Misfire/Idle air valve

Chris Baker

Well-known member
Sep 12, 2019
81
1
Bolivar, Tn
I have gradually been working the bugs our of my 04 Discovery 4.6. At a cold start it will either idle up really high like close to 2500-3000 RPMs or it will jump up and down or stall. However, no matter which one it does, after about a min. it will smooth out. I am still getting random miss fire codes as well as the code for the idle air control valve.
I am pretty sure a bad idle air control valve may have something to do with the idle problems. However, since this does have something with the air and gas mixture, will it also contribute to the misfires? I ask because I have put all new plugs, wires and coils on it.
If I replace the idle air control valve, will this help my issues?
The code I am getting is P0505.
 

simondewing

Well-known member
Feb 10, 2020
59
11
Texas
Chris that's possible but only if its opening and closing very rapidly (a partial blockage or intermittent maybe) as it adjusts the amount of air bled around the throttle butterfly.
Mixture is controlled by fueling tables in ECU and injectors.
Do you have access to anything other than a basic scan tool? (e.g. hawkeye or L/R testbook).
There are diagnostic test sequences for injectors and for the IAC valve.

If not check the connector to the IAC valve is clean and dry, then check the condition of the hoses and make sure there are no air leaks.
(I have had the T on the plastic intake pipe that goes to the IAT split at the joint, almost invisible but it did cause issues).

If you have a spare IAC or a nearby scrapyard try swapping it out.

May also be worth pulling the spark plugs and checking they are all clean(ish) and a similar color to rule out any consistent misfiring.
 

Chris Baker

Well-known member
Sep 12, 2019
81
1
Bolivar, Tn
Chris that's possible but only if its opening and closing very rapidly (a partial blockage or intermittent maybe) as it adjusts the amount of air bled around the throttle butterfly.
Mixture is controlled by fueling tables in ECU and injectors.
Do you have access to anything other than a basic scan tool? (e.g. hawkeye or L/R testbook).
There are diagnostic test sequences for injectors and for the IAC valve.

If not check the connector to the IAC valve is clean and dry, then check the condition of the hoses and make sure there are no air leaks.
(I have had the T on the plastic intake pipe that goes to the IAT split at the joint, almost invisible but it did cause issues).

If you have a spare IAC or a nearby scrapyard try swapping it out.

May also be worth pulling the spark plugs and checking they are all clean(ish) and a similar color to rule out any consistent misfiring.

I have another IAC on order, should be here by Saturday. I will continue to check the hoses as you suggest.
Once the IAC comes in and I install it, will see what I have at that time and go from there.