How does Idle Air Control Valve Work?

[bkfitz]

Check the coolant temp sensor. when it goes bad and sends a signal to the ECM that the engine is cold, the ECM will control the engine with increase fuel flow and increase in idle.

Wow... you may be the winner here. Great thought. Ever since I bought this truck, the engine temp meter in the dash barely moves... previous owner said it just runs cool... maybe it's been the coolant temp sensor all along. Here is a 'cold start' vs. 'been running in 90 degrees for 30 minutes' difference:

Cold:

Hot:

 

[dallen341]

2 different things. The gauge uses a sending unit to vary the needle. The engine uses a coolant temp sensor to tell the ECM how hot the coolant is so it can control idle and fuel.

The 2nd one is the one you want to check and/or replace.

 

[snobdds]

I would also check the temp sender (for the gauge) while your doing this, that reading does not look right.

 

[bkfitz]

Did you meter the air temp sensor? It has a pin (2 pins?) in the AFM harness.

If it meters OK, I'd make sure it's getting its signal back to the ECU.

That WILL do odd things to idle if the truck thinks it's -40 or +130....


But given all that goo, jeez, I'd really confirm that passages aren't clogged up
inside there.

hth

t

I did...

Everything from this FSM page checked out pretty good... For the third row, I did get ~ 1.2KOhms when it was ~ 75 degrees out, but I figured that was close enough:

Everything from this FSM page checked out perfectly:

 

[bkfitz]

2 different things. The gauge uses a sending unit to vary the needle. The engine uses a coolant temp sensor to tell the ECM how hot the coolant is so it can control idle and fuel.

The 2nd one is the one you want to check and/or replace.

10-4 - I'll look through the FSM for instructions on testing the "coolant temp sensor" and the 'sending unit'...

Update: I see references to the "water temperature sensor" FSM pages FL37, FL77 etc - is this the same as the coolant temp sensor?

Thx!

Update:

Here is the test I found in the FSM I'm guessing you are referring to:

 

[bkfitz]

Just thinking... if the water temp sensor fails, it will fail indicating a low temp (-20 degrees) which will just increase fuel flow accounting for a cold start yes? The result of this wouldn't be a high idle, but just increased fuel consumption yes?

 

[bkfitz]

Also thinking... 650 rpm idle is in Neutral right? Not in gear?

 

[dallen341]

years ago the wife had a FJ-62. I had a high idle situation that crept up over the course of several days getting progressively worse. I suspected the Coolant Temp Sensor and sure enough that was it. Not only does it cause the ECM to pump in more raw gas but it increases the idle as well. As the engine warms, fuel is pulled back and the idle drops as well.

 

[dallen341]

Not sure nomenclature on the 2. believe the gauge will refer to it as a sending unit however.

10-4 - I'll look through the FSM for instructions on testing the "coolant temp sensor" and the 'sending unit'...

Update: I see references to the "water temperature sensor" FSM pages FL37, FL77 etc - is this the same as the coolant temp sensor?

Thx!

Update:

Here is the test I found in the FSM I'm guessing you are referring to:

 

[dallen341]

Disconnect it and see if the gauge works. if gauge still works you have the sensor I believe may be faulty. If gauge quits working, then its the other sensor!

Not sure nomenclature on the 2. believe the gauge will refer to it as a sending unit however.

 

[pappy]

The gauge sender is on the thermostat housing, driver's side.

The EFI temp sensor is also on the thermostat housing as shown in the FSM drawings.

There is a third switch also on the thermostat housing called the Cold Start Time Switch that controls the cold start injector.

 

[pappy]

The gauge sender is on the thermostat housing, driver's side. It has a one wire connector.

The EFI temp sensor is also on the thermostat housing as shown in the FSM drawings.

There is a third switch also on the thermostat housing called the Cold Start Time Switch that controls the cold start injector.

 

[TobyB]

...or if the truck is just running stonkin' cold, that will confuse the ECU, too... although maybe not to that degree...


t