So I'm hoping someone can make some sense of this. I've had my FD3S up for sale and a few people have asked for compression #s. So I purchased this kit (http://amzn.com/B000U58B56) and tested today. Every rotor face is showing less than 25 lbs psi.

If that was correct, my engine should not even run, right?

However my car runs very strong, idles smoothly (cold or hot) at 900-1000 rpm, with or without load (a/c, electrical). My vacuum at idle is -18/-17 inches.

Bad gauge? I'm doing it wrong?

 

I know that sometimes those compression testers remain on the lest measured pressure. Because when used on a regular piston engine you're always testing the same chamber at once, where as in the rotary you are testing "three chambers" through the same spark plug hole. There is usually a little pressure release button that will reset the needle to zero. When using it on a rotary you should keep the pressure release button pressed while revving so it resets every time a new rotor side passes, and you look for fluctuations in the measurements.

But I know some people don't even bother with the pressure testers, and just use the old finger-in-the-hole method. If you finger blows out of the hole...you're good.

I would assume the compression could be also be low because of a bad alternator or starter...not necessarily a bad seal.

You did buy a cheap tester...so I wouldn't rely on it, especially when you say your engine runs fine

 

If you just hold the release valve on the side, you won't get good numbers. You have to remove the schrader valve from the end of the tester, it's like the core in a tire valve stem.

Also, make sure the throttle is wide open.

Really, if you have good vacuum (which you do) and 3 even bounces on each rotor face you're good. Seeing how clean your car is I doubt you're trying to sell some blown engine car.

Dale

 

I guess I'm just surprised that some people are saying no compression #s no sale, so I'm trying to get them to sell the car. Now it looks like the engine is trash reading these #s.

 

Cold Start Video from this morning (so I could warm it up for compression test)

 

I take it nowhere close by has a Mazda rotary compression tester?

The funny thing is you can have amazing compression and the moment a coolant seal goes bad you're looking at a rebuild

 

Great info, thanks Dale!

 

Looks like your idle voltage is at 12V? Might wanna make sure your battery is getting enough charge.

 

Yeah, I think my alternator is toast. New one is on the way so new owner doesn't have a fuss.

 

Yeah you really need a mazda comp tester to be truly accurate. Alot of people just go off the throb at idle. When my comp went bad in one chamber it sounded like the car had bridge port port and the whole car would have noticeable vibrations through it.