Well, long story long, five years ago the idle on my ' 89 150 (302, stick shift, two wheel drive, 165K, bought it new) went up to 1400 from its usual 680rpm. A week later it went up to 1700 where it remained until last month, now it's 2100. I'm able to sense when the computer turns on the fuel when slowing. Just like German cars, it used to send in an idle mix at 1250 when slowing in gear. I'd feel a little bump acceleration. I've read the book on Bosch electronic fuel management and that's what it says. At 1250, the computer turns on the idle mix again. Now, I get it at 1750 consistently. Years ago, I replaced the coolant temp. sensor, no change. Sprayed wd-40 all over the intake manifold with the engine on searching for an intake leak, nope. Sprayed halon all over the heads looking for an intake leak, zero. And in line with that, it never back fires through the intake or anything. Tons of torque. This week, I replaced the throttle position sensor, nada. Replaced the idle air valve, no change. So-o I'm thinking, this points to a management problem. I disassemble the kick on the driver's side and start unhooking the electrical boxes there one at a time. The first one's plastic, kinda small, truck starts no change. Second one looks like the computer, steel case, like 14 pins, etc., nope, truck starts right up, acts like it'll run fine. Third one is plastic cased, truck starts fine. Truck runs with the three "black boxes" sitting on the bench seat. So, sorry for the length of this, I think it's the cold start feature of the engine management, does anyone know where that is? Thanks for the time.

 

I think you are looking at this too deep. If you unplug the Idle Air Control (IAC), located on the side of the throttle body does the idle still remain high? By unhooking the IAC you are taking the PCM out of the equation.

The steps you took to try finding a vacuum leak are perfect examples why neither method is reliable. I prefer to use a handheld vacuum pump/gauge to test each vacuum circuit. There are numerous vacuum reservoirs on these trucks that are quite capable of causing a vacuum leak, yet the tests you performed pass.

The first is the large metal reservoir on the passenger side fender. It looks like a juice can. It rusts through on the bottom side. The second is a small metal reservoir located behind the driver side head light for the vacuum operated cruise control. The third is a small plastic reservoir mounted on the blower case for the heater/AC/vent. It is plastic, but it can crack. Do not forget the yards of plastic vacuum tubing under the hood. Each one can pass the "spray" test but in reality they can have dozens of small porous cracks along their length that when added up cause a large vacuum leak.

I found the cruise control and HVAC reservoirs were the largest contributors to the high idle I inherited when I purchased my 1992 F350 a few years ago. The rest of the contributors were those pesky plastic vacuum lines and a bad upper intake gasket.

 

i had a similar problem with my truck. i went through 3 iac valves before the problem was fixed. i threw so many other parts at it until then. unplug the iac and see what happens. make sure to use motorcraft parts.

 

Wow, thanks, I'm on it.