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Old 10-04-2010, 18:57   #1 (permalink)
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98 DOHC Taurus idle issues w/codes

My recently acquired 98 wagon has had a few issues since I got it. The IAC was unplugged so I hooked it back up, cleared codes and drove a bit to get ready for smog. On the test drive it feels almost like the cruise is on (ie letting off of the gas pedal doesn't allow for much coasting or slowing down). If I shift into neutral while at highway speed rpms will hit around 3000-3500 with the throttle closed. If I allow this to continue for a a bit I get a code P1120 which means TPS input is lower than MAF. I still get decent idle control in gear stopped, at low speed and in neutral parked. I have checked the TPS signal at the ecm pin with my voltmeter and it is within range, however the scantool readout shows the TPS voltage to be responding to change a lot slower than all the other sensors. Eventually the scantool reads actual tps voltage, it just takes a while (ie 5 seconds from closed to wot, 5 more seconds back to closed) to get there. PCV and all vacuum systems appear working and not leaking.

I'm thinking the IAC is either worn/malfunctioning or maybe the ecm is faulty and sending the wrong signal. My scantool doesn't really let me monitor the IAC commands. Any ideas or suggestions would be much appreciated. Thx.
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Old 10-04-2010, 19:27   #2 (permalink)
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Re: 98 DOHC Taurus idle issues w/codes

Try cleaning the IAC and see if it helps. If it does, you'll need to replace it. How was it running before you hooked it up?
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Old 10-04-2010, 19:44   #3 (permalink)
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Re: 98 DOHC Taurus idle issues w/codes

I forgot to mention I cleaned it out today with carb spray and q-tips. It didn't change the results. I did note that in its resting state the plunger wasn't fully closed. I don't know if that's normal. I never drove the car with it unplugged as it had a slow idle/stall issue until warmed up. I popped the hood and immediately noticed the unplugged connector.
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Old 10-04-2010, 19:58   #4 (permalink)
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Re: 98 DOHC Taurus idle issues w/codes

Look around for a possible vacuum leak. Check the PCV and hose to it.
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Old 10-04-2010, 20:25   #5 (permalink)
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Re: 98 DOHC Taurus idle issues w/codes

I replaced the pcv and the hose on the vacuum side of the valve. I'm sure my air leak is through the iac as it shows on the MAF. I'm not sure, however if my ecm is commanding the IAC signal based on bad inputs, if the IAC is worn internally causing it to open a bit too much in a given duty cycle, or if the ECM is just bad. The whole experience while monitoring the tps voltage seemed a bit off to me. I will try and hook up to the IDS at work tomorrow and look into it further.
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Old 10-04-2010, 20:43   #6 (permalink)
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Re: 98 DOHC Taurus idle issues w/codes

Keep us posted on what you find.
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Old 10-05-2010, 15:17   #7 (permalink)
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Re: 98 DOHC Taurus idle issues w/codes

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Keep us posted on what you find.
Hooked up to the IDS at work today and confirmed the ECM was not reading actual TPS values. It was showing "partial throttle" mode most of the time as well so that's why no idle down at highway speed. I ordered a used ECM for $35 and will have it in around a week. Will post when completed.
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Old 10-12-2010, 20:13   #8 (permalink)
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Re: 98 DOHC Taurus idle issues w/codes

got my used ECM in today and after battling the PATS system for hours I am in good working order. A short test drive and all my readiness monitors have cleared for smog. Thanks Bert for your input on this one.
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Old 10-12-2010, 21:02   #9 (permalink)
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Re: 98 DOHC Taurus idle issues w/codes

That's good news. Thanks for the update.
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