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Discussion Starter #1
I've been searching and and reading threads about IAC problems, and for now I'm fairly certain my IAC is dead. There is still some uncertainty in this and things I'd like to clarify to myself. That's more like engineers curiosity. I bolded those things bothering me the most.

So, one day I drove the car home with no problems and the very next day it does this starts and dies immediately. Can be forced to idle with slight throttle. Stays running when warmed up, but on ~400 rpm (no gear, A/C off).Once during the tests listed below I even heard that "Moo" sound which, I believe, also points to the IAC valve.

Done so far
-Valve cleaned and oiled with WD-40.
-12 V is present.
-Duty cycle jumps constantly up and down between about 25 and 40% and occasionally higher. One time the motor stalled during this test and duty cycle freezed to 60.4% for two or three seconds before showing 0.
-Coil resistance is 9.7 Ohm in both directions and also diode test showed 0.004 V both ways. There is a diode and it shouldn't behave like this, right?
-The valve sits partly open when unaffected and travels only very little towards more open state when connected to 12 V. I'm not sure how this should be, but doesn't sound right to me.

Any thoughts on this before I go get a new valve (and find out it helped nothing)?
 

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I've been searching and and reading threads about IAC problems, and for now I'm fairly certain my IAC is dead. There is still some uncertainty in this and things I'd like to clarify to myself. That's more like engineers curiosity. I bolded those things bothering me the most.

So, one day I drove the car home with no problems and the very next day it does this starts and dies immediately. Can be forced to idle with slight throttle. Stays running when warmed up, but on ~400 rpm (no gear, A/C off).Once during the tests listed below I even heard that "Moo" sound which, I believe, also points to the IAC valve.

Done so far
-Valve cleaned and oiled with WD-40.
-12 V is present.
-Duty cycle jumps constantly up and down between about 25 and 40% and occasionally higher. One time the motor stalled during this test and duty cycle freezed to 60.4% for two or three seconds before showing 0.
-Coil resistance is 9.7 Ohm in both directions and also diode test showed 0.004 V both ways. There is a diode and it shouldn't behave like this, right?
-The valve sits partly open when unaffected and travels only very little towards more open state when connected to 12 V. I'm not sure how this should be, but doesn't sound right to me.

Any thoughts on this before I go get a new valve (and find out it helped nothing)?

On the troubleshooting aspect, if it still the original IAC, I'd say go ahead and buy a new Motorcraft unit, as well as Throttle Position Sensor. The third party ones are hit or miss, and it's a very common, very easy part to replace.

What are your symptoms when driving? On my car, if I hit a bump in the road my car would go to a very high idle, so that when in park or neutral idle would be 2000 rpm, and when stopped at a light, the car would pull against the brakes. Mileage suffered dramatically as well.
 

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Discussion Starter #3
On the troubleshooting aspect, if it still the original IAC, I'd say go ahead and buy a new Motorcraft unit, as well as Throttle Position Sensor. The third party ones are hit or miss, and it's a very common, very easy part to replace.
Not sure, but it very well might be the original. Ford logo and part number present. OK, I'll try to find a Motorcraft unit. It's not that obvious here in Finland. May have to be ordered from Rockauto and that'll take some time..

What are your symptoms when driving? On my car, if I hit a bump in the road my car would go to a very high idle, so that when in park or neutral idle would be 2000 rpm, and when stopped at a light, the car would pull against the brakes. Mileage suffered dramatically as well.
I haven't driven it since and don't feel I'd want to. High idle I can handle, but no idle with an automatic sounds way too laborous, even a bit dangerous if brake booster effect is lost :|
 

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IAC Fix

I too have fought with the IAC Valve on my '97.Changed out Valves, checked TPS, never could get it to stop sounding like a fog horn.......I finally put some electrical tape over the inlet and the outlet of the plastic muffler, bumped up the idle by slightly bending the stop tab on the throttle (950 RPM with A/C on) and haven't had a problem since. If you don't need A/C just adjust the idle accordingly. I did not unhook any wiring just plugged the hose.
This was a last ditch effort.
 

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Discussion Starter #5
New IAC helped only little and a short while. Not even sure if it was the cause, anyway. Been driving the car some short distances. It goes fine for a while and occasionally drops torque and RPM and at the same time I can see a drop in MAF reading. This reading may or may not correspond to drops in RPM. If not, it'd imply an air leak. Haven't found any leaks, though. Now, a horrible thought crossed my mind.. How can I tell if a head gasket is leaking? Oil cap is clean, not that foamy white stuff which I've seen in some other engines with busted head gasket.
 

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Discussion Starter #6
Some new info. I retested the PWM duty cycle. At cold start it shows zero and makes some occasional jumps to 5-7%. After a minute or two it jumps high, to about 95%, and at the same time idle reappears. For what I've heard both values are way off the scale. Duty cycle should be somewhere between 25% and 60%. Can someone confirm this?

Also made the engine run about 1500 rpm and unplugged the IAC wires. Revs dropped to ~800 rpm. The same happens when PWM duty cycle drops to 0. So, it seems that PWM signal is occasionally lost. But why? Wires in the engine compartment seem to be OK for what I can tell without tearing those thick wire bunches apart. Is my EEC toasted? Any other ideas besides EEC?
 
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