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Discussion Starter #1
:confused: Did a compression test on my 88 Shelby Z 2.2 Turbo 2. Numbers I got were surprisingly low. 100's across the board. Put a little oil in each cylinder and the numbers did not change. I'm a little confused because the car runs awesome and boosts to 12psi without issue. The car pulls about 16 on the vacuum gauge at idle. Car smokes a little when warming up. Not blue or white just a bit of light grey smoke when warming up. No smoke at all on accelerating or decelerating. I had thought the smoke may have been caused by a fouled catalytic converter because the o2 sensor was shot when I bought the car. No noise coming out of the car that would indicate an issue with the valves.
Car had high number on NOx on an emissions test but the other numbers HC and CO were low.
No coolant loss or any other headgasket issue is showing up either.

Previous owner said the engine was rebuilt 20K ago and I tend to believe him. He was very upfront with every issue the car had and the car runs strong all the way to 6000rpms.

Do all those symptoms add up to anything? Cam timing or issue with the valves were my thoughts.

By the way compression test were performed correctly according to what I know. Warm-up car. Disconnect HEP,plugs, and coil. Open the throttle. I tired the test with a closed throttle at first and numbers were the same. I also tried the compression gauge on another car my 93 Daytona with 85K on it. Got 125's on it cold which makes sense so I think the gauge is good.

Thanks for your time.
--Chet
:confused:
 

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All the plugs were out too right? Not just the cyl being tested? How many times did you turn it over? I do mine 5 times. Does it keep increasing every time it does turn over?
 

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Discussion Starter #5
Only pulled the plug being tested but all plug wires disconnected. Cranked it at least 5 times sometimes more but no change. Unfortunately, I did the test alone and could not see the gauge during the cranking only the result.

Thanks,
--Chet
 

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Discussion Starter #7
100s seem low compared to some of the other compression readings I've seen. My 93 has 85K miles on the original engine and is still hitting 125 cold.


--Chet
 

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as long as they are even, it should be fine. but, under 115 i would be looking at valve seats(on high mileage engines), or (most common) cam timing.
if it doesnt have major oil vapors out the oil fill cap (blo-by), then i wouldnt worry.:)
 

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Discussion Starter #9
I'm going to check the cam timing again. I checked it with the timing light trick but I was not sure how evident it would be if it was off. Shined the timing light in the timing cover top hole and saw a mark at 10-11 o'clock.

Retarded cam timing would cause low compression correct?

Thanks,
--Chet
 

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There was a great thread on the proper procedure for checking compression...The basics were as follows:

Open the throttle, pull the HEP connector, pull all plugs, and check the compression.
 

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Just don't forget to disconnect the HEP. With the pulg wires out, the spark tends to jump in the distributer, right through the HEP. The HEP doesn't like that and niether does the ECU.

Leaving the plugs in shouldn't effect the reading that much, but you should do it at WOT. I would look at cam timing and such. If it all checks out, then I wouldn't worry about it too much. If you aren't using oil or blowing out your dipstick, the bottom end is probably OK. As Adam suggested, it could be tired valves/seats.
 

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Discussion Starter #14
I disconnected all the spark plug wires so the reult would have been the same. Other then that I followed the testing procedure(WOT, disconnected HEP and coil, engine warm). Any other thoughts? I will check cam timing this weekend. No oil blow-by.

Retarded cam timing would cause low compression correct?


Thanks,
--Chet
 

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I got 160 across all four. New gauge, warm engine, all plugs out, hep unplugged, WOT. Thought mine was a little high. 800 miles on rebuild. any ideas with this?
 

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I've gotten 150psi withe a 2.5L running new Mahle pistons, but I typically get about 125psi on an unrebuilt 2.2L engine with 150K+ miles. It sounds like the lower compression is probably due to the cam timing being a tooth off. I've had broken piston ring lands and still obtained around 100psi with a dry compression test.
 

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Discussion Starter #18
Bottom end seems a little weak compared to what it should be and the low compression numbers bother me. If there is some more performance to be gained by a simple timing adjustment then I'm all for that.


So back to my question.

Would retarded cam timing would cause low compression and less bottom end but more top end?

Which way do I move the cam sprocket(clockwise or counterclockwise) to advance timing by one tooth?

Thanks again,
--Chet
 
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