MX-3.com

It only onsets when the car is cold and for the longest time i figured it was just a stuck open injector. Anyways while the car was running i disconnected the BAC and the surging stopped. Ordered the last one of eBay (used) but don't really trust the condition. I don't really have any other options. I should mention when i performed a resistance test it was only 1 ohm out of spec (above).

The purpose of the Bypass Air Control Valve is The idle speed control system regulates the amount of bypass air entering the engine to provide for quick warm-up and to maintain idle stability and good driveability. Operation is The thermowax type air valve is open when cold to allow extra bypass air into the air intake system. This raises idle speed to shorten warm-up time and ensure good driveability. As the thermowax element is heated by engine coolant, the wax expands, gradually closing the air valve. After the engine is warm the powertrain control module engine (PCME) regulates idle speed with the idle air control valve. The Construction is The BAC valve consists of an air valve that functions only when the engine is cold, and a PCME regulated idle air control valve that operates throughout the engine operation range. Sounds like its doing its job as to opening up, but I would assume that its not closing as it gets to temp, hopefully when you get the new (old) one it will work the way its supposed too

mazdaspeed23 wrote:The purpose of the Bypass Air Control Valve is The idle speed control system regulates the amount of bypass air entering the engine to provide for quick warm-up and to maintain idle stability and good driveability. Operation is The thermowax type air valve is open when cold to allow extra bypass air into the air intake system. This raises idle speed to shorten warm-up time and ensure good driveability. As the thermowax element is heated by engine coolant, the wax expands, gradually closing the air valve. After the engine is warm the powertrain control module engine (PCME) regulates idle speed with the idle air control valve. The Construction is The BAC valve consists of an air valve that functions only when the engine is cold, and a PCME regulated idle air control valve that operates throughout the engine operation range. Sounds like its doing its job as to opening up, but I would assume that its not closing as it gets to temp, hopefully when you get the new (old) one it will work the way its supposed too

You have most of that right, but are miss understanding some of it... The PCM controlled portion is actually only an "idle up" valve... Your idle in its self is regulated by setting your base idle settings threw the throttle body and tps adjustments... Its used for stability and to keep the engine from stalling when extra load is put on the motor, such as the cooling fans kicking on, turning at low speed (power steering load), AC clutch turning on, having your lights and lots of electrical load from the interior, ect on...

You can disconnect the PCM control portion and leave it disconnected, The car will idle fine so long as the base idle settings are set correctly... Just when the cooling fan kicks on at idle the car might stall, or else you'll notice a drop in idle rpms....

Before replacing the BAC valve its good to verify no vacuum leaks, and coolant levels...

Yea but if im not mistaken he has already bought the BAC, so I would just replace it and check for Vac leaks, if your unsure if its a vac leak, get some brake cleaner and spray around the TB and other vac hoses, if idle changes when sprayed then its a leak usually. fix the leak and see if it helps with the idle

Did replacing the IAC resolve the issue? I have a similar issue.

Try using a gasket sealant on the rubber gasket between the TB and IAC, I had the same problem. I read somewhere that since it's a squish gasket it could leak if you take them apart. Worked for me, anyway.

I like that, "squish gasket".

Lol.