Well, is it time for a rebuild? PLEASE READ
 

Motor was built by rotary reliability and racing 25k ago. New housing, rotors...the works. The prev owner paid 6k for it.

I can't find any vac leaks for starters...after I drive the car and park it, you can hear what sounds like boiling/bubbling coolant in both the AST and the Filler neck/thermo housing. Coolant pisses out the overflow container for a short time and creates a puddle. BTW, I have a sprung cap on both the AST and the filler neck. I know I need an unsprung on the filler neck...could this car the problem? Thermostat is a new OEM one. Car seems to drive and boost well with the exception of a 3k hesitation. Idle is complete crap. TPS is adjusted correctly. At operating temp, motor wants to idle now around 650-750...but VERY VERY ROUGH. Throttle position screw on front of TB is backed all the way out so butterflies are closed. I have adjusted the air idle screw, but not getting any better. On idle, if I hold revs at about 1500, it will intermitantly drop to 1100, then on its own will return to 1500, then back down to 1100. 2 plugs at the front rotor were black and had a residue on the surfaces. The rear rotor's plugs were dry. Seemed like oil to me. Not soaked or dripping..just a slight filmy residue.

Anways...what do you all think? I have been building supras for about 6 years...my first rotary however. I know that rebuilding a rotary is much simpler than in I6.

Could the boiling coolant be a coolant seal? I don't have the proper tool for compression testing. My boost gauge broke so I have a nwe one coming so i don't know the vacuum.

Trev

Get a compression test done.

I work for a dealership that owns a mazda dealership and I will see if I can borrow the compression tool.

Trev

One of your coolant seals may have given out.

If your car is exhibiting symptoms similar to blown head gasket on a boinger, this is normally the case. Rebuild time!

Instead of getting all worried, to be absolutely sure, go to a garage that has emmissions testing equipment. Get a quick test done for hydrocarbons in your cooling system by placing the sensor / hose to sensor of the testing equipment into the filler neck. Any trace hydrocarbons = combustion gasses getting past the coolant seals into your cooling system and pressurising it out he overflow tank.

Coolant may also be getting into the combustion chamber in this case, accounting for your poor idle. Your compression may not be affected at all (at this stage) in either case.

Don't mess with the idle screw adjustment unless you are certain adjustment is required, there are many other factors which affect idle. Because you don't have a boost gauge it is difficult to diagnose things properly.

I think it is not fair to say that building a rotary is much simpler than anything else. If it was, engines wouldn't blow at 25k miles.

Before jumping to conclusion you do some troubleshooting...

Except for idling issue, last month my car displayed similar cooling issue as you. AST was gurling, coolant pouring from overflow tank.

Turn out I had a bad alum ast, not sealing at the cap.

Test your by removing the rubber overflow tube at the AST, fill the filler neck, coolant should not leak out of the AST at the overflowing nipple.

Good Luck.

Could this be a problem...I have a .9 bar sprung cap on the filler neck...is it supposed to be unsprung?

Trev

The cap on the filler neck is typically a flat cap , but having a thermostatic cap on it will not make any difference.

Another easy check for a bad coolant seal is to pull the 2 leading plugs and rotated the engine backward by hand. If coolant comes out of either plug hole, then you have a bad coolant seal. this should be done after bringing the car up to normal operating temp, then allowing to cool over night.

got it....thanks for the tip