MX-3.com

i dont really think this is a problem but i'm curious as to why in the morning when i start my car the car doesn't idle high.. it idles around 900 rpm until it warms up then its around 650..

of course i drive around under 3000 rpm and very carefully until it warms up.. and really carefully when it's cold.. i dont let it sit to warm up longer than about 3 mins in the morning because i know it's not really going to warm up that much sitting there at around 900 rpms (i do let it warm up because the transmission shifts quite hard the first few shifts when it's cold - anyone else with an MTX have this problem?) although whenever i clutch in and put it in neutral the revs would shoot to about 1300ishRPM for a two seconds or so and then drop down to about 900 rpm again for the idle when cold..

Your IAC plug probably came disconnected. Does your car try to stall when you turn on your air conditioning?

i got no air conditioning, where's the IAC plug?

I believe there are two controllers concerned in this; the Mazda literature is very poor on this point but it mentions the BAC valve and the ISC valve, both in the lower part of the Throttle Body. ISC= Idle Speed Control, controlled by the ECU. BAC I take to mean Bypass Air Control. I believe the BAC is a thermometric device, which is open when cold, and gradually closes as the coolant heats up (This is why removing the coolant supply to the TB is not a clever idea).

Now it may be that your car has the TB coolant supply removed -I think that would account for the low idle speed when cold - you might think it would be high all the time, but the air bypass adjuster has probably been moved to counteract the effect of the BAC being open when the engine is warm - that would result in too little air when cold.

Another possibility is that the BAC valve is stuck.

thanks, you went in great detail IanL , i dont mind my car not idling high when it's cold so i'm leaving it like that but i'll keep in mind what u have said if I ever decide to let it idle high when it's cold