I just bought my 91 F150 XLT Lariat two months ago with 90k miles. When I bought the truck the previous owner informed me that there was a problem with the idle and that it will go away if you recharge the AC. I thought that was a strange but as time has passed on I am curious as to whether there is something there or not.

The truck has a hard time finding it's natural idle point, which should be around 700-800 rpms. Normally it will wander between 800-1400 rpms. The most troublesome aspect of the problem is that between gears (5 speed manual) the rpms will shoot up as high as 2800 rpms when the clutch is pressed to the floor. It seems to be related to how high the rpms where when it was in gear. For example, if I'm driving along at 1500 rpms and go to shift gears, the rpms will shoot up to 2000-2100 when the clutch hits the floor. But if I'm driving along at 2000 rpms and go to shift gears, the rpms will shoot all the way up to 2800 rpms.

There was a problem with the thermostat so I replaced that. The throttle body and cables seem to be free and in good shape, so I've ruled them out. I just recently replaced the air idle control valve and the problem remained the same. At this point I'm confused and wandering what else I should be looking at. And I'm curious about how recharging the AC would have made the problem go away before for the previous owner.

 

I do not see any thing to the A/C other then it may load the engine some and keep the rpm from going that high.
The only thing I can think of that might cause your problem is a bad TPS or FPR.
First go ahead and read your CM codes and post them back here.
Here is how.
To read your CM codes or check for other types of codes and to run the self test go to one of these links below:

This is good for the KOEO test:
http://fordfuelinjection.com/?p=13

This one is good for the KOER test:
http://fordfuelinjection.com/?p=94
OR
http://www.broncodata.com/tech/codes.htm

Or get a code reader, here is a good one.
The EQUUS #3145
http://i54.photobucket.com/albums/g9...CodeReader.jpg

 

The CE light came on for a while yesturday and then went away. That was the first time it had ever happened. I'm taking the truck to the garage to get some old nerf bars put on and I'll have them check the codes while it's there.

 

TPS needs to adjusted on replaced.

 

Went to the garage and had them check the TPS and check the ECM codes. The TPS is working normally but here are the codes from the ECM:

Code 25 - No knock signal
Code 31 - EGR circut out of range
Code 32 - EGR signal low
Code 41 - EGO not switching, lean exhaust bank 1
Code 52 - Power steering switch open

I'm not exactly sure where to go from here. Any suggestions would be helpful.

 

Service Code 25 says most likely he did not do the Dynamic Response test during the engine run test.

Service Code 31 indicates that the EGR Valve Position (EVP) sensor signal is less than the Self-Test minimum value of 0.2 volts.

NOTE:
Because the EVP sensor is preloaded when attached to the EGR valve, a failure in the EGR valve causing the EVP sensor to lose preload may also set code 31.

Possible causes:
-- Damaged EVP sensor.
-- Damaged EGR valve.
-- Open harness.
-- Grounded harness.
-- Damaged processor.

Service Code 32 is about the same as the 31.

Service Code 41 system indicates lean, bad PCV valve?
Remove the PCV valve from the valve cover. Inspect both rocker cover hole and PCV for damage, sludge build up, blockage and movement of valve plunger.

Service Code 52 Again the engine run test was not done right. In short most likely he did not turn the steering wheel during the test. This is code is for the 5,0L engine and not for all engines so he may not have been aware of this.

 

I replaced the EVP sensor and nothing changed. I am going to replace the EGR valve and the PCV valve later in the week.

One thing that I don't understand is that the RPM's can be wandering like they do, but if I turn the truck off and start it again the problem seems to go away.

 

I would go along with what dvldog458 said above for the idle and replace the sensor.

Did the code 31 go away after you changed the sensor?
Did you disconnect the battery for 15 minutes after you changed the sensor?
You need to do this after you replace any sensor.

 

The code stayed, but that may be because I did not reset the system after I replaced the sensor. I'll give that I shot this weekend before I give up hope on the EVP sensor. I'm also going to check out the TPS.

 

At this point I have replaced the PCV valve, the TPS, the IAC, and ordered an EGR delete set-up from Ford Fuel Injection. Through all of this the idle problem has changed, but stayed none the less.

The idle will stay high on start-up (1100rpm's or above) and go up to 2000rpms or higher through every gear UNLESS I turn the truck off and then turn it back on. When I do that the truck runs normally (CEL from the EGR Resistor).

I'm starting to run out of ideas on this one. Maybe the O2, ACT, injector problem, maybe something in the fuel delivery? I've check all the vacuum lines I can find and they all seem to be fine.

 

Have you pulled codes since you replaced the parts? For the record, just becuase you dont have a CEL doesnt mean it wont show codes.

 

I don't have a code reader but I take it to the garage after every change. The only code that comes up is #33, which is basically just the EGR resistor telling the computer that the EGR valve is closed. Ford Fuel Injection says that the ECM will throw this code when using their EGR delete kit.

 

The problem still has not been solved. The 02, TPS, MAP, ACT, and the coolant sensor have all been replaced.

I noticed that there is something weird happening with the IAC. When I pull the plug on the IAC when the engine is running, sometimes the truck will want to shut down and sometimes the idle will be unaffected. I have replaced the IAC but it still does the same thing. Is that indicitive of a vacuum leak somewhere?

 

I've had my 89 f 150 with a 302 for 13 years. Does the complete same thing your describing. I've replaced,tested and bitched at every sensor on the truck. My trouble light has not come on in many years so I can't really pull codes,and yes i have tested the light to see if it works. One thing I have noticed is if you start the truck and immediatly take off,I have a 5 speed too, it will not do what you describe,but if you start it and let it idle a while you will have to turn the key off then back on when you take off,pretty easy with the 5 speed! I can't say i have heard this problem with an automatic trans,so i'm blamming it on a glitch with the ecm on the 5 speed trucks.