Low Compression?

[dboosted1]

Today I did a full tune up and before installing the new spark plugs I checked the compression of the cylinders? I got 90psi across the board and was like what in the hell? I do not think this can be correct. My car just hit 38,000 original miles? What can be causing this? I could understand if my car felt slow or had no power but i am still leaving everything behind with only 15psi (Now if I was runnig around with 20psi it would be another story) A compression check is pretty idiot proof but did I screw up? Thanks

 

[WarpSpeed]

Did you prop the throttle blade open?

 

[dboosted1]

No...should I have?

 

[FJM568]

Yes...without the throttle blade open, you won't be getting enough air in the motor which gives you lower readings...

 

[wicked6]

I am going to have a compression and engine leak down test performed in the next coming week.

What numbers should I be looking for?

 

[GN-231]

I find it hard to believe that leaving the butterfly closed will sway your reading that low....90psi . I have left my closed before and still got 140 psi, but maybe that depends on if your IAC is plunged open??

I have gotten 160 psi +/- 2psi compression on all cylinders with stock 87 gaskets/ 87 heads. With 1 steel shim and 87 ported heads, I got 170 psi +/- 2 psi.
Mainly you need to make sure all your cylinder compressions are within 5% (maybe it is 10%???) of each other.

On a leak down test all cylinders held within 4 psi of the set test pressure. I don't know what you should be looking for, but I felt like my results were good.

 

[mycarsucks]

How are the battery and starter? I know it wouldnt make for that low a reading, but add that to not having the throttle propped open and maybe a few other variables. Do you have your injectors turned off? If you dont, when you crank the injectors pulse and wash the oil off the cylinder walles. Without oil it doesn't seal well and you can get faulty compression numbers. Normally though, with this problem, your numbers woudl start high and end up lower. (Unless you have been cranking this for other reasons)

 

[Steve Wood]

throttle blade wide open so the engine can suck air into the cylinders? Engine warm?

 

[dboosted1]

I have only checked compression on two other cars in my life my 72 malibu and my typhoon booth only requried removing the power wire to the cap. for my T i just disconeected the fuel pump and the wires from the cap. Maybe i got low readings because i changed the oil right before doing the check and did not drive the car around. What should i do step by step? thanks

 

[Steve Wood]

warm, the engine, remove the plugs, disconnect the orange wire by the battery...prop the blade wide open, install the compression gauge, crank the engine and note how many strokes it took to reach maximum compression reading. Do it for each cylinder.