is anyone good enough to answer this one?


Old 03-17-05, 04:05 PM
Thread Starter
Join Date: Sep 2004
Posts: 238
is anyone good enough to answer this one?

I asked a similar question earlier but no one could help.

The car: ’93 Mazda Protégé, 5 spd, 1.8l sohc w/ 91K+

The Problem: Car stalls under the following conditions only
1. AC must be on
2. Air temp must be above 70
3. Car is at normal temp (after 10 min. of driving)
4. Throttle is quickly closed at RPM >2500
5. Clutch pedal is completely depressed
6. Car is in gear

This happens while driving when approaching as stop sign for example. I step on the clutch while simultaneously releasing the gas pedal. The rpms drop quickly to zero. The car will start instantly by letting up on the clutch or using the starter.

This NEVER happens if the AC is off, or the air temp is 70 or less, or the car is still warming-up, or the engine speed to begin w/ is less than 2500 rpm or if I slide the shifter into neutral.

The process can be simulated in the driveway as follows:
With the car warmed up, AC on, clutch pedal on the floor, tranny in any drive gear if I rev the engine above 2500 RPM and quickly let up on the gas the car will immediately die or drop down to about 200 rpm and sputter back to about 750.

With the car in NEUTRAL and clutch in any position the rpm will quickly drop to 1100 plateau and then drop slowly to 750. The whole process while in neutral is as smooth as glass.

The computer gets input from a neutral switch on the tranny. When in neutral, this switch is closed. If I unplug the switch and tape a jumper onto the end of the wire I have no problem and the car drives fine.

On this car the throttle position switch only tells the computer two throttle positions, wide open or fully closed. If I unplug the TPS, my problem goes away. The switch is adjusted correctly and functions fine. I checked with feeler gauges and ohmmeter.

The dashpot seems to function fine and I extended the rod all the way, but that did not help. It does appear to slow the throttle closing.

The engine does get a slight boost when the AC is switched on. It goes from about 700 to 750 per the tach in the dash.

I checked the plugs, the timing and the fuel pressure. The air flow meter checked out ok too.

While checking the timing I noticed that the idle is set as low as it can go but is as per spec. The air bypass screw is turned in all the way.

The idle control valve has a mechanical feature that closes slightly as the coolant flows through and warms up. With the electrical connector unplugged and starting from a cold engine the idle may drop from 1200 to 1100 rpm once the engine is at operating temperature. When I plug in the electrical connector the idle drops back to 700 rpm. I wonder if the mechanical portion is not closing enough. Such a small drop (100 rpm) seems hardly worth having.

Why does the car die in the above conditions?
Why does unplugging the TPS or jumpering the neutral swith so the computer thinks the car is always in neutral or never has a closed throttle help?

Thanks for helping.
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Old 03-18-05, 03:57 AM
Join Date: Apr 2002
Location: Pittsburgh,Pa
Posts: 2,417
Did you test the switch on the trans?Have you checked and rechecked for vacuum leaks.The only reason for the screw to be adjusted all the way in is either a vacuum leak or a problem with the idle air control.Also give the throttle body a good cleaning.
Old 03-18-05, 07:20 AM
Thread Starter
Join Date: Sep 2004
Posts: 238

Thanks for responding.

The tranny switch is fine.

Any idea why the car needs that switch and why it runs fine in neutral or jumpered to appear to be in neutral?

All vacuum hoses and fittings are ok. I sprayed carb cleaner on all visible joints on intake manifold and the idle never changed. I have heard of doing the same thing w/ propane. Should I try it?

Can I verify an intake leak w/ a vacuum gauge? I own a gauge and there is a convenient port downstream of the throttle. If yes, how do I do it?

I forgot to tell you that the car tends to just barely pass the annual idle emissions test. It usually passes at 200 ppm HC with 220 being the highest allowed.

Is there any way to verify the performance of the idle speed control valve other than what I outlined in the original post?

Thanks again for your time.
Old 03-19-05, 04:01 AM
Join Date: Apr 2002
Location: Pittsburgh,Pa
Posts: 2,417
Before we get in too deep have you tried to back the bypass screw out a little and note any change?
Old 03-19-05, 07:19 AM
Thread Starter
Join Date: Sep 2004
Posts: 238

If I back out the screw the speed increases. I have never taken the screw all the way out. Maybe it is the wrong screw or there is a piece of crud at the bottom making the adjustment for me. There was no cap, plug sealant or paint mark on the screw as I would have expected.

Last night I diconnected the idle air control valve electrical connector. With a cold engine the car idled at 1100 rpm. After the cooling fan cyled once the speed dropped to 1000. That is the full range of control with coolant alone flowing through the valve. The valve was hot so coolant was getting to it.

For grins I hooked up a vacuum gauge and got 17"mg with maybe +/- 1/2" pulse at idle and any other speed with the throttle held steady. The vacuum would increase to about 22" as I increased speed an level off again at 17"after a few seconds.

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