Good Morning,

I have a 2001 Ford F150 SuperCrew with the 5.4L engine in it. The issue that I am having is that when the truck is at idle, the idle fluctuates while the heater is on. It basically acts the same as it would if the A/C was on. I have looked, and the A/C compressor is not engaging during this time. I have replaced the fuel filter, plugs and coils all within the last 5k miles.

Thanks for any suggestions,

Joey

 

How many rpm's does the idle vary?
Does it only happen when the heat is on?

If you're sure the AC compressor is not engaging (defrost on) i'd first eliminate a vacuum leak.

.

 

It will jump 200-300 RPMs about ever 5 seconds or so. I have looked for a leak, but might not hurt to go back through it.

A/C is not engaging for sure, I would expect it when the defroster is on, but it mimics that same behavior.

Thanks,

 

First thought, throttle body leak or IAC valve but that wouldn't explain why it only happens when the heat is enabled.
Don't know offhand if the HVAC system under the dash uses motors or vacuum.
If its vacuum operated and can locate the main vacuum hose going into the cabin, might try plugging it and see what happens.
Could be a vacuum leak, disconnected hose or a vacuum servo with a blown diaphragm in the HVAC system under the dash.

Other than that no clue, maybe someone else here has some other ideas.Thread bumped again.

 

Where does the engine idle with the HVAC turned off without change? If the engine idles higher than the baseline test at times, something is detecting a signal that the compressor is engaged and is directng the IAC to raise the idle speed to compensate.

Plactic vacuum line are especially problem-matic in the cold, but IME don't cycle good to bad...I may just not have seen this yet.

Brad