Compression test results, need opinions please...

[JZano]

I found a fairly nice and clean coupe with about 170k miles on the motor. Did a compression test, but I might have done it wrong, or so I am thinking.

I was taught to kill the spark, pull one plug, test, put plug back in, move to next one.

With that I got between 135-145... But what i'm reading is 190 is best but shouldn't be lower then 150.

Car still ran strong though...

Thoughts?

I know I posted this in the KA section, but I figure this place gets more traffic and I'm short on time.

[badbob2121]

if you did it right... that compression is low

[PapaSmurf2k3]

Did you do the test with a warm engine?

[JZano]

Engine was slightly warm, was cold by the time I got to the last 2 cylinders.

I didn't release fuel pressure, throttle wasn't at WOT and all spark plugs were in except whichever one I was testing.

[ghx407]

Do the compression test again with a fully warm engine and a good battery. Remove the fuel pump fuse and unplug the coil to dizzy wire. Removing all the spark plugs at once makes for easier cranking, but I guess the engine might perhaps cool down a bit quicker. Hit the ignition, keeping it at WOT, and don't let go until the needle stops climbing.

The biggest issue is large differences (over 15-20 psi) between cylinders. If you get 145-155 psi you'll probably have a good runner for a long time, but don't try forced induction without a rebuild.

[JZano]

Yeah, it's hard to do these tests, car sat for a while, hard to keep battery charged... So I might bring a pair of jumper cables and keep the battery juiced that way.

If the car output between 135-145 with added restriction (plugs and closed throttle) the compression can only go up... I don't see doing the test the right way showing worse results, only better. Right?

[ghx407]

yup.

[AZ89two4Tsx]

Hey, is it a dual or single cam?

The single cams have lower compression. 155 or around that is good for a single cam.

But as stated above, the most important factor is the p.s.i. difference between cylinders. You want the numbers as close together as possible.

[motoman399]

to do a compression test right, you need to warm the engine to operating temp (180-195 degrees) then pull the fuel pump fuse and let the car die on its own. then pull all the plugs and use your compression gauge. when cranking with gauge in the spark plug hole you want to hold throttle wide open till the gauge reaches its highest measure. that will be your accurate reading.

The biggest issue is large differences (over 15-20 psi) between cylinders. If you get 145-155 psi you'll probably have a good runner for a long time, but don't try forced induction without a rebuild.

this is correct. the factory says no more than 15 psi difference (i believe). im sure with all the plugs removed you would have decent numbers. but might be worth testing again.