Great compression on 3,5,and 6, but 2psi on 1,2, and 4...what to do?!?!

G60SuperCharger · 09-15-2010, 05:05 AM

I have a 73.5 911T CIS that has sat for the past three years. When I bought it four months ago, the guy said (of course) that it ran fine when he parked it. Well, I did all the usual things when restarting a long-stored car (fresh fuel lines, boiled fuel tank, fresh fuel, marvel mystery oil in the cylinders for a week). When I turned it over for the first time, the starter sounded as if was turning over the engine with the spark plugs out...really fast!

So I did a compression test and got 150-160 psi on cyl 3,5, and 6, and got about 2psi on cyl 1,2, and 4. It barely hissed when I released the compression guage pressure!

I did a valve adjustment before turning it over, and all of the valves tolerances were in spec or very close. So where do I start? With zero compression, it has to be a valve problem, right? Wouldn't I get more than 2psi with totally shot rings?

Where can I buy a fitting/hose that will screw into my spark plug hole and connect to my air compressor? This way I can hear where the air is escaping to and diagnose the problem.

Thanks for any ideas as to what might be causing this.

deathpunk dan · 09-15-2010, 05:10 AM

a leakdown tester can be had at harbor freight for short money and it's relative accuracy will be sufficient for this.

Shot rings I think you'd see more like 90 psi or something.

Flat6pac · 09-15-2010, 06:18 AM

I have a customer car, running on 2 cyl. and making all kinds of racket. 1,2,3, zero compression and 4,5, and 6@ 150# 4 is dead for some other reason.
My diag. is it has jumped time on the left and its not a worry at this point the #4 will be there when all the heads are done.
It doesnt matter what the diagnosis is in the car, its running on 2 cyl, it has to go on the stand.
Bruce

BK911 · BK911's Garage

I would do a leakdown first. You may have messed up the valve adjustment. A leak down will tell you where the issue is, then you can see if it can be fixed.

Laneco · 09-15-2010, 07:25 AM

Maybe you're just lucky enough to have a nice bit of carbon sticking the valves open on those holes. Not uncommon on an engine that has sat a long time.

Maybe...

angela

G60SuperCharger · 09-15-2010, 12:51 PM

Thanks all, I'll give that HF leakdown tester a try.

Green 912 · 09-15-2010, 02:20 PM

Quote:

Originally Posted by Laneco

Maybe you're just lucky enough to have a nice bit of carbon sticking the valves open on those holes. Not uncommon on an engine that has sat a long time.

Maybe...

angela

...and if that is the case you must redo the valve ajust after you clear the carbon out.

G60SuperCharger · 09-18-2010, 09:36 PM

OK...I bought the HF leakdown tester ($31 w/tax! Had a 20% off coupon) and indeed I have major air leaking through the exhaust AND intake valves on cyl. 1,2, and 4. I could actually feel the air coming out of the exhaust pipe and the CIS intake flap!

It's so bad that I'm wondering if the valve adjustment I did went wrong somehow...could an improper adjustment screw things up this badly? If BOTH valves are leaking air on three cylinders, the possibility of carbon blockage or having six burnt valves is unlikely, don't you think?

I'm thinking my next step should be to completely redo the valve adjustment...wise move or a waste of time?

James Brown · James Brown's Garage

See if you can rotate the engine around till you get a good leakdown # If you can't, then check into the valve timing or stuck open valves. A good rap on the valves might seat them. Squirting some oil in each cylinders might help. My guess, this one is due to come apart, cleaned up, new seals and then it's good to go. Time, moisture, bugs, non operation really take there toll on an engine.

BK911 · BK911's Garage

What were the numbers?

retrogarage · retrogarage's Garage

I had the same issue after my 911 sat for 22 years. I ended up buying the valve spring compressors and redoing the valve stem seals.
While putting air into combustion chamber I had two or three intake valves that wouldn't seat. I undid the keepers on those valves. I put a piece of fuel hose on the valve stem and used it as a handle and "slapped" the valves into their seats. I seen a race engine builder do this. I also rotated and spun the hose as to sort of lap in the valves. Note this is with engine assembled and out of the car.
It worked perfect. Absolutely sealed up great. NO need to remove heads.

Bobboloo · Bobboloo's Garage

Put a dial gauge on it and check cam timing or if you don't have a dial gauge pull both chain housing covers and see if the cams are timed together. i.e. Both dots up at TDC #1

G60SuperCharger · 09-19-2010, 09:00 PM

Some great ideas...will report back. Thanks!

T77911S · 09-20-2010, 07:05 AM

cam timing would effect all 3 one one side. he has at least one good one on each side.

recheck the valve adjustment. i have gone to do #4 and actually tried to do #5 and could not figure out at first why it was tight. just a brain fart at the time. open the adjuster up and try another LD test if you have to. carbon in the port is possible.

johnsjmc · 09-20-2010, 07:57 AM

I,ve also seen similar results with rust on the stems on a boat engine(obviously a lots of moisture environment). I flooded the cylinders with diesel fuel let sit a day and then bounced the valves with a deadblow hammer. Compression all came back up. It took a while as some unstuck and seated easier than others.. Probably the ones which were open further when stopped had more rust to loosen than others.

bpu699 · 09-20-2010, 08:21 AM

I found it shocking how small of a piece of carbon can screw up your leakdown...

When adjusting the valves on the 930, 1 cylinder was registering 60%... my heart sank...

Looked at the leaking valve, and it looked fine (motor out of car). Shot some wd40 onto it, and it hissed like crazy...

I took a small bristle brush that is used to clean out paint guns, and ran it along the valve/inlet. A couple tiny pieces of carbon came off. Retested, 98% on the leak down

.

It really doesn't take much to lose pressure...

Try running a pipe cleaner along the valves/seats if they are reachable...

But having no compression on one bank suggests a valve timing issue...

911pcars · 09-20-2010, 09:45 AM

What are the odds carbon is blocking the in. or ex. valve in each of the three cylinders on one bank? IMHO, almost nil unless 4-5-6 were all encrusted with oil (burning lots of it).

Repeat the compression test, but squirt some oil in the cylinder first. If the compression increases, the rings may be shot. Perhaps the PO fixed one cylinder bank, but not the other.

How do the spark plugs look? Any sign of oil fouling?

If no difference w and w/o oil, check valve timing as suggested by others.

Sherwood

KingSalmon911SC · 09-20-2010, 10:11 AM

FWIW, I'm in agreement with Sherwood.

Richard

Bobboloo · Bobboloo's Garage

Quote:

Originally Posted by T77911S

cam timing would effect all 3 one one side. he has at least one good one on each side.

Oops. Looks like Sherewood read it the same way did. i.e. 1,2,3, good 4,5,6 bad but after re-reading I see that's not the case. Never mind on cam timing.

911pcars · 09-21-2010, 12:00 AM

Quote:

Originally Posted by Bobboloo

Oops. Looks like Sherewood read it the same way did. i.e. 1,2,3, good 4,5,6 bad but after re-reading I see that's not the case. Never mind on cam timing.

Yes, oops too. I should pay more attention to them cylinder numbers too. The only other thing I could think of is that certain valves are stuck in the open position. Are you sure the valves are closed when setting the valve clearance? If so, pull the heads and take a look.

Sherwood