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Old 04-17-2010, 16:58   #1 (permalink)
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Strange probs after IAC change

Hi All I'm a Noob to the forum and to all this EFI stuff as a matter of fact

Now here is my big problem I'm Having trouble after changing a faulty IAC valve. I had a problem with idle fluctuating and changing the valve fixed that but now have another problem.
When I start the car it will start normally then after about 30 seconds sometimes more or less I can hear the computer click and the car almost dies (sometimes does) then changes its running, sometimes will rev very high or will start to run very rough (even if I move the revs up it runs roughly) after a while it seems to smooth out most times.
When it is hot if I turn on the ignition and don't start, the fuel pump will run and stop, but then after about 3 seconds I get a click a click at the computer and it runs again.
I have put a new MAF sensor in but after clearing the codes then running at idle for a few minutes it comes up with a 157 code (although each time after about two minutes the engine will stop and need a restart).
I tried to run a cylinder test and the reader came up with a 998 code then just went to continuous buzz.
I did dis-connect the battery after the change.
When I changed the IAC I had to undo all the wiring from the engine side as I can't get to the plug an the eec side, I'm wondering now if this problem could be a broken or damaged wire.
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Old 04-17-2010, 20:25   #2 (permalink)
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Re: Strange probs after IAC change

That's what it sounds like to me. A connection problem. Recheck all the ones you removed and see if you get it going again. What year is it? Crown Vic?
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Old 04-17-2010, 20:33   #3 (permalink)
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Re: Strange probs after IAC change

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Originally Posted by rtg143 View Post
That's what it sounds like to me. A connection problem. Recheck all the ones you removed and see if you get it going again. What year is it? Crown Vic?
Thanks Bert I am in the middle of pulling it all apart again ( that gives me a bit more confidence
!993 but it is in a 46 Ford, and I did have to disconnect a lot due the way the wiring is set up in the 46
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Old 04-17-2010, 20:51   #4 (permalink)
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Re: Strange probs after IAC change

You'll probably have it fixed in no time.
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Old 04-18-2010, 02:16   #5 (permalink)
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Re: Strange probs after IAC change

Nope no luck

Thought I had it but nope, Had it idling beautiful but after a while went off again. Started it then after a few minutes it suddenly stopped and on re-start it ran rough ( almost like a motor not getting enough fuel) at no matter what revs.
The two goes with the fuel pump only happens when the engine is hot (running temp)
The bad running seems to happen randomly, and after it, it seems to go to the "cold" running cycle again.(revs high for a while then backs down to normal idle)
I'm thinking one of two things.
1. Is the IAC I bought faulty (all happened after replacing it)?
2. Has my computer stuffed up?
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Old 04-18-2010, 08:34   #6 (permalink)
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Re: Strange probs after IAC change

If you have the old IAC, put it back on and see if it does what it did before changing it. Do you have the OBD connector?
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Old 04-18-2010, 15:56   #7 (permalink)
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Re: Strange probs after IAC change

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Originally Posted by rtg143 View Post
If you have the old IAC, put it back on and see if it does what it did before changing it. Do you have the OBD connector?
The trouble is with the old IAC it would hardly idle at all at it is a very tricky job to change as my engine is almost touches the firewall.
Not sure on "OBD"? but from what I understand mine is EEC=IV if I'm on the right track, (I have a code reader that flashes and beeps)
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Old 04-18-2010, 17:19   #8 (permalink)
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Re: Strange probs after IAC change

Do you still get a code 157? 998 is just that it didn't pass key on engine off test. Have you tried running it with the MAF disconnected?

Here's the test for the MAF if you need it. Ford’s MAF has a voltage output that increases with engine air flow. The mass air flow sensor has 0 volts output with key-on-engine-off and 0.50 to 0.80 volts at idle. The specifications will give the specific voltage at idle. The output must increase with engine RPM. A test drive is a good diagnostic test for MAF sensors.
Snap wide open throttle test: MAF sensors that become contaminated with deposits don’t respond quickly to changes in air flow. An easy test is to do a quick wide open throttle acceleration test. The signal voltage must go over 3.7 volts. Check sensors that don’t respond properly.

A test drive is the best method of testing the MAF sensor. The MAF signal should follow smoothly with changes in engine speed. Watch for sudden changes when engine speed is relatively steady. The MAF output must be a smooth even increase as engine speed increases.

MAF Values For a Typical Test Drive

Engine Off-Key-On 0.0 V
Idle 0.7 V
30 MPH 1.0 - 1.3 V
55 MPH 1.6 - 2.2 V
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Old 04-18-2010, 21:00   #9 (permalink)
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Re: Strange probs after IAC change

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Originally Posted by rtg143 View Post
Do you still get a code 157? 998 is just that it didn't pass key on engine off test. Have you tried running it with the MAF disconnected?

Here's the test for the MAF if you need it. Ford’s MAF has a voltage output that increases with engine air flow. The mass air flow sensor has 0 volts output with key-on-engine-off and 0.50 to 0.80 volts at idle. The specifications will give the specific voltage at idle. The output must increase with engine RPM. A test drive is a good diagnostic test for MAF sensors.
Snap wide open throttle test: MAF sensors that become contaminated with deposits don’t respond quickly to changes in air flow. An easy test is to do a quick wide open throttle acceleration test. The signal voltage must go over 3.7 volts. Check sensors that don’t respond properly.

A test drive is the best method of testing the MAF sensor. The MAF signal should follow smoothly with changes in engine speed. Watch for sudden changes when engine speed is relatively steady. The MAF output must be a smooth even increase as engine speed increases.

MAF Values For a Typical Test Drive

Engine Off-Key-On 0.0 V
Idle 0.7 V
30 MPH 1.0 - 1.3 V
55 MPH 1.6 - 2.2 V
Yeah I still get 157 after running at idle, for a while, in the ket on engine off test
Only get the 998 when I do engine running test, then the reader just crashes and goes to a steady buzz.
Where do I hook up to do the maf test, I don't have a breakout box
Thanks Woody
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Old 04-18-2010, 21:51   #10 (permalink)
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Re: Strange probs after IAC change

You can check it at the MAF. tan/lt blue. Here's adiagram of a similar system

Ford Fuel Injection � 4.6L harness
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