yoblink17

I was wondering a few things about a compression test I did on my 91 n/a. I took out my EGI fuse and didn't disconnect my crank angle sensor. When i did the test I had it at WOT and cranked it over 3 times on both rotors. It read 85 psi on one and 80 psi on the other, but when I cranked it over 7 times on each the first rotor read 85 psi and the second one read 95 psi. Is this normal or is there something wrong. Please reply back with any info that will help.

Shane

WAYNE88N/A

Now how do you know that you're cranking the motor over EXACTLY 3 times or 7 times?

And are you using a conventional compression gage, with the check valve removed?

yoblink17

bump

yoblink17

i can hear the psst noise 3 times or 7 times
Im using a normal/standard guage
i just screwed it into the spark plug hole and cranked

Sesshoumaru

iTrader: (1)

you have to take the shreader valve out (check valve)

samperry007

why do you have to take it out?

SureShot

Leading plugs in, trailing plugs out, EGI fuse out (to cut off fuel).
If the engine has been sitting for a while, spray some WD40 in the trailing holes on all rotor faces to free up any stuck seals & build compression.
Install the tester & crank the engine at WOT.

Here is the tricky part.

Open the bleed valve on the tester just enough to see the needle bounce up & down between each face.
Observe the sets of three pressure peaks.
I have read that 120's are possible in a perfect engine. I’ve never seen one.
100's - good
90's - OK
80's - about worn out
My J-spec shows 87-87-87 front & 85-85-85 rear.
My hurt A-spec was 85-85-83 front & 80-50-50 rear.

Project84

On a piston engine, you are checking each piston, one at a time. The check valve holds the pressure on the gauge so you can see what it was.

On a rotary, one rotor has 3 faces, each of which are combustion surfaces. The rotor rotates, and if you have the check valve installed on the gauge, this is what happens. The gauge will show you the highest pressure of the 3 faces. So in you could have a 100psi face, and two 0 psi faces, but never know it cause the gauge will shoot to 100 and stay.

If you remove the check valve, the gauges will bounce. The peak point to which it bounces is the pressure for one of the faces. 3 bounces to 100psi means that all 3 faces are 100 psi. 2 bounces to 100 and a bounce to 70 psi shows you you have good combustion on 2 of the faces and the third is kinda weak. Make sense?

Oh, and the leading plug hole (lower plug) is where to put the gauge. Leave the upper plug installed so the compression doesn't escape from that hole.

samperry007

thanks for the info

turboii87rx7

y do you check it at wot???
how far off would the compression be if you didnt give it wot, and in what direction (lower than your result, or higher?)

SureShot

Damn - all this time I've been using the trailing holes.
I'd better go back th the FSM & reread.

apreludem

i thought compression check was supposed to be done at normal operating temperatures, meaning the car had to be warmed up and stuff....or did i just make this up? o.o Also, can't you just hold the check valve open?

rxseven

yeah, the test has to be done when the engine is hot. A cold engine will give higher compression numbers. Also, with a regular tester, do the test in the leading plug holes instead of trailing.

SureShot

Excuse me - the compression testor goes in the trailing hole.

I just looked it up in the FSM - engine section - page C7.

Check it out..

totallimmortal

yes trailing hole and engine must be warm, remember 5th grade science? heat makes things expand, cold apex sleals don't seal as well.

rxseven

Yes i know what the factory manual says, thank you. It also says to use the MAZDA compression tester for it. All I am trying to say is from my experience, the compression readings from the leading plug holes are closer to the actual numbers with the Mazda tester, that's all.

SureShot

The next time I'm in there I'll try both L & T to see how they compare. - Interesting...