Having a problem that I cannot figure out.

Truck: 1994 F150 XLT 302 Fuel Injected V-8, Automatic Transmission

Here are the symptoms:

1. In cold weather, the electronic choke (controlled via the computer) will run the engine at about 2000-3000 rpms. On some days, the choke will stay on for 10-15 minutes. When the engine is running between 2000 – 3000 rpms, the truck does not have enough braking power to stop. Very unsafe and results in a quick shift to Neutral

2. In Warm weather, the truck will run fine for a little over an hour. After an hour of highway driving, the truck will get sluggish (like I have a clogged fuel filter). Initially, I did not notice the sluggishness because I was motoring in overdrive down the highway, what I did notice is that the transmission would downshift and then immediately upshift. As a result, the truck had extensive Transmission work to the tune of a complete replacement (including the wire harness that FORD has a factory notice about). After about 10-15 trips to the transmission shop, I am now convinced that it is not a transmission problem as the before and after is the same.

Here is a list of things I have done… to no avail:
1. Tune Up (including new wires, cap, and rotor)
2. New Fuel Filter
3. Complete Transmission Rebuild
4. Installation of a Transmission Cooler
5. Replaced all the electronics in the Automatic Transmission
6. The Transmission shop once got an engine code. The code was “311 Thermal Air System not working Bank 1”. Error code was not repeatable. In any event, an engine shop advised me that was the smog pump. I changed out the smog pump, no luck.
7. Installed a rebuilt Long Block for another reason, however, symptom same with new long block as with old.

Not sure where to go now… does anyone have any clue what might be going on? I am pretty sure I have some type of Thermal event going on.

Thanks

 

The computer looks at the inputs (ECT,IAT,) for tempature adjustments of fuel mixture and idle speed. It then controls the output(IAC or idle air control motor) to get the desired idle speed. You need to get access to a scan tool and look at ECT and IAT after it has sat overnight. They should read very close to the same on first start up. I would also unplug the IAC and see if it idles too fast, if so you have a vaccum leak, maybe upper intake. Also monitor the TP voltage. It should be about .9 volts - closed throttal. You could also have a sticking throttal body.

 

Thanks Canoe. Guess I may be having multiple problems.

1. Is the IAC the same as the IAB on older model Ford 302? I think you are referring to the solenoid that is attached to the side of the throttle body.

2. As for the scan tool, Will the simple models that you can purchase at the auto store do the job? Ones like Sunpro, etc. The Snap-On scanners that the auto shops use are pretty expensive.

3. Looking through some of the other posts on this board, for the “311 Thermal Air System not working Bank 1”, I am leaning towards an Oxygen Sensor replacement. It was not repeatable, but those sensors have a bunch of miles on them.

4. One thing I have never had a clear answer on, is how do you monitor TP voltage? Do you have to splice into the wires to hook up the multi-meter or is there another way.

Thanks again for your help.

 

1 Yea IAC is the same as IAB

2 I only have experince with factory scanners, dont think Auto Zone ($200 or less scanners) will read live data stream. You could use a digital voltmeter. You would just have to get a good book to change voltage redings to tempature. Snap will work but expensive.


3 Never heard of thermal air system. More likely Thermactor air system. Make sure your Thermactor(smog pump) and related plumbing are intact. There should be a pink and maybe white vaccum line(check you sticker) going to the diverter- bypass valve.

4 Check TP with a scanner. If not available use a digital meter and backprobe the wires. (3 wires) With key on 1 of the three will alway be around 5 volts(Vref). Another will be ground 0v(signal return). The 3rd will vary from about .9 at closed throttal to 4.5 volts at wide open throttal. (signal wire)
With 1 meter probe on the (signal ) wire and the other on (signal return) wire, the reading should change as you open the throttal.