Testing your own engine compression
 

We all know that compression is a vital statistic to understanding how well our engine is operating. Is there a way or device that can let us check this ourselves? I would think a compression guage, like a boost or oil pressure gauge would be useful for our cars. Anyone have any experience with this? Given that early detection of low compression could help save us from some expensive repairs, I would think that someone would have created a device for owners.

yup. the trick is being able to spot the 3 spikes on the gauge while you turn it over... i guess 120 or 130 psi would be good compression for such a new motor.

Good guess wakeech. :)

The compression pressure for the 13B-MSP is 123 psi.

Should be a piece of cake. The compresion tester I have holds the presure from the spikes until you want to release it. I usually let it hit about 4 times on piston engines. You should be able to find a gage for this at your local auto parts store. They come with a flexible tube that you screw into the spark plug hole.

Which is fine for a piston engine where each spark plug hole serves only one combustion chamber, but the rotary has three chambers per housing, and a gauge that holds the pressure between pulses will only show you the compression of the best of the three. Factory technicians use a fancy graphing gauge, but the home mechanic can make do with a regular dial gauge as long as you remove the non-return valve that holds the pressure between pulses.

The trick is to watch the dial as the engine is cranked, looking for variations between the pulses. If all pulses are about the same, all is well; if every third pulse is lower, you have a bad side seal; if every third pulse is higher, then you have a bad apex seal.

Do you need anything special to read off both removed spark plugs? per roter. Do you need to disconnect any parts of the ignition system other than removal of the spark plugs?

Remove only the leading plug, and use the guage in that hole. (But don't forget to disconnect the plug wire from the trailing plug!)

REMOVE THE ECCENTRIC SHAFT SENSOR CONNECTOR BEFORE DOING A COMPRESSION TEST!!!!!!

This is easier to do from under the vehicle. If you just disconnect the coil, you still have fuel being injected, and risk flooding the engine. Do NOT disconnect the EGI relay either, the electronic throttle will go into failsafe.

You are better off with a rotary compression tester though. Remember, you are not just looking for low compression, but for a variance in compression between chambers. The specifications only allow 21 psi difference between chambers on a single rotor, and a maximum of 14 psi between the lowest chamber on one rotor, and the highest chamber on the other rotor (compression difference between rotors). Maybe it is possible to record the values of all six chambers with a normal dial type compression tester, I have never tried it, but it sounds quite difficult.

Oops, still thinking like a piston. Thanks for the correction. This sight is great for learning!

Anyone know where to get a rotary compression tester that will register all three points. Vendor? Web Site?

THX Mr M I will look into how to disable the fuel system to keep from flooding the car. Have you done this with a Renesis?

Mazda sells a special compression tester just for rotary engines..... dunno how much it costs though, lol

It costs mucho $$$. Nothing close to a regular piston engine compression guage.