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Fast idle problems Mark VIII 4.6 DOHC


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#1 mark of distinction

 
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Posted 30 September 2007 - 10:24 AM

A friend at another board suggested that I come here. This is my first posting to FlatrateTech.


I'm having fast idle problems with my '93 Lincoln Mark VIII. I'm idling at 1200rpm hot and cold.


I have replaced the TPS, IAC valve and ECT sensor. These changes have not helped.

Some history: The '93 engine, which always idled at 800rom had to be replaced in April. The replacement engine was a 76K mile '98 Mark VIII DOHC 4.6. All relevant '98 parts were removed and the '93 parts were installed on the '98 "long block".

The '98 mill idled at 1200rpm the day I picked the car up from the shop. It runs fine, other than this damnable 1200rpm idle.

Something else you need to know: I owned this car for one full year of winter and summer driving without problems before I found the IAC valve was unplugged. I plugged the IAC in and was greeted with pulsating idle so I unplugged the IAC, left it unplugged and was quite happy for the next 1.5 years until I had to have the engine replaced.

Same this with the '98 engine. It idles terrible with a new IAC plugged in and does not help the fast idle problem.

I keep the IAC unplugged.


Any thoughts and ideas would be appreciated.

#2 carnut

 
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Posted 30 September 2007 - 12:00 PM

I am not a tech, just an avid do it yourselfer. I had a similar problem on an 89 F250 (302 with Fuel Injection). Mine was caused by a vacuum leak between the upper and lower intake manifolds. It could be a vacuum leak anywhere (most likely under the hood). You may have to disconnect and block the vacuum lines one at a time until you find the source. I am assuming no one has messed with the throttle body stop screw.
Jay Kellogg

#3 easttexas

 
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Posted 30 September 2007 - 03:55 PM

QUOTE (carnut @ Sep 30 2007, 12:00 PM) <{POST_SNAPBACK}>
I am not a tech, just an avid do it yourselfer. I had a similar problem on an 89 F250 (302 with Fuel Injection). Mine was caused by a vacuum leak between the upper and lower intake manifolds. It could be a vacuum leak anywhere (most likely under the hood). You may have to disconnect and block the vacuum lines one at a time until you find the source. I am assuming no one has messed with the throttle body stop screw.



ditto
James Freund
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#4 mark of distinction

 
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Posted 01 October 2007 - 08:00 AM

QUOTE (easttexas @ Sep 30 2007, 03:55 PM) <{POST_SNAPBACK}>
ditto


#5 me6894

 
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Posted 01 October 2007 - 10:28 AM

ditto

also make sure the wires going to the IAC aren't ripped or anthing, and the terminals are tight

#6 mark of distinction

 
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Posted 01 October 2007 - 03:54 PM

QUOTE (easttexas @ Sep 30 2007, 03:55 PM) <{POST_SNAPBACK}>
ditto


I considered the possibility of a vacuum leak since the '93 manifold was moved over to the '98 engine. I could not find any leaks, but to be sure, I removed the manifold, soaked it in a 5 gal. can of carb cleaner and removed ALL the gunk. I reinstalled it with new lower and upper gaskets and torqued it to factory specs using the factory torque sequence. Once reinstalled, I still have a 1200rpm idle.

Wiring: It is not a wiring problem. I went to the trouble of replacing the engine wiring harness when I removed the manifold(yep. the entire engine wire harness---it's not as hard as it looks)with a different harness I had in the shop. Different wiring and I still have a 1200rpm idle.

I have triple checked all vac lines and replaced the power brake vac lines, including a new rubber grommet in the power brake booster. I have no vac leaks.

I can, however, remove the PCV tube at the throttle body and create a massive vac leak which would lean out the air-fuel mixture. With the extra air supply coming in through the open PCV tube and leaning out the air/fuel mix, the idle goes down.

It's a puzzle.




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