I need to do a compression test on my 89 GTUs. Do you just leave one plug in and test teh top spark plug hole? Whats the Proper PSI? Thanks a Bunch

 

Not totally sure on how its done, but I think like 70psi is bare minimum for acceptable. All I know is to do it when engine is warm and make sure when you do the test you do it at WOT. Thats what I know. :P
Good luck.

 

Search my man.. anything lower than 80 id trash.

 

FSM states to remove both trailing plugs. Approx PSI should be 85. Course it says to adjust for starter cranking speed too. How you do that I have NO idea. Engine should be cranked for 5-10 seconds.

Jarrett

 

Thanks you guys, so i am shooting for 80 to 85 PSI? and test on the Traling Plug holes. Anybodie else go any good info.

 

Get the engine nice and toasty warm. You are gonna need leather gloves btw. Pin the throttle wide open. Take out the trailing plugs. Remove the pin on the comp gauge that holds pressure in the gauge. Hop in the car and crank it. Youll see the needle bounce around. All pulses should be even. For the factory compression recommendations see your free FSM at www.fc3s.org. I believe Mazda says 85psi is bare minimum. You can drive it when its lower. But when engines blow up its going to be alot more expensive to rebuild it.

 

yeah you should be trying for 90+ i'd say. 80 is starting to get a little low.
but keep in mind your rotors wont be spinning as fast as they would while the engine is running so a bit of the compression will slip through the cracks and have a little more time to get away before your pressure gauge sees it.

 

couldnt you technically pull one of the trailing plugs and stick your pressure gauge in the hole and start the car? you can run the caron justthe leading plugs, so i dont see why you couldnt run it on 2 leadings and one trailing as long as you dont use a meltable pressure hose... i mean there would be alot of heat, but if you used a steel hose or something wouldnt it be possible? can anyone answer that?

 

when i had a compression test done i got 115 and 120psi with less than 5psi varience between each face

 

are you on a rebuild cracky? new housings?

 

Umm, thats not the proper way to do a compression check. You only need to crank for 5-10 secs. Plus you need to crank slow enough to see variances in rotor faces.

Jarrett

 

me??

if you were asking me,

Nope, original engine at 156K

 

yup..I was asking you. Thats pretty impressive.

 

i know its not the correct way to do it, i was just asking if it could be done... RAT

 

And I answered!

Sorry!

Jarrett

 

Jacob ... you dont want the car running during a compression test. I dont think the rubber hose on a comp gauge would like the combustion temps or pressure.