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Compression Test. Help Diagnose.


tashakes

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I'm still trying to sort out completely my engine hesitation. I have a 318i EFI conversion that came with the car. From 2000 rpm and up I get some hesitation and I don't feel like the engine is developing full power. I did a compression test and the result is the pic attached. It goes top left 1, 2 then the lower left 3 then 4.

I have read that all cylinders must be the same and anything over 125 is good. So help me diagnose if the engine is good and I still need to fiddle with the FI.

Thanks.

post-39605-0-04273300-1379997654_thumb.j

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Do it again with your foot hard on the throttle , this opens up the butterfly in the throttle body so that you get unrestricted air flow, rotor button out and all plugs out.

+1

must have throttle wide open. has nothing to do with EFI.  unrestricted airflow to the cylinders is important for a consistent accurate reading.  engine should also be warm.

 

fyi - gas pedal does not control the injectors or fuel flow. all it does is control the air flow.  really should be called an "air pedal;".... ;)

 

your initial numbers, if the test was done right, are actually pretty bad.  they are all low and there is a huge variance.

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except for the determining throttle position/ desired load part... but yes. throttle open. anything more than a 20% variance is too much.

if you suspect worn cam lobes, you can pull the valve out of the bottom of the compression tester, put it in like you're doing a compression test and start the engine. it's called a running compression test.

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Another compresstion test trick:  after testing compression on all four cylinders (cold engine, throttle fully open), then squirt a little motor oil (one shot from a pump oiler is plenty) down each cylinder and try it again.  If the numbers come up significantly, then your rings are worn.  If it stays pretty much the same and you're still burning oil, then you have worn valve guides/stems/seals.

 

cheers

mike

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I did my test on a warm engine that's what I read. Should it be with a cold engine? With WOT? I will try your test Mike. I am burning plenty of oil. Would that cause the hesitation?

I did my test on a warm engine that's what I read. Should it be with a cold engine? With WOT? I will try your test Mike. I am burning plenty of oil. Would that cause the hesitation?

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Always WOT for compression test, warm or cold, dry or wet. Warm engine best, but cold gives some info if it engine won't run. Either warm or cold, adding a bit of oil and doing it a second time (wet test) gives info as said.

 

BTW, the gas pedal on a tii control both air AND fuel :)

 

--Fred '69 & '74tii

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did you read the responses in this thead?

Sure did, this is my first time doing a compression test and want to learn to do it right, I was not clear if hot OR cold, and WOT did not make sense, but now I'm learned....

 

Always WOT for compression test, warm or cold, dry or wet. Warm engine best, but cold gives some info if it engine won't run. Either warm or cold, adding a bit of oil and doing it a second time (wet test) gives info as said.

 

BTW, the gas pedal on a tii control both air AND fuel :)

 

--Fred '69 & '74tii

Thanks, this pretty much narrows it down.

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On a 318 conversion, I'd look at the distributor-

it's easy to get it spinning the wrong way, and

that will give you decent low torque, but power falls

off as the distributor retards (instead of advances)

the spark.

 

Your problem is not the compression- while it's not ideal, it's not

bad, and certainly not bad enough to cause your symptoms.

 

hth

 

t

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