GSL-SE idle drop with AC on, car in gear
 

I'm having a very weird issue right now where my idle drops low and bounces back up repeatedly when the AC is on and the car is in gear with the clutch pushed in. This is especially noticeable at stop lights, as the RPMs drop a ton when I put the car in first. I don't see any obvious connection between the AC being on and the car being in gear, so I don't know what causing the problem. It's not when I push the clutch in – it's when I put the car in gear, with the clutch still held in.

I actually recently replaced the clutch in part to get rid of issues with the clutch not engaging/disengaging properly which caused the car to stumble like this. I'm surprised I still have this problem after a new clutch. What could cause this?

My SE does the same thing. I end up only using the AC on the freeway because at stop lights the RPM drops so low it sometimes stalls. I've been told it's probably the Air Control Valve, but I replace it with another used one, no change. New ones are no longer available.

Search on "SE Idle Surge + LongDuck" and you'll get a number of hits. Short version, it's a combination of your Idle Air Control Valve which needs to be cleaned out and the steel throttle body rods galling in the aluminum throttle body, causing a feedback loop in the bypass air circuit.

Thank you LongDuck. I actually had a number of idle issues when I bought the car and pieced together 3 separate throttle bodies to get one that didn't bind and worked correctly. I'll do more searching and look into cleaning the Idle Air Control Vale (is that on the driver's side of the plenum?)

I'm still at a loss why this only happens with AC on, when the car is in gear. Is there some link between the transmission being in gear and the throttle body that I'm not aware of?

The extra load from the tranny in gear and the comrpessor means the idle needs to
bump up a bit. Theres usually a vacuum actuated relay that does this in some way.
Not sure on an SE what that would be or where to find it.

Welcome :)

Yep, drivers side of the RE-EGI chamber is the Bypass Air Control Valve, and is electrically switched with the A/C compressor to allow a slight bump in idle speed when the A/C comes on. This is actually a 2-part valve and solenoid, but cleaning out the carbon may or may not fix it. As someone else pointed out, these are long since NLA through Mazda channels, so cleaning is your best nly option.

A workaround is to adjust your idle speed to 1k RPM with no load, which gives you a margin when the compressor starts up. A sticking throttle body rod will cause the feedback loop and surging at all idle conditions, too.

I cleaned the BAV about a year ago. I'll do it again, as well as clean out the throttle body and re-adjust the idle and see what happens. The car seems to idle a lot smoother around 800, so I'd prefer not to bump up the idle too far if I can help it.

Thanks for the help. I'll give this stuff a shot this weekend

Where is the idle speed screw located? (if one wanted to try setting the idle higher as a temporary solution)

It's hidden on the right side of the throttle body and part of the cast iron block which is darker in color than the aluminum throttle body as you're looking in from the passenger side. The factory idle speed adjuster is mechanical (not bypass airflow) and pushes against a stop on the primary butterfly rod. Turning clockwise opens the throttles slightly, increasing idle RPM. Expect that there's also a stop nut to hold the setting, but I've found that it's not required to retain your setting. Most I've seen have removed the factory skinny "all-thread" and replaced it with a longer bolt with a wider head for easier adjustment with standard size screwdrivers.

The FSM method to play with the flat-blade air adjustment valve on the top of the throttle body is marginally effective and usually exacerbates the BACV feedback loop, requiring more tinkering than most want to put in.

You're talking about this tiny screw?

Attachment 610372

That's it!

It won't budge. What is the size of that locknut? I don't seem to have anything that fits it and boy is it hard to get to. I really think Mazda didn't want us to mess with that thing LOL

It's the ever-popular 7mm.

It's worth the hassle, trust me. As stated, remove the lock nut and get a nice, long bolt that you can reach easily from the top and thread that sucker in there.

Thanks for the picture, you win the internet for a day!