Erratic High Idle
Thread Starter
|
Cross-Country
Joined: Jul 2013
Posts: 83
Likes: 1
From: Hanover, MD
Erratic High Idle
'99 4.6L exhibiting erratic high idle. Usually its fine, but occasionally it will idle at as much as 3k. Blipping the throttle will sometimes clear it, but not always. If I unplug the IAC connector it drops down to normal levels. Replace the IAC? I don't believe there are any vacuum leaks, and no codes are being thrown...
Elder User
Joined: Oct 2010
Posts: 602
Likes: 1
From: Maryland
Rockauto.com usually has the best prices on a lot of stuff. On top of that if you google rockauto discounts they always have a 5% off discount code. They have name brand parts, OEM Ford parts and cheaper brands.
Thread Starter
|
Cross-Country
Joined: Jul 2013
Posts: 83
Likes: 1
From: Hanover, MD
Trending Topics
Thread Starter
|
Cross-Country
Joined: Jul 2013
Posts: 83
Likes: 1
From: Hanover, MD
pulling up an old thread as its back, although I'm not sure I ever really fixed it. As previously reported I had replaced the IAC, seem to fix it initially but it came back and still would do it occasionally. Still only occasionally get the high idle, and I've swapped in a known good motorcraft IAC off another vehicle. What else controls idle that would be forcing the computer to high adjust the idle through the IAC without normal input? TPS, crank position sensor? cam position sensor? Computer issue? I am at a loss and its frustrating as the high idle is dangerous on the road when it occurs. I've cleaned the throttle plate and it doesn't appear that there is any binding that could be causing it to stay open. No codes.
Assuming tach gets signal from the crank position sensor - I haven't notice loss of tach signal when the idle surges.
One other thought. Sticking throttle cable or linkage? I've played with it and don't think its doing this, but being intermittent who knows. Is there a reasonable way to lube the cable and linkage to rule this out?
It does seem like the problem mostly occurs after the motor is warmed up...
Assuming tach gets signal from the crank position sensor - I haven't notice loss of tach signal when the idle surges.
One other thought. Sticking throttle cable or linkage? I've played with it and don't think its doing this, but being intermittent who knows. Is there a reasonable way to lube the cable and linkage to rule this out?
It does seem like the problem mostly occurs after the motor is warmed up...
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
Top 10 Fords at 2026 Carlisle Ford Nationals
Joe Kucinski
3 Best / 3 Worst Parts of Modern Ford Ownership
Brett Foote
10 Amazing Upgrades That Solve Common Ford Truck Owner Headaches
Pouria Savadkouei
Every 2026 Ford Engine Explained
Brett Foote
10 Ugly Ford Trucks That We Still Kinda Love
Joe Kucinski
10 Things Every Truck Owner NEEDS (2026 Edition)
Michael S. Palmer
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Verdad Gallardo
Top 10 Most Expensive Ford Trucks Ever Sold on Bring a Trailer
Joe Kucinski
2027 Ford Super Duty Buyer's Guide (Every Model, Engine, & Package)
Brett Foote
Thread Starter
|
Cross-Country
Joined: Jul 2013
Posts: 83
Likes: 1
From: Hanover, MD
I really need someone who understand the engine control enough to know whether a bad reading from the TPS could influence the computer in such a way as to open up the IAC to cause high idle. I'm convinced the IAC itself is OK as it has exhibited the same issues with 2 different ones.
Logistics Pro
Joined: Jan 2001
Posts: 3,740
Likes: 37
From: Central NYS
I am assuming you have no check engine light?
Try pinching off the PCV line when the idle goes high and see if that brings the idle down.
You May have a PCV valve that is sticking wide open and over powers the IAC’s ability to bring idle down.
Try pinching off the PCV line when the idle goes high and see if that brings the idle down.
You May have a PCV valve that is sticking wide open and over powers the IAC’s ability to bring idle down.
Lead Driver
Joined: Sep 2006
Posts: 7,907
Likes: 143
Cooling system/intake low air temperature false signal.
Cross in leads with another circuit.
A Limp mode response.
Possible TPS fault.
Advanced ignition timing caused by a fault.
Look at these data signals with a Scanner.
Coolant temp indication too low.
Intake air temp indication . too low.
Other readings out of normal.
Cross in leads with another circuit.
A Limp mode response.
Possible TPS fault.
Advanced ignition timing caused by a fault.
Look at these data signals with a Scanner.
Coolant temp indication too low.
Intake air temp indication . too low.
Other readings out of normal.
Thread Starter
|
Cross-Country
Joined: Jul 2013
Posts: 83
Likes: 1
From: Hanover, MD
Well replaced the TPS last night on a whim as I pulled it off another truck. What a bear to get those dang screws out!!! Not sure if it helped, no idle issues during the test drive but about half way though the heat cut out and noticed temperature rising. Pulled over and filled up the overflow bottle and made it back home. Couldn't see the leak specifically in the dark but its in the area of the water pump. Try to figure that out tomorrow before dark.
