low low compression

[Mr. Rustypwnz]

hey guys I did a comptest on my stang and the numbers go like this, 100, 90, 90, 80, 90, 80, 90, 100. what is going on here?? I didnt think that it would even go that low and run, it still runs like a raped ape, smoking the tires thru 2nd and holding idle verywell and at 50psi of oil cruising, so what do you guys think, im using a gauge that one of my dads friends hasnt used in a while, do you think the gauge could be out of whack? I found why it was smoking white, the fuel press is at 45 psi hooked up, that means that it is at 53 un plugged, it should be at 40 correct?

Thanks josh

 

[TomCat]

How exactly did you do the test? WOT? How much compression built up on the first crank?

 

[Mr. Rustypwnz]

I just put in on there and turned the starter, The tester only holds one comp, stroke at a time, does the test at wot, raise it or something? oh crap, I dont have the egr spacer and the tb on right now, would that make a diff.

 

[R2D2]

pick a low cylinder, squirt a little oil in the cylinder, repeat test, if the compression jumps a bit you know the problem, and its not the gauge

 

[Mr. Rustypwnz]

what if it dosent do anything? and if it goes up the rings are bad? shoot all the cyls are low. LOL

 

[HISSIN50]

Was it the kind that you hold over the spark plug hole (it has a rubberish tapered end that fits over the spark plug hole)? Those kinda suck.

This was done on a stock bottom end on an 86? I ask in case you didnt do the test on a hot motor and you have forged slugs from a PO.

The FP stock is about 38 PSIG unplugged and 32-33 when you plug the vac line back in. That's some high pressure.

Good luck.

 

[Mr. Rustypwnz]

bottom end of a 93, I havent drove it in a couple of days, so that might help, but I took a cylnder that was at 80, put some oil in it and it shot up to 120. So my rings are bad? or I need to drive it a little and then test it? this is my first time doing the test so im kind of lost, the gauge screws into the plug threads,

thanks josh

 

[HISSIN50]

You should have hyper pistons in that 93 bottom end. I would atleast heat it up and retest one or two cylinders. If they stay the same, chances are the first test was kinda valid. If the numbers go up, I'd redo the test.

You normally want a hot engine. Disable the ignition and fuel system. Prop the throttle wide open. Put a battery charger on the battery and do your testing.

Good luck.

 

[Mr. Rustypwnz]

will do, hopefully I can save this poor 5.0, will update tomorrow. Thanks guys!!!