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Intermittent stalling

2K views 12 replies 4 participants last post by  Lizzie94 
#1 ·
Hey guys, sorry to be a bother but I'm still trying to figure out my intermittent stall during decel. It seems to happen after a long decel in third or second gear (about 20-30 seconds) with no gas input. Push in the clutch pedal, engine dies. Restarts normally and runs fine otherwise. Sometimes it catches itself before it stalls and surges up before returning to a normal idle. Any ideas? 1990 1.9L CFI/MTX-III transaxle.
 
#4 ·
If this only happens after decelerating, my suspect would be the Idle Air Control Valve or a sticky injector.

With my manual trans 2nd gen Escorts, if you have your foot off the throttle but the engine is spinning faster than about 1400 rpm, the computer doesnt fire the injectors at all. If you then push in on the clutch, the engine seems to recover just fine - as the crankshaft speed drops below somewhere in the 1400 rpm range. And by recovering, I mean the injectors are now being triggered and the Idle Air Control Valve is keeping the rpm at the normal idle speed.
Its been several years since I tried this, (with an l.e.d. wired in to show the injector trigger event), but I dont recall any surge in rpm.
 
#5 ·
The TPS and idle control motor both tested good. There are no vacuum leaks and the plugs, wires, cap and rotor are all new. Distributor was rebuilt last week. I swapped out the injector but that didn't fix it.
 
#6 ·
Not that this will help, but:
I dont know about 1st gen IACV's, but with the 2nd gen kind - I have no way to test them really. It would take a signal that would drive the valve from fully open to fully shut. The one time I connected an oscilloscope across the IACV leads, with the engine idling, I got a more or less square wave signal, about 1 or 2 kilocycles, and about 5 volts peak to peak. The 'duty cycle' of the square wave (the width of the on-pulses versus the off-pulses) varied as I twitched the throttle slightly. The amplitude of the square wave didnt change.

One of my 94LX's had an infrequent high idle speed after a long drive.
When I went to check out a TPS for it, the old one had some intermittent regions of resistance as I turned it. (My digital VOM readout went nuts. My analog voltmeter just showed some jumping around of the needle on the OHM scale.) The new TPS one showed an even change of resistance with both meters. I should have written down what the resistance readings were at max and min; cause I cant now recall what they were.
I put in the new TPS, but the car seemed to run the same as with the old one. After 2 or 3 hours of highway driving, it may still not idle down at the bottom of an off-ramp.
 
#7 ·
The 1st gen CFIs use an idle control motor to control idle speed, except the HO, which uses the same type of IACV as a Mustang (basically the same part).

First thing would be to check for error codes in memory. That'll help.

If you're having stalling problems while coasting down, this is very likely the ICM failing to respond quickly enough. It may be responding too slowly, and gunked up. The ECU test only confirms if the tip switch on the ICM responds, and that the motor responds, not necessarily the speed. They don't respond super fast anyway.

Try watching the motor respond when it's at cold, and then at warm idle, goose the engine, and see how things respond. You may need to take the air cleaner bonnet off to get a better view. I'm going to start there since it's the most likely culprit.

The car can even be driven without the ICM in place, as the CFI has an idle set screw, but it'll stall out between shifting, or when coasting down.

If it's still happening, and you have zero error codes, you may have a worn/loose ignition switch that's disconnecting randomly, and sometimes reconnecting, which would allow the engine to stall/surge as mentioned. I've had that happen before.

Codes first, if any, and that'll be the starting point.
 
#8 ·
The 1st gen CFIs use an idle control motor to control idle speed, except the HO, which uses the same type of IACV as a Mustang (basically the same part).

First thing would be to check for error codes in memory. That'll help.

If you're having stalling problems while coasting down, this is very likely the ICM failing to respond quickly enough. It may be responding too slowly, and gunked up. The ECU test only confirms if the tip switch on the ICM responds, and that the motor responds, not necessarily the speed. They don't respond super fast anyway.

Try watching the motor respond when it's at cold, and then at warm idle, goose the engine, and see how things respond. You may need to take the air cleaner bonnet off to get a better view. I'm going to start there since it's the most likely culprit.

The car can even be driven without the ICM in place, as the CFI has an idle set screw, but it'll stall out between shifting, or when coasting down.

If it's still happening, and you have zero error codes, you may have a worn/loose ignition switch that's disconnecting randomly, and sometimes reconnecting, which would allow the engine to stall/surge as mentioned. I've had that happen before.

Codes first, if any, and that'll be the starting point.
There are no codes. I've checked every time after having a stallout and it passes the KOEO and KOER quick tests.

Perhaps I could try driving it with the ISC motor unplugged and the stop screw adjusted to the normal idle value, just for diagnostic purposes?

The ignition switch is new OE. I replaced it before the rebuild and the symptom didn't exist then.

I'll watch the plunger at cold and warm idle and see if there's a difference. The ISC is new and came as part of the reman throttle body (reconditioned by Ford).
 
#9 ·
The idle screw shouldn't need to be changed from the factory; it's already set.

It could be a defective ignition switch, or the connection is dirty. The ICM could have sat on a shelf for a while, and gumming could've happened. That won't throw a code, but it will act sluggish.
 
#10 ·
I have noticed that the idle does act pretty normal under all conditions when the engine is cold (drops normally during shifting, etc.) but it's when it gets warm that the problem happens. It's if I let the rpms get down to about 1000 in second gear and then clutch to stop that it falls on its face. In fact it just stalled on me yesterday.

How can I check for gumming in the ISC? Do I have to take it apart?
 
#12 ·
I can understand an engine that has a high idle now and then; because its getting more air than should come past the throttle valve and the Idle Air Control. That is; it depends on getting air.
I dont understand any other event causing it. The spark advance isnt likely to have that large an effect on the engine speed (when your foot is off the pedal), and the amount of fuel getting dumped in by the injector isnt likely to cause a high idle. I would still be looking for a vacuum leak or other air leak; which could be loose bolts on the intake manifold, on the throttle body, or the vapor canister, a brake booster bladder, a leaking PCV circuit, etc.

With my Jaguar, a common failure resulting in a high idle, was a leak past the sliding metering piston in the SU carburetors. They would stick in the slightly raised position - causing a high idle. It was partly dirt between the metering piston and the housing, partly wear of the piston and housing. This usually wasnt a really serious problem, because the Lucas ignition system gave enough problems of its own (where the car would not run at all) to make you forget just having a high idle.
 
#13 ·
Over the weekend I replaced the intake manifold (one of the throttle body mounting ears was broken) and the old, worn-out, and now broken spacer. I thought SURE that was the source of my idle drop problem.

Not.

I don't know where to look next.
 
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