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2.5L ('98-'01) P0442 rough idle and stalling. Everything checks out.


franklin2

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I would not turn the idle stop screw without first marking where it is, and then doing the unplug the IAC test and see if the engine is barely idling. The problem with just turning it without unplugging the IAC, the computer is going to be in the picture trying to adjust the idle while you are turning the screw, and you can get this screw open too far, and that messes up your throttle position sensor.

The computer has 3 different modes of operation, and they are determined by the throttle position sensor. It has a idle mode, above idle running mode, and a wide open throttle mode. The problem comes with the idle mode. It is when the throttle position sensor is around 1v or less. It's very easy to turn the screw too far, and at rest the TPS is above 1v and this confuses the computer and it will not run correctly.
 


-Nate

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THANK YOU for the reminder ! .

Never touch the 8MM idle stop screw with the IAC connected .
 

ben_2_go

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I would not turn the idle stop screw without first marking where it is, and then doing the unplug the IAC test and see if the engine is barely idling. The problem with just turning it without unplugging the IAC, the computer is going to be in the picture trying to adjust the idle while you are turning the screw, and you can get this screw open too far, and that messes up your throttle position sensor.

The computer has 3 different modes of operation, and they are determined by the throttle position sensor. It has a idle mode, above idle running mode, and a wide open throttle mode. The problem comes with the idle mode. It is when the throttle position sensor is around 1v or less. It's very easy to turn the screw too far, and at rest the TPS is above 1v and this confuses the computer and it will not run correctly.

When I unplug my IAC with the engine at operating temp and nothing on, the idle settles in at a near steady 500rpm using my scan tool. With the old cleaned up OE IAC, the idle is anywhere from 850 to 1100 rpm. Sometimes it will drop to close to 500rpm after a long drive and stall. It doesn't settle in at 750 to 800 like it use to. I've done all the tests including a smoke test looking for vacuum leaks. All seems well. Testing the TPS while the engine is running gives me all good voltages, .89 idle and 4.95 wo. The T Series IAC was the one that hung my idle at 3000 rpm and sent me through a stop sign almost into a house. My truck is an auto and it happened so quick I didn't think to knock the shifter up into neutral. It took me by surprise. My OE Ford IAC has very little movement from closed to fully open. The T Series IAC wouldn't close and left a larger gap than the OE IAC. I made the mistake of trashing the no name IAC I removed around a month a go or a little longer. It seemed to work fine after I installed it. I only replaced it because I was doing a lot of work chasing an idle and continuous spark knock problem which ended up partially being a bad purge valve. I'm still chasing the fluctuating idle issue.
 

franklin2

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I think you figured out you have idle air control valve problems. Once you settle in on one you think is working right, if you still have a changing idle speed, I would get a timing light and check the timing when the engine is idling. If it's moving all over the place when the idle speed changes, there may be a problem there. I have never worked on a 2.5, so I do not know if they still use a distributor or not. If they do it could be wore out. If they don't, I would make sure the crankshaft position sensor is ok and nothing is loose in that area.
 

ben_2_go

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I think you figured out you have idle air control valve problems. Once you settle in on one you think is working right, if you still have a changing idle speed, I would get a timing light and check the timing when the engine is idling. If it's moving all over the place when the idle speed changes, there may be a problem there. I have never worked on a 2.5, so I do not know if they still use a distributor or not. If they do it could be wore out. If they don't, I would make sure the crankshaft position sensor is ok and nothing is loose in that area.
It's a Ford DIS system. They've been using them on the 4 cylinders in the Rangers since at least '93. Timing is what it is with these systems. I just did a timing belt and replaced the crankshaft sensor connector. That really made a big difference. The wire insulation has started to get brittle and break apart. I'll keep chasing it until I get my Kubota engine ready to drop in.
 

-Nate

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The moving timing point at idle is normal .

This is part of the anti - knock in the ignition .

Thanks to you alls keeping this thread alive I finally got off my duff and took a Foo-Foo can of brake cleaner on my next errand and after it was fully warmed up I sprayed here, there and every where, I used the snorkel to spray in between things and up underneath, no idle change so I'm off to look for another ACV .
 

franklin2

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The moving timing point at idle is normal .

This is part of the anti - knock in the ignition .
Hmmm......never heard of that before. If the timing is changing, the idle speed will change also.
 

-Nate

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No, it won't and that's the entire point : to reduce emissions by stabilizing the idle speed and simultaneously retarding the timing when under load if it detects a sub audible knock .

This is (or used to be) all standard stuff learned at the basic SMOG test school .

The older guys (me now) who didn't believe in In Service Training used to say 'oh, you can't set the ignition timing on new cars because it jumps around so much at idle' .

There's a method or device (depends on the manufacturer) to disable the knock sensor so the timing remains fixed at idle .

I'm guessing distributor less ignition systems no longer need worry about this .
 

ben_2_go

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Base timing is locked in and the EEC/PCM changes timing as needed based on sensor inputs. If there's a steady idle, the timing shouldn't change until a load is placed on the engine.
 

-Nate

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So ;

I'm thinking I need an iac valve, what brands should I look for or avoid ? .

I can't find an N.O.S. Ford / Motocraft one anywhere .

TIA,
 

ben_2_go

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So ;

I'm thinking I need an iac valve, what brands should I look for or avoid ? .

I can't find an N.O.S. Ford / Motocraft one anywhere .

TIA,
Outside of Ford/ Motorcraft it's very hit or miss. My first replacement was a no name offshore unit that came in a small white box with no labels. I bought it from Amazon for around $30 is I remember right. The last one was a T series that's been recommended on several forums and it was bad straight out of the box. I don't know what it cost because I bought a lot of other stuff at the same time.
 

Jazzer

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According to RonD, the Hitachi IACs are comparable in design to the Motorcrafts, IIRC.
 

-Nate

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Yabbutt ;

I can't find one to save my life .
 

ben_2_go

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Yabbutt ;

I can't find one to save my life .
Rock Auto is where I go to when I can't find stuff locally or on Amazon.
 

-Nate

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They only have Chinese ones .

I've been searching for an N.O.S. Motocraft one on E-bay awhile now, no luck .
 

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