Weird Compression Readings

[zcoker]

Hey all,

Did a compression test because I've been having some strange plug readings, black on the passenger and normal burn color on the driver's side. Onto the compression numbers, warm engine, throttle plate open: the driver's side, all three cylinders are 110psi; the passenger side, however, all three cylinders are 90psi. I've no clue as to what was ever done inside of the engine, but it has been dropped before and gone into as evident by twin plug cylinders. This offset compression points to cam timing, in my opinion. Car runs okay, a bit down on power, no smoke. What do you all think, or has anyone had this problem before? Perhaps this is a primitive form of VTEC.

[JohnJL]

Cam timing or valve clearances are a good start. Check for uniformity.

[DonE]

When I am adjusting valves for people, I find it interesting that these guys are more interested in getting .004 valve lash rather than checking cam timing. Granted this is not a big problem on stock motors, but any that have had new cams or top end jobs, etc, this can be a real problem. The worst I've seen were off by 3mm which is probably not too bad compared to others. In any event, the correct timing and lash made a nice difference.

[JMA-RUF]

Agree, if it's not smoking, then it won't be piston rings. I would say valve clearances, or as DonE suggests, CAM timing.

That reminds me, I need to get my valve clearances all set properly too

[zcoker]

I'm leaning more toward cam timing, as it is about 20 psi difference between the right bank and left bank. I can't see valve lash making that huge a difference equally amongst three cylinders like this. I was just wondering if anyone else has had a similar issue with the same symptoms.

[les_garten]

Originally Posted by zcoker

I'm leaning more toward cam timing, as it is about 20 psi difference between the right bank and left bank. I can't see valve lash making that huge a difference equally amongst three cylinders like this. I was just wondering if anyone else has had a similar issue with the same symptoms.

10-4 on that!

.

[zcoker]

Hey guys,

Worked on the engine a bit and checked the cam timing for TDC overlap with a dial indicator and it checked out 0.73mm for both sides. I was hoping to find it out of sync on the right bank with the lower compression cylinders, as I was sure that was the problem but it wasn't. All cylinders hold pressure and I cannot hear anything in the way of major leaks except for the usual ring gap leakage. Only speculation at this point. Thanks.

[DonE]

I assume you are using a leak-down tester when testing the cylinders now that you've eliminated cam timing. You can listen to the exhaust, intake and case for tell-tale air leaks.

[les_garten]

I've been thinking about this a little and have another suggestion. First a question though. You say the plugs are black. Are they dry black or wet black?

The other think I could thing of is that the deck was set improperly. Although usually you would expect higher compression not lower. 110 sounds right. 90 is low. The consistency is what's strange.

Did you do a wet and dry Compression check(squirt some oil in)? The other thing is that you might have a bunch of broken compression rings on one side.

When I built my motor, every compression ring in the engine was broken. Every shop I took it to said it ran fine. Felt weak to me. I never did a check though, I just tore into it.

Since it's not Valve timing, a top end Job is in your future it seems.

.

[zcoker]

Thanks for the input, guys. The plugs on that side (right side) are a dry/black. I see no evidence of burning oil, either on the plugs or from the exhaust. I did not have my leak down tester handy, so I just pumped up the cylinders to 100psi. I did hear very faint air leaks from the inside of the engine (valve covers off) and could smell the odor of the internals. Intake and exhaust were quiet. I did this cold, so it could've been clearances and ring gap. I did a dry compression but will do a wet to confirm what I think I already know and as some here have suggested: compression rings. Seems common with many who have done top-end jobs on these engines. Strange, though, how they just let go on that one side.

les_garten, were your cylinders damaged at all from all of the compression rings being broken?

[les_garten]

Originally Posted by zcoker

Thanks for the input, guys. The plugs on that side (right side) are a dry/black. I see no evidence of burning oil, either on the plugs or from the exhaust. I did not have my leak down tester handy, so I just pumped up the cylinders to 100psi. I did hear very faint air leaks from the inside of the engine (valve covers off) and could smell the odor of the internals. Intake and exhaust were quiet. I did this cold, so it could've been clearances and ring gap. I did a dry compression but will do a wet to confirm what I think I already know and as some here have suggested: compression rings. Seems common with many who have done top-end jobs on these engines. Strange, though, how they just let go on that one side.

les_garten, were your cylinders damaged at all from all of the compression rings being broken?

Strange thing. Every ring in the engine fell out in multiple pieces. Not one mark on the Cylinders. They had the original cross hatching in them and looked perfect. Shows how slick Nikasil is, and the quality of Mahle Gear.

.

[zcoker]

Any other engine would be wiped out or eaten up. Like you said, even some of your mechanics said everything was fine when in fact you found out otherwise. I've heard, too, people who's motors were strong and then to find out during a tear down all the broken parts yet still holding together. It almost appears that Porsche designed not only a good running motor but a motor with failure thresholds. It amazes me how many can "boost-on" for years with broken parts like rings or head studs. Wild stuff!

[les_garten]

Originally Posted by zcoker

Any other engine would be wiped out or eaten up. Like you said, even some of your mechanics said everything was fine when in fact you found out otherwise. I've heard, too, people who's motors were strong and then to find out during a tear down all the broken parts yet still holding together. It almost appears that Porsche designed not only a good running motor but a motor with failure thresholds. It amazes me how many can "boost-on" for years with broken parts like rings or head studs. Wild stuff!

Indeed! It will be interesting to see what you find.

Les

[DonE]

Try to borrow a leak-down tester. It will at least tell you if the rings are in spec and you can eliminate rings, pistons and valves.