I own a 1990 f150 351W with an idle speed related problem. The idle speed doesn't fluctuate during trips, but changes between trips. For example, on one trip the truck might idle at 600 rpm, but on another it idles at 1000. Any suggestions? Thanks guys.

 

What's the mileage and is it auto or manual? Other than the idle fluctuation, how does it run? The most common causes would be sludge build up in the throttle body (TB) and/or sticking idle air control valve (IAC). If you haven't cleaned the IAC or TB, I would definitely recommend cleaning them with a can of carb spray and a old toothbrush. Clean the TB and remove the IAC valve to clean. If it were mine, I also would pull the throttle body to clean and replace the TP sensor while it was off due to it's mounted on the bottom side of TB and one of the most common sensors to cause intermittent driveability issues such as you have described - look at it as preventive maintenance. Also disconnect the battery to reset the base idle strategy. But it could also be a intake vacuum leak - with the truck idling, spray around the upper to lower intake area to see if the RPM changes. But your issue could also be related to a faulty sensor such as a TP sensor, EGR valve, O2 sensor, ECT, MAP, etc...you first need to scan for codes and go from there.

 

Thanks for the reply, cts! The chassis has over 200K miles and has an auto transmission, but the rebuilt engine I put in has under 5K. I recall cleaning up the TB pretty well during that time. The IAC, O2 sensor, catalytic converter, EGR valve/sensor, spark plugs/wires, rotor, distributor cap, and TPS each have under 12K. I've never replaced the distributor or the MAP. I believe the problem didn't start until after I replaced the engine. Other possibly related symptoms the truck's experiencing include a drop in power up hills, rough idle, and poor fuel economy (these trucks on average are supposed to get about 12 mpg while mine's getting about 9.7). I ran the codes a few months ago and I got "EVAP control system fault / voltage higher than closed limit". Do you know what this might mean? I'll run the codes again soon and will disconnect the battery for a while as you suggested.

 

Our EECIV trucks didn't have an EVAP system until the '96 MY which was EECV. This leads me to believe the code you were getting was a 34 - EGR valve position (EVP) voltage above closed limit. This would definitely cause all the issues you are having. Does your truck run and idle normally when cold but mostly acts up when at operating temperature? If so, then your EGR valve is not closing completely, which would set the code 34. You could have a faulty EGR valve (physically stuck open), wiring concern, faulty EVP sensor (on top of EGR) or incorrect one (should be grey), EVR solenoid or PCM (not likely).
The EGR Vacuum Regulator (EVR) has a filter which is located under the cap - which is nothing more than a piece of foam but it can get clogged with dust. It filters incoming air for the release function of the EGR. If air cannot enter the EGR diaphram, it won't release once opened by vacuum.
Make sure that your EVP sensor is grey and is not over tightened and that it has the oring under it. If not, then it will cause the the closed voltage to be higher than the 0.67 volt minimum and, as a result, code 34 will set. However, I don't believe this is your issue because your truck also has a driveability concern.
I would temporarily disable your EGR by disconnecting and plugging the green vacuum line on the EGR valve and see how your truck runs.

 

You are correct, it was a code 34. I'll try disconnecting it as you suggested and I'll get back you with the results. If the problem is as simple as cleaning the EGR valve, that would be great!

 

Hey, cts. I reset the codes, drove the truck for a while, then checked the codes. Got Code 34 again, so I went ahead and replaced the EGR-VPS and sprayed out the EGR valve with carb clean. Mileage seems to be better and it doesn't seem to be having that wildly fluctuating idle issue between trips anymore. Thank you for your advice.