22 replies to this topic

[Randy Thieme]

The patient is an SM built from a '93 California model (different ECU and injector harness). Had the AFM tuned a while back. Immediately after installing the tuned AFM the car would not idle below 1600-1700 RPM. Idle adjustment screw is turned all the way in (CW rotation). As soon as I turn it out (CCW) the idle speed picks up. Have heard a fast idle is common with tuned AFMs so ignored it. But when last dyno'd it ran slightly lean. It almost never hunts except when first started and very cool (<<50 F). Usually does not stall either except, again, when first started and very cool.

Tried looking for a vacuum leak with no luck yet. Tried spraying starter fluid around the intack manifold and throttle body to no effect. Detached the hose to the brake booster and plugged it with a metal rod to no effect. Did not hear any obvious vacuum leak either. Purchased a used AFM from Craig's List. The AFM appears to be unchanged (the seal is intact) and the seller was not a racer. Installing what seems to be a pristine, un-tuned AFM made no difference in idle speed!? Checked the MazdaSpeed website and it does not show a different AFM part number for the '93 California model so I have no reason to think any AFM would not work. The Mazda parts diagrams show the bottom of the throttle body is an idle air valve (part # B61P-20-660). Can those things go bad?

First A Sanity Check: The air filter is basically one big vacuum leak. So for a vacuum leak to affect idle speed it must be downstream from the throttle butterfly. Correct?

I admit I have not gone through the diagnosis steps in the shop manual yet and I need to do that. But before hand has anyone experienced this? If so can I learn from your experience on where to look next please?

Have lived with it for a while since I heard this happens with tuned AFMs. But it's a problem now since the car won't tune out completely. Plus it's hard on the engine starting cold after not running a few weeks and it immediately wants to idle at 2000 RPM without oil pressure built up. I've been pulling the injector relay so I can turn it over to get oil pressure first but that's getting tiresome. In frustration I'm about to install a toggle switch to control the injector power from the cockpit although that would probably be an illegal modification (harness modification). (What happened to the banging-your-head-against-a-wall emoticon?)

[Mark]

Randy,

Try bypassing the neutral switch that is located near the top of the trans with a jumper. The connectors are high up on the right side of the trans IIRC. I had a car that was doing this and it killed me as I worked for hours trying all kinds of things to bring the idle down. Bypassed the switch with a jumper and idle dropped right down. Seems they go bad once in a while. In a discussion with Louis at Rush I mentioned it and he says 'oh yeah... '. Woila my problem solved. Not an intuitive solution for sure.

Mark

[Randy Thieme]

Randy,

Try bypassing the neutral switch that is located near the top of the trans with a jumper....

Mark

Thanks Mark I will try that.

[davew]

Check the throttle cable, is it taught, thus holding the throttle slightly open?

Check the dash pot(little vaccuum thing on the throttle linkage), is it binding, not releasing, again holds the throttle open slightly

Check the throttle stop screw. This is located on the throttle body and is the absolute minumum idle speed. Uses a 7mm nut and tiny allen wrench to adjust.

Check the throttle "wheel" Is it bent? Happens often in side impacts or when a motor mount is broken.

Dave

[Randy Thieme]

Check the throttle cable, is it taught, thus holding the throttle slightly open?

Check the dash pot(little vaccuum thing on the throttle linkage), is it binding, not releasing, again holds the throttle open slightly

Check the throttle stop screw. This is located on the throttle body and is the absolute minumum idle speed. Uses a 7mm nut and tiny allen wrench to adjust.

Check the throttle "wheel" Is it bent? Happens often in side impacts or when a motor mount is broken.

Dave

Checked the throttle cable and it's loose. Wheel is not bent either. I will need to check the throttle stop screw. Not sure what the dash-pot is. That's not 13-W80 in the drawing is it? If it is then it's not binding. There's a gap between it and the small adjusting screw below it.

[davew]

Correct, 13-w80 is the dash pot.

The idle stop screw is not pictured, but points straight back near the lower out board mountong bolt for the throttle body. Hard to see, hard to adjust

Dave

[Randy Thieme]

Thanks Dave. Have class tonight so might not be able to check your's and Mark's tips until tomorrow. Will post the results when I do.

[Funracer]

Randy,

Try bypassing the neutral switch that is located near the top of the trans with a jumper. The connectors are high up on the right side of the trans IIRC. I had a car that was doing this and it killed me as I worked for hours trying all kinds of things to bring the idle down. Bypassed the switch with a jumper and idle dropped right down. Seems they go bad once in a while. In a discussion with Louis at Rush I mentioned it and he says 'oh yeah... '. Woila my problem solved. Not an intuitive solution for sure.

Mark

Correct me if I am wrong, but won't this also alow the car to start in gear?
If so something to keep in mind.

[Mark]

Nope. Car will start just fine. I have run into two cars now that needed this switch bypassed to bring down the idle.

Mark

[davew]

The switch refered to is the "car is in neutral" switch on the transmission (2 long wires with square spade connectors). Not to be confused with a "neutral safety switch" used on automatic transmission to prevent starting in gear. Or the "clutch safety switch" that is on the clutch pedal that prevents starting with the clutch pedal up. Or the "backup light switch" that is also on the trans (two short wires with bullett style connectors).

The car in neutral switch will raise the idle slightly, via the ecu, when the car is in neutral. Push the clutch pedal down and move the shifter from neutral to first gear and there is a slight increase in rpm. Maybe 150 rpm.

dave

[Cy Peake]

Or the "clutch safety switch" that is on the clutch pedal that prevents starting with the clutch pedal up.

So far the best modification that I've paid for that's in my car. It allows me to lean in a window and start the car. I always shut down in neutral. I also like to think that it prevents premature wear to the TOB.

[William Keeling]

you can check the IACV by warming up the engine and then disconnecting the plug -- you should hear a click and the idle should jump up by 700 or 800 RPM. if this does not happen I would go thru the FSM check for IAC.

[Randy Thieme]

Yes, check the IAC too.

Did not get home until 10 PM so won't check work on it until tonight. Did a search on www.miata.net and all kind's of things could cause a fast idle to one degree or another. Makes me think, back in college, my old MG Midget with the dual Solex's wasn't so bad after all.

[Randy Thieme]

Not fixed yet but closer. Checked the nipple in the back of the intake manifold and it's there. Did another audio check using a hose with one end in my ear and poking the other end around the manifold and TB. Did not find anything. Intentionally created an external leak as a reference point by removing the small nipple on top the manifold near the TB. Doing so kicked the idle up another 300 rpm or so and the leak had a definite whistle. Having never used starter fluid to find a leak before tried baselining that as well by "finding" the known leak. Spraying it actually had very little effect on RPM. Based on my reference point have decided if it was an external leak anything big enough to double the idle speed would be unmistakable. Also decided the starter fluid trick is not that great especially considering the fire hazard.

Tried bypassing the neutral switch which had very little effect. Tried turning out the throttle plate stop screw at the bottom of the TB. That had a definite impact. Now idles around 1200 instead of 1700. I could live with that but since that screw was never touched I'm not sure it's not just compensating for something else being broken.

Removed the IACV and stuck it in the freezer for a while. Noted the position of the internal springs and widgets. Tried blowing through one end and it was definitely open. Stuck the end with the hose nipples into a pot of boiling water and watched the internal springs and widgets move around. Tried blowing through it to see if it was now closed completely. Does everyone else know aluminum conducts heat really well? After realizing that spending years of eating hot sauce has paid off afterall I found I could still blow through the IACV. Don't have a baseline if it should be completely closed or not, and if so, at what temp. Hopefully the FSM has some way to evaluate the IACV.

It's either an internal leak involving the IACV, a misbehaving ECU, or combination of out-of-whack TPS and butterfly stop. Pretty confident I can narrow it down this evening and, if needed, get some parts ordered in time for next weekend's race. Thanks for the help everyone.

[Keith Novak]

Dumb question Randy, but a mistake I made before...how are you measuring the RPMs? If it's your new AIM or a timing light, you're accounting for the extra spark right?

[Randy Thieme]

Dumb question Randy, but a mistake I made before...how are you measuring the RPMs? If it's your new AIM or a timing light, you're accounting for the extra spark right?

Factory tach. But I know what you're referring to. Did have to go in and ajdust the multiplier in the AIM settings to match a four-banger.

[William Keeling]

I was talking about 20-660 in the above diagram

[Randy Thieme]

I was talking about 20-660 in the above diagram

You're right. My mistake.

[Keith Novak]

Factory tach. But I know what you're referring to. Did have to go in and ajdust the multiplier in the AIM settings to match a four-banger.

Had to ask the obvious since your RPMs seemed about double normal and it would fix a quick facepalm if that was it.

[FTodaro]

Randy did you ever find the answer to this question.?

ever since I advanced my timing and adjusted my fuel pressure I notice my idle is about 1400. I tried to back out the idol adjuster with the nut on it but its all the way out. could not find or hear a Vacuum leak. the cable is free. It seams to be correlated to when I advanced the timing. Any further thoughts.