so today me (87 with 125,044 miles) and my friend (93 FD with 54,00 on replaced motor) did a compression check on our cars. we used a snap on compression tester for pistion motors and we tried damn near every way we could think of to get some decent numbers but something just wasn't right. on my front rotor i got regular bumbs of 77 psi and 74-75 on the back. the FD got 80 on both. we tried both top and bottom plug hole with no real difference. i know 125 is perfect for my motor and 100 is for the FD and we were nowhere near that so we put in the little pressure gauge that holds the pressure. i got 125 psi on both rotors and the FD got 115. something just doesnt seem right here so i was hoping i could get some feedback from the guru's around here. any info would be great, thanks.

 

was ur engines warm?

and no u didn't do it wrong... the fact that we use "piston" engine testers is gonna leave some error.. and not all testers are alike...

check out this link to answer some of your questions: compression test

 

thanks for the site, and yes, the engines were warm, egi fuses pulled and pedal floored

 

try another compressor... or go to the dealer and have them do it correctly with correct tester

 

[QUOTE=Ottoman]was ur engines warm?

and no u didn't do it wrong... the fact that we use "piston" engine testers is gonna leave some error.. and not all testers are alike...

I think I'm going to have to disagree with this ^

It's not that it's necessarily inaccurate. It's just that the 'bounces' are difficult to read.

To the original poster; did you make sure that the compression tester was FULLY tightened in the lower spark plug hole? If you have a decent/new tester, I don't see any way that it could be THAT inaccurate. But the fact that you put in the check valve, so that it 'holds the #' and you got 125 is pretty strange...

I really have no idea. If you're really worried (or merely curious) about having low compression, just go the the Mazda dealership. It's kind of expensive though...

 

Yep, I made sure that thing was nice and tight, and I didn't have to much trouble reading the bounces. The only thing that got me worried is that I keep seeing that 75psi is time for a rebuild, but both rotors and all faces only varied by a few lbs. so I wont lose any sleep. and to clear things up a bit, when I put the check valve back in, it didn't go strait to 125, it took 3-4 pulses to get that high, then stayed there.

 

The 100psi and 125psi numbers are for a Mazda spec compression meter.
Very few (piston) compression gauges will show numbers like that.

It's is more important to have three even peaks versus maximum pressure.


-Ted