ludeowner

Hi guys,

Pretty soon I will be shopping around for an FD. I understand that it is a good idea to get a compression test done. Are there portable testers that I can get and bring with me?

Also is there like an "Idiots guide to using compression testers" or something? I've never had to use one of these, so I dont quite know how to work them. If someone can give be some quick instructions, I'd appreciate it.

TIA

DomFD3S

Just go out and get a regular compression gauge. You can find them at any automotive store like Pep Boys, Kragen, AutoZone, etc. Sears would also be a good place to look for one.

Just make sure you get a standard compression gauge w/ a hose and different fittings. Don't get the "cone" type compression gauge. These "cone type" compression gauges basically have a "cone" shaped rubber bushing on the gauge. You'll see the difference.

Some come w/ a carry case (Snap On, MAC Tools, and Matco come w/ one). You can easily keep one in your glovebox or in the trunk.

ludeowner

I thought that rotaries needed special compression testers. Or maybe i'm thinking that rotaries will give different readins or that they are read differently.

And sorry for my Noobishness, but how do you use them?


thanks

maxpesce

To PROPERLY check the compression on a Rotary you do need the MAZDA tester - It records each face of the rotor independantly - However you can use a Regular tester - Hold the pressure release open as you are testing - Each time a rotor face passes the plug hole, the guage needle will bounce. You won't get accurate pressure readings BUT you will be able to tell if all the faces are even, which is the most important thing - Two WEAK bounces & One STRONG indicates a Broken Apex or Corner seal. If the Bounces are all even the Seals are either all good or equally bad (or 2 could be bad & 1 good - still BAD). Next test again with the pressure release closed and check the max compression, 90+ psi is good, 75-90 is ok but getting tired, less than 70 the motor is on the fast path to a rebuild.
Repeat on Both Rotors.

ludeowner

thanks for the instructions on how to read the gauges, but where exactly does the tester "plug" into?

TIA

DaedelGT

Spark Plug holes.

ludeowner

Ok so I did some searching on the net and I managed to find some instructions.

Reading is both altitude and RPM dependent. You can remove the schrader valve from a standard tester to test compression yourself. Readings come very fast so you must have a quick eye.

To perform the test:

-Check that battery is fully charged.

-Warm up engine to normal operating temperature, then stop it.

-Allow 10 minutes for the exhaust manifold to cool.

-Remove the front and rear trailing side spark plugs.

-Disconnect the circut-opening relay. This is the relay inside the engine compartment near the left front strut. The relay cover should be labeled. I believe mine is green.

-Disconnect the igintor connector. The ignitor is a small (2.5" x 3.5" x 0.75") silver box with cooling fins located on the left fender near the engine firewall below the cruise control cable mount bracket. Dimensions are approximate but this should get you close. When I do this test, I drive the car to operating temp, then perform 4, 5, and 6 during the cool down period.

-Fully depress the accelerator pedal and crank the engine for 5 to 10 seconds. Note readings.

So is that pretty much it? for the last step, do I just step on the accelerator, or do I have to step on the clutch too?

I wish I had taken auto shop in HS..

DomFD3S

Set the car in neutral and just rev the car. There is no need to step on the clutch.

BLKTOPTRVL

What I remember from auto shop (many many moons ago) is that a car does not 'run' let alone 'rev' during a compression test. It is just the starter cranking that drives the guage.

Have things changed that much?

shred

If you are compression testing a car, like the email that was posted

This will keep the engine from starting. Since you are removing the trailing and leading plugs, you wouldn't want it to try to start anyhow.

Not so sure that this is the best test for this. Plugs are not the easiest thing to take off on these if you don't have pratice, not to mention that this isn't your car and you are going to start ripping into it (you have to in order to get at the plugs from the top, unless you got really small hands). Personally I would check the manifold vacume at idle. Much better way to identify bad seals. You just get a vacume gauge from Sears that goes up to 20lbs, take the nipple off the far side of the upper manifold, plug it up and there you go!!

Check the vacume at idle, this will tell you tons (especially if it is stock). When the rotor pulls air into the chamber, the seals have to be working right as the rotor wobbles back to suck in air (this makes your manifold vacume). You will see that it isn't if the manifold vacume is low (around 7-10lbs, I am sure that the sticky newby link has the correct vacume levels for a good and poor performing engine Link ) right away. Unless there has been some serious porting work done, low manifold vacume is a key indicator of needing a rebuild.

The only difference on the compression tester is you will be measuring the apex seals under pressure when the rotor wobbles to create pressure.

Both numbers are good to know but (and some of you old hats at this can correct me if I'm really off base here) you will almost always see low manifold vacume on engines needing rebuild because of bad apex seals.

yy4u

when mazda hooks up its compression tester does it take out the trailing plugs as well?

The reason i ask is that i'm planning to have a compression test done but i also need to change out the plugs.

Can i do it myself? Seems like some of the posts ahve been contradicting. Seems to me if you are removing plugs you wouldn't not want to start the engine, but at the same time how would you get measurements for each rotor face unless the car is running and the eccentric shaft is turning?
THanks for any info!

-Rob