I'm having a bit of trouble with the feedback carburetor on my '82 F-150 with the 300-6 engine (w/130,000 miles). Although the truck runs and drives well--reliably enough to be a daily driver--I cannot seem to get the idle stabilized below about 1,300 RPM's (adjusting it any lower causes to it oscillate or die). Now, I should mention that this is the dreaded California version of the MCU-controlled feedback carb...

I have pulled the codes from the MCU, and it throws only one: "63--CROWD Vacuum Switch is Closed Constantly." What precisely that means...I have been unable to determine, but it may or may not be relevant to the particular problem at hand.

Additionally, I can find no blatant vacuum leaks; I found a small leak at the carburetor base gasket--which I replaced. All of the feedback sensors/doodads seem to be in place, along with their color-coded vacuum lines, but since I do not fully understand how they all operate, much less how they operate in concert (and no local mechanic I've yet come across seems to either), I worry that it is some aspect of this Rube Goldberg-esqe system that is the source of my troubles.

Is there a way that I can narrow this down to whether it's the carburetor itself that is the cause of this, or one of the ancillary feedback mechanisms? Does anyone know what the actual function of the CROWD vacuum switch is, and where exactly it is located (my shop manual doesn't even mention it)?

Thanks in advance for any insights/responses!

 

This link says the 63 code is the throttle position sensor.

 

Those are the EEC-IV codes, Gary; he has an EEC-III system from California; we need Ralph for this one....

 

How much slop is in the throttle shaft of the carb? They can wear and the throttle blades won't seat consistently, and air can leak by the shaft.

 

Just unplug the feedback solenoid and see if it runs better.

Jim

 

Sorry. I missed that.

 

Thanks for all the replies thus far!

ctubutis: I believe I actually have the MCU, unless I'm mistaken and/or that's functionally the same as the EEC-III?

Franklin2: I don't detect significant slop in the throttle shaft, but how much are we talking? And what's the best way to get an accurate feel for this? I didn't remove the carburetor when I checked, but can if need be.

Jim: A few months ago, the truck began running horribly, belching black smoke out of the exhaust. Ultimately, I discovered that one of the wires going into the bottom of the feedback solenoid had broken off, and after repairing it, things were back to normal. Which is to say--it doesn't run well at all when it's disconnected, although I appreciate the suggestion.