Idles good, but after revving, drops below idle & stalls

[MattZig]

My 93 MX3 has an annoying problem right now. After warming up, it'll idle around 750 RPM (in between 500 & 1000) just fine, but if I rev it up and let it drop down, it'll fall below idle and struggle to stay on, sometimes stalling. I've taken the TB off and apart and sprayed the crap out of it with brake & carb cleaner. That seemed to make the idle more stable but it's still dropping. The vacuum is a solid 17 inHG. Spark plugs were recently changed, along with rotor/cap. Spark plug wires are all within spec.

Does this sound like a sensor problem? No CEL from it, but it is OBDI so no surprise there.

[Trevor]

Adjust your TPS that should take care of that problem

[Ryan]

Try another airflow meter if you have one handy...

When you rev it up for a sec, even just tapping the pedal the vane jumps a LOT. If it gets stuck, or the spring inside sucks, or the contacts are wearing out, it will hold that air signal and flood the crap out of the engine for a moment until it sorts itself out again.

Try rapidly pumping the brakes at idle, too. See what that does.

[MattZig]

TPS seems to be adjusted properly. I'll try fiddling with the airflow meter, that sounds like it could be the problem. I can always grab one from pick n pull for 10 bucks.

[OROutdoors]

Have you checked the timing during this operation? My thought, just a suggestion, at idle, the vacuum advance sets advance. When you open throttle, the vacuum is equalized and the advance relies on mechanical advance. If the vacuum advance doesn't recover correctly -- when the throttle closes, and the vacuum re-establishes in the manfold, and the RPMs drop --then there may be no timing advance. The results may be what you are describing. Checking the timing with a timing light would show this.

[MattZig]

Ok the rabbit hole goes deeper than I thought.

On the clutch pedal there are two switches. One is a safety to start the engine, the other is to shut cruise control off when depressed. Simple right? Not so fast.

When I push the clutch pedal in and both plugs are connected, the idle drops from 1000 RPM to 650. If I hold the clutch in and rev it, the revs go up, drop and settle nice and perfectly at 650. Works great, you can see the revs smoothen out at 1000 and slowly fall to idle, no problem.

Now if I take my foot off the clutch pedal in neutral, the idle shoots up to 1000, and if I rev it, the revs drop way below even 650 and the engine almost and sometimes stalls.

What gives? Why does the cruise control clutch switch affect idle? What's going on?

[MattZig]

99% sure it's my netural switch. One guy said this:

You have a switch on the clutch pedal and a neutral switch on the transmission. If either switch is activated the engine computer controls the idle by modulating the idle speed control valve. With the clutch switch broken when you put it in gear the ECU thinks you're going down the road and doesn't need to control the idle.

In summary:
Idle behaves in neutral, misbehaves in gear -- clutch switch bad.
Likewise, idle behaves clutch in in neutral, misbehaves clutch out in neutral -- neutral switch bad.

I went out and tested my neutral switch - stays open even when in neutral. Definitely faulty, and makes complete sense!

[Ryan]

Makes sense to me too.

If you look through the engine relationship chart, clutch switch and neutral switch do affect idle. All part of Mazda's desire for that peppy feelin'

But a pain to diagnose

[Daninski]

Good thread, I just had the same issue. Coming to a stop and releasing the gas pedal the engine would stall sometimes. After about 3 weeks of this my fuel pump went. Repleaced the pump and the car runs fine right now but I'll give that switch a check to be sure. Additionally, when I would hold the idle at 1600 the engine would cut out momentarily, idle would drop for a second then it would rev up to 1600rpm stall idle drop then back up to 1600rpm and it would do this until I increased or decreased the rpms away from 1600. With my fuel pump replaced it doesn't do that any more. Weird.