Super low idle
 

Helping a friend trying to diag this issue with his car, and have reached my wits end. Car is an S4 NA. He idles around 300-400 when hot or cold. Listed below is what all we've tested, what we haven't tested, and what I believe are possibilities for the root of the problem. If anyone thinks it's something else i don't mention, please feel free to chime in!

What was tested:
-BACThis was the first thing we tested. Clicks when 12v is applied. Connector side gets 5v with ignition on
-TPSThis was next up. Set the TPS using the voltage method. 1k ohm at rest, ~4.5k ohm at WOT.

-Variable resistor and idle adjust screwTried adjusting idle via FSM with the variable resistor, and air bleed screw on top of the throttle body. With both wide open, idle still won't go above 400

-SparkPlugs and wires are fairly new. Used known good wires from my car. Idle still low.

-AFMSwapped in a known good unit, idle still low


What was NOT checked:
-Primary injectorsShouldn't be clogged, but can't rule that out
-TimingNot sure how this could be the issue, as timing was set last time he had the CAS out. We can't really jump timing, so it'd be odd of this was out of spec
​​​​​​-CompressionTo clarify, we don't have compression numbers from a tester. But, with spark plugs removed, you can hear 3 distinct pulses. No dead spit's


What I think could be the issue:-BACWe did test to make sure it clicked, but did not check resistance across the pins

-AFMLess worried about the unit itself, but more so the wiring. FSM doesn't give much info on what voltages and resistances to expect on the harness side of the connector. Just gives spec for the AFM itself
-FuelI mean this as a broad term, because it could be a number of things. Clogged injectors, dirty filter, fuel sock, etc.

-Electrical GremlinCan't rule this one out. It is an RX7 we're talking about. May be a short somewhere in the harness or something.




That is the extent of where I've gotten with this. Any thoughts/advice are much appreciated!

Definitley double check the timing just to eliminate a variable.

Assuming you have already checked for vacuum leaks, adjust the primary throttle blade hard stop to desired idle, then adjust the TPS.

Will do that. Gotta re-stab the CAS and get a timing light on it
wouldn't a vaccum leak make the idle higher?

Also, by the primary throttle hard stop, you mean the one right above the TPS, with the lock nut on it that Mazda says not to touch?

Vacuum leaks can cause all sorts of weird issues.

DOes Mazda say not to adjust that throttle stop? Honestly its been a while since I've worked on a FC.

Have you adjusted the idle mixture as per the FSM?

EDIT: I see that you did that. Did you jump the connector?

Interesting. I always thought it could only cause high idle. Will check for leaks then.

Maybe I'm wrong...I could have sworn the FSM said not to adjust it, and that the Mazda dealership should put a sealant on the screw, so it couldn't be tampered with. But looking back at it, this is all it says. Just an idjustment procedure. Maybe I'm not looking in the right part of the FSM:

EDIT: yes, the connector near the leading coil was jumped. The air bleed screw is all the way open, and idle mixture turned all the way clockwise. Still sits at 300 rpm.

https://cimg0.ibsrv.net/gimg/www.rx7...fa887232a1.jpg

I would honestly throw a timing light on there first. May just have to turn the CAS a bit.

As other suggested, definitely check timing and report back.

When I restab my CAS and start, my idle is always low until I hit it with a timing light and line things up. Once marks are lined up, idle is good.

Roger Roger. Will give that a try and report back. Appreciate the insight guys!