Low Compression?, What am I doing wrong? Options

[silentsprintero]

So i take my car into my auto mechanics class today to do a compression test. Remove all 4 spark plugs (which look perfect) and the fuse for the fuel. Keep in mind the gauge we have is old and that is what I think is the problem, but it read 85psi on the spot across the board. How important is it to have the car warm, i only drove it across the parking lot. also, i notice that my dipstick pops up about 1/2" almost every time i open my hood, what's causing that? i know my compression is higher than that, it pulls strong, so what am i doing wrong?

-Chris

[turbohcar]

The dipstick popping out is a common problem, especially with higher mileage motors. It means you've got some blowby in the crankcase. I usually just grab some pliers and pinch the dipstick tube a bit at the top (while the dipstick is all the way down) and that keeps it from popping out again.

I believe that you have to get the engine to near operating temp to get an exact compression test. Not sure though, I forget. Rob help me out on this one.

[silentsprintero]

Yea the motor has 92k. I took the hose from the valve cover to the intake off and capped that and put a filter on the hose, would that effect the comp. reading? also i had the throttle at WOT, but would it help if the TB elbow was off or would it not matter at all. thanks again! i feel retarded for asking such simple questions

-Chris

[AWD DSM 1]

Three major things that affect compression.... rings, valves, and the head gasket. Having the engine warm is fairly important, can typically boos compression readings by 20-30 psi. Basically just trying to get everything to expand so that it seals as it would if the engine was running.

I'd make sure you get a good gauge. If the engine runs and you are only getting 85psi, I'd guess the gauge is bad.

[black92_tsi_awd]

Having equal readings across the cylinders is a good sign, like AWD DSM said, get a new gauge.

As long as the TB is a WOT, your OK. The breather hose won't affect compression readings.

[silentsprintero]

Thanks for the advice!! I'll borrow my friends (new) gauge and check it out sunday after work

-Chris

[awd4kicks]

BTW- An engine will not usually even fire when the compression is below 90psi. The only way to make up for that would be with boost like your turbo and you have to at least have a few cylinders firing to get the turbo to pump boost. So I'd say that the reading IS faulty since you didn't mention that problem.