2nd Generation Specific (1986-1992) 1986-1992 Discussion including performance modifications and technical support sections.

Just did a compression test...

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Old 06-29-04 | 10:13 PM
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From: St. Louis/Columbia, MO
Just did a compression test...

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
Old 06-29-04 | 10:29 PM
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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?
Old 06-29-04 | 10:36 PM
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Old 06-29-04 | 10:40 PM
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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
Old 06-29-04 | 11:54 PM
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you have to take the shreader valve out (check valve)
Old 06-30-04 | 09:41 AM
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From: tatooine
why do you have to take it out?
Old 06-30-04 | 09:54 AM
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From: Orange Park FL (near Jax)
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.

Last edited by SureShot; 06-30-04 at 09:57 AM.
Old 06-30-04 | 10:14 AM
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Originally posted by samperry007
why do you have to take it out?
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.
Old 07-01-04 | 12:05 AM
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From: tatooine
thanks for the info
Old 07-01-04 | 01:38 AM
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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?)
Old 07-01-04 | 08:19 AM
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From: Orange Park FL (near Jax)
Originally posted by Project84
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.
Damn - all this time I've been using the trailing holes.
I'd better go back th the FSM & reread.
Old 07-01-04 | 11:21 AM
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From: houston
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?
Old 07-01-04 | 01:17 PM
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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.
Old 07-01-04 | 02:08 PM
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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..
Old 07-01-04 | 02:28 PM
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yes trailing hole and engine must be warm, remember 5th grade science? heat makes things expand, cold apex sleals don't seal as well.
Old 07-01-04 | 03:43 PM
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Originally posted by 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..
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.
Old 07-01-04 | 03:53 PM
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From: Orange Park FL (near Jax)
The next time I'm in there I'll try both L & T to see how they compare. - Interesting...
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