Mine normally idles at 500.

 

Hum, then maybe that's not my problem, but I could swear it normally idled around 650 to 700 rpm. Hard to tell by looking at the factory tach.

 

Mine idles between 500-600rpm.

 

I had a leaky brake booster cause this on my 86 Jeep. Plug the hose from the brake booster to the vacuum source and see if it changes? The only other thing I can think of would be a shorted brake light circuit that's putting a load on the alternator and stalling it out or bad battery. At idle it doesn't take much.

Thinking out load here?

Does having the A/C on help by making the idle higher?

Having the headlights on make any difference?

 

Yes, when the A/C is on, I don't have this problem.

 

Try disconnecting the vacuum hose to the brake booster and plugging the hose. Does the brake pedal still cause a stall? Using the A/C increases the RPM and will make such a leak less apparent.

 

Thanks for the tip and I'll try that the first chance I get and report back.

 

I'd clean the idle-air passage.

 

OK, where's that located?

 

Which engine?

 

Sorry, it's the 4.7L.

 

Ok, I found the Idle Speed Control Motor on the side of the throttle body and took it out and cleaned it up along with the vent section leading to it. Still no improvement so I found an OEM one on eBay and purchased it hoping this takes care of the problem. Otherwise I'm lost for other possible alternatives.

 

You need to also make sure the passage is clean. If I recall from my 4.7L (and there may be a post here from me on this), the idle passage is clearly visible when you take the intake tube off the throttle body. It's a slot-like area.

My general process is (with the engine idling):

• 
  Start gently spraying carb cleaner into the idle passage.
  If you're careful, you'll be able to trick the PCM into raising the idle to match what you're spraying in
  Do this for several seconds to get a fair amount of cleaner running thru the passages.
  Then just let loose with the cleaner at full blast, which will stall the engine
  Fill up the passage with cleaner and let it sit for maybe 15 minutes
  Restart the engine.

You'll have to feather the throttle a bit until the PCM figures out which end is up and idle might be a little shaky for a few starts.

This may or may not solve your problem, but it's easy and quick to try.

?tom

 

Thanks Tomk

 

Typically a 4.7 will increase RPM if there's a vacuum leak, at least I know pulling the hose on the back of the throttle body will make the RPM go to 2K.

It could be as Tom suggests the IAC is just screwing up and giving too little air to allow idle, but then there's the brake pedal interaction. I wonder if a new battery would help. Brake application does two things. Turns on lights and uses some vacuum in the booster. Could just be a flaky battery and, when the lights come on at low idle, the voltage dips and the CPU gets confused because of the voltage drop or noise.

 

All good points for me to investigate. I'm going to start by replacing the IAC and then check out the rest.

Thx!