Hi

I did check compression on my EVO today and resulsts are:
FRONT: 120 PSI
REAR: 90 PSI

Is it normal ?
Shouldn't they be the same or very close values ?

Regards,
Piotr

 

Rear cylinder is too low.

Couple things: did you open the throttle all the way ( or prop open the slide in your CV )? If not, do so, and repeat your test. Your compression readings should be within 5-10PSI of one another.

Put a few drops of oil in the rear cylinder - if the compression #s go up, then your rear-cylinder's rings are weak. If no difference, the likely culprit is a valve that's not sealing (could be burned exhaust valve, carbon on the valve seat, etc)

 

thanks for reply. I will try again tomorrow and see how it goes.

 

Ditto 1984 ., be sure you are doing the comp. test correctly .

Warm up the motor to operating temp.
Remove both spark plugs and ground out the spark plug cables or disable the coils .
Be sure the choke is fully open , be sure the throttle is fully open , be sure the carb. sliders are fully open . You need a completely open intake system to get true readings. Turn off the fuel pet **** .
Use a threaded screw in comp. gauge that screws into the sparkplug hole , the press ons are a pain and not reliable for true readings.
Be sure the o ring on the comp. fitting is ok.
Roll over the engine and note not only the highest pres. reading but how long it took to reach the highest pres. in each cyl.
Compare the values read between each cyl.
If you find a big difference between the two cyls. do as 1984 advised ., pour a tea spoon amount of oil into the testing cyl., wait 5 min. to let the oil migrate then comp. test again . Note the values as compared to the no oil tests.
Check any website re: evaluating engine compression tests.
Hope all is well .

 

Compression checks done again.

Condtitions:
Warm engine
Both spark plugs out
Throttle fully open
10 turns to get highest pressure in both cylinders

Front: 138 PSI
Rear: 130 PSI

I guess this readings are goog, aren't they ?

 

For a stock compression engine w/stock cams - this is a decent reading. I don't have the book in front of me, but, I *think* that the threshold is 125PSI. As long as you're above that - your cylinders are healthy.

 

Your in good shape .

 

not more than 10% difference between cylinders is ok!