JEBSR5

Found my FSM and code 51 is "switch signal" for system, diagnose was "air conditoner switch on, idle switch off during diagnosis check" and trouble area is "a/c switch, a/c switch circuit, a/c amlifire (how it's spelled in manual), throttle positon sensor circuit, throttle position sensor, ecu."

AC was not on at any time during diagnostic or during no start.

Last time it had this symptom was code 12 "rpm signal for system and diagnose "no ne and g signal to ecu witin 2 seconds after the engine cranked". Trouble area was distributor circuit, distributor, start signal circuit, ecu. Cleaning fixed it that time, but this time didn't help.

Code 12 turned out, I thought to be fixed by cleaning connections to coil and ignitor. Didn't get that code this time, so that cleaning could have fixed the code 12.

Former had same symptoms, but different code.

None of this points to AFM, but fooling with it by moving leads to maybe misconception it is AFM.

I don't know.

dlbrunner

unless she is blowing white smoke, thank the 3.0 gods it is keeping it's oil and water seperate.

Mine idles up and down a little too, but I chalk it up to needing a tune up, or the inherent 3.slojuju

rdharper

Two more thoughts. The codes don't always point you in the right direction... in fact that is understating it. This is partly because something can cause a problem which the ECM tries to compensate for, causing something else to "code".

Also, if it is the AFM, it will get worse with time, finally to the point where you are lucky to get it through an intersection when the problem shows up.

Thought you said you moved the plate. If so, that would have moved the pickup from the discontinous point (if there is one). So next time you get a no-start, try that again, as it is relatively easy to do. Also, you may notice hiccups while driving from the same problem... which would get more common with time. So that is my experience with the AFM.

In my own case, when I was finally convinced it was the AFM, I dug out the RTV and lifted the black rubber plate on top. Once that was off, it was obvious what the problem was. The resistive plate the contact slides on as the gate moves, had a groove worn in it. So, as I said, if that is the problem, you will have lots of further head-scratching opportunities. If the problem doesn't change, then you can figure its something else.

I could not fix the internal plate, although I tried, finally paying full retail to make sure I got a new one, as it was clear they wear out. In the case of the '85, there was 200k on it. So they last a long time (one way to look at it).

Jay351

My truck likes to do that.

Will usually stick around 1000rpm but somtimes at a light it will stay at 1100-1200. or after I turn it on after a longer trip it will idle at 600-800.

Truck has a fresh tune up and whatnot, always thought it was normal as the truck runs great........great for a 3.slow anyways

JEBSR5

Funny you should mention that, mine does that also. Runs great, idles a little high on cooler mornings, even after 50 miles. Later in the day it settles down to 800rpm. I just always figured the coolant senser was loosing it's values from age or something. I know it's getting good flow around it, checked that.

Don't know if that type sensor can deteriate slowly or not, but sures makes sense. Probably not true though.

JEBSR5

Already have that idea in mind to move the AFM flapper first if it does that again. Really runs great now when it's running. Smooth, no hesitation, rarely hear a spark knock.

I'll wait and see before digging into AFM lid.