No compresion on #3

[Matthew]

Well all my 1971 1.7L 914 now has a complete new ignition system (point's, condensor, rotor, cap, plugs, plug wires and coil) I have fantastic spark but the car still would not start. A buddy loaned me a compression tester and I got the followin reading Cylinder 1 135 #'s, 2 5 #'s (now this is a deffinet problem) 3 145 #'s and 4 143 #'s. So I have come to the conclusion I have a dead cylinder. What could the cause of that be? The car turns over fine no nasty grinding noises. When it would start (for like 5 to 10 seconds at a time)my friend noticed that when it died he could feel "back pressure" coming out of the "snorkel" that leads to the air filter. As always any help would be greatly appreciated as I try and get my new found beast to rumble. P.S. If you want to take a look at my poor abused baby you can see it at: http://www.fortunecity.com/boozers/cambridge/285/porche.html

[Dave at Pelican Parts]

Air coming out of the intake probably means that #3 intake valve is having a big problem.

JJ Slowik, on the Porschelist e-mail, suggested pulling off the valve cover on that side to look at the rocker arms. One time I dropped a valve seat, that showed me that something was very wrong. The other time, I only figured it out after I had taken the head off of the motor. (Saw damage in the surface of the combustion chamber, saw light around the sealing surface of the valve.)

Carefully examine the valve stems (etc.) for cylinder #3. Check for debris stuck in between the rocker arm and the valve, check for anything fishy with the rocker arms, springs, pushrods, etc. Get a magnet and pull the lifters out of the case. You'll need to remove the pushrod tubes to do that with the engine in the car.

If nothing is visibly wrong, then your problem is inside the motor. It will be time to tear it down. If the engine has anything close to 100K miles on it since the last major overhaul, it might be worth going into the bottom end as well. Ditto if your oil pressure isn't all that it could be.

Sorry to be the bearer of bad news.

--DD

[Wayne 962]

Before you do anything, make sure that your valves are properly adjusted. If your #3 valve is off, or open all the time, then the engine will not run, and you will get compression levels that are really low. Also, make sure that you perform the compression test with the throttle all the way open, otherwise you won't get really good readings.

If you want to figure out if it's the piston rings or the valves, remove the valve adjusting screws from the rockers and see if you can turn the motor easily through the stroke. If there is a lot of resistance, then it might not be a valve problem...

-Wayne