hey, I got a 1986 ford f150 that I just bought. it has a fuel injected 302 v8 with an AOD auto tranny. its got 68000 original miles an runs an goes good, but it has one propblem. whenever you start driving down the road and let off the gas to come to a stop, the engine will idle way down and stall out. it will start right back up and idle all day just sitting in park or in neutral. whats wrong here? is there some problem with the torque converter? please help me, I have many old fords and some moderate experiance with these age trucks but this one just bogles me. thanks

 

First thing to do when diagnosing any computer-controlled engine is ask the computer what it thinks is happening.

Here's how:

Ford Fuel Injection » How To Run a Self-Test
Ford Ranger/Bronco II EEC-IV Testing

Sounds to me like the TPS or maybe IAC might be involved.

 

And when you run the codes expect to see the one about the Idle Air Controller. My guess is that's the issue. But, as ctubutis said, you do need to pull the codes as you can spend a lot of money throwing parts at it and still have problems if you don't do it systematically.

 

i have already been checking the idle air control sensor. i read up on it in the haynes manual last week, and the connector was reading 12 volts which the book said was normal, and the sensor had 10 ohms of resistence, which the book said was also within the normal range. so i took out the entire sylonoid and checked to see if it was stuck, i put some penetration oil in it and moved the rod around in it, it seemed to be fine. i replace the sensor with another used one off of another 302 motor of the same year I have, and it made it start and idle better, but it still stall when coming to a stop. i will check the codes tonite, and perhaps just buy a new iac sensor. thanks

 

Throwing parts at it can get expensive (and is a waste when doing so doesn't fix the problem).

 

well i pulled the codes with my code scanner, and codes 16 and 23 came up. and when i looked in the book that came with the scanner it says that code 16 is idle speed control RPM out of self test spec. and code 23 says throttle positipon sensor signal voltage out of self test spec. it also put out code 13 which said RPM out of spec during normal idle operation. so does this mean I have a bad TPS? if so than it looks like im going to have to pull the entire top half of the intake off just to change that sensor.

 

Executive Summary:

Yes, I'd suspect the TPS.

Longer Version:

I have never used a code puller, only counted flashes and then used the error definitions from those two sites I posted above.

Different documentation uses somewhat different text to define the errors and that can be helpful.

In any event, given your codes combined with your tests of the IAC would lead me to look at the TPS.

I don't recall how to test them, there's an ohm range they're supposed to exhibit as they move but it should be in the archives here or online someplace.

There is also my having played with this stuff on my own DD, I was getting similar errors as you yet I was fairly confident the IAC was OK.

I replaced the TPS with one I got from the junk yard and the errors went away.

I don't recall what is needed to get at yours but there is a helpful tool that's basically an extended Torx wrench; not sure if it's needed to just get at upper stuff or not, it might be needed only to remove the entire lower intake manifold.

If I were to throw a part at it, this is what I would replace.

In any event, this guy says it best but I think the price has gone up $5 since posting this:

IIRC somebody said Harbor Freight has a set for significantly cheaper but don't know for certain.

 

Racingaxe - did you follow the testing process when you pulled the codes, inc the one to blip the throttle at the appropriate time? If not the you would get some of those codes even if the hardware is good. Been there, done that.

 

yeah i pulled the all the codes the way the booklet that came withthe scanner told me, even. i went out and got a TPS and as it turns out i did niot have to take the intake off, just the throttle body, an wellah shes fixed! she starts right up and doesnt stall any more when coming to a stop, and it eeven kick down into the next lower gear like it should when you give it a little gas, before you had to matt it get it to shift. ive got to say thank you Ctubutis, and Gary Lewis for helping me.

 

When you were at stops, did your idle surge up and down, kinda like it was hunting for the correct rpm range?

 

Glad you got it fixed. It can be a real bummer to have that problem, but when right they sure run nicely.

 

Thanks for the update, axe! Glad we helped.