here. I had a 58 super with very similar numbers & it drove me nuts. Since I had another motor, I decided to just drive it until something broke. Three years of hard driving later (including 2 L.A.-Seattle trips) it was still running like a (low compression) Swiss watch. I realize I'm being over-simplistic here, but a good percentage of 4cyl Porsches out there probably have similar numbers & for street use will probably continue to run well into the future.
As for compression-testing a Porsche, I've seen good results where the plug is substituted for a fitting (made from an old plug with clamped hoses) and each cyl (TDC) is charged with air from a compressor which allows listening in various places for leaking air with a stethoscope. I once watched an aircraft mechanic do this to a Continental 4 . It was on a bench and he had a little rubber bulb with a tube connected to it (sort of looked like the little hand blowers used to clean camera lenses.) It had a very fine talc in it and when he shot it in an area, he could "see" air leaks in various areas. Since I smoke a pipe, all I have to do is aim my puff at a suspected area.
Bill.

Bill:
You are absolutely right. My engine will probably go for many miles as is. I just hope it's not due to a broken head bolt, bad rings I can live with!

FYI
I found a site with info on the Leak Test for VW's on Ken's Buggy Barn. http://www.w-p-c.com/buggy.htm

Thanks again,
Ron