I have a strange problem with my '91 f-150. My truck has a 302 and a 5-speed. Hear's the problem, whin I start the truck it idles about 1500 rpm. After about a minute it slows down to about 1100 rpm. Now whin driving the truck and I go to shift gears insted of the motor reving down, it stays at 2300 rpm and it will stay there for about two minutes. Ive checked the throttle cables and nothing is sticking that i can see. Now hears the weird part. Whin I start the truck, if I rev it up just a tiny bit. The idle goes down to 800rpm and the truck runs perfect. Shifts are all good. Is this a common problem? Any advice you guys got would be verry much aprechiated. Thanks.

 

I've had this same problem, but I fixed it. One of the butterflies in the TB was sticking. A thorough TB cleaning will fix it.

 

The IAC could also be sticking, or perhaps the EGR valve. Most anything that can dump usable air into the intake will cause fast idle, but normally introduces some rough idle and if bad enough, severe bucking when throttle position is changed because the vacuum drops sharply and the EEC doesn't know how to trim the timing and fuel mix for the excess air. If you're having some driveability issues at low RPM in higher gears, take a peek at my gallery and see what I found on mine. This cleaned up the low-end miss and weird idling problems on mine. Teardown takes about an hour, and re-assembly about 2 hours. The clean-up takes the longest to get rid of all the build-up in the throttle body and EGR/IAC ports. If you have the time, go ahead and make one other modification (which I didn't do). Search for PCV mods in here or the 302 forum, and you'll find a recommendation for relocating the PCV hose from the back end of the plenum to a central location. This is supposed to help stop the crankcase gasses from going entirely to #4 and #8 cylinders and distribute it to all of them. I wish there was a way to do the same with the EGR as most of that seems to go to #1 and #5 because of location. This looks like a pretty simple modification, but I just didn't have the time when I had mine down. It's my daily driver, so I have to be picky about mods I do based on time available.

 

ONE was sticking? How can that be? They're on the same shaft. If one is open, both are open, unless something really weird has happened to that shaft.

 

One was getting stuck, impeding both butterflies on the same shaft.

 

I had a similar problem and i cant remeber what the exact name of the part was, something like air temp sensor. It is a small sensor that screws into the manifold and detects the incoming air temp and adjusts the idle accordingly. Either that if could be a sticking IAC which can be cleaned or replaced.

 

Not to ask a stupid question but what exactly is the IAC?

 

No such thing as a stupid question, except the one that doesn't get asked.

The IAC is the Idle Air Control Valve. It's mounted on the side of the throttle body and straddles the butterflies in the throttle body. When the engine is at idle, the butterflies should be completely closed. This is where the IAC comes in. The EEC will monitor RPM, and if it drops below 672 RPM, it will open the IAC to bypass the butterflies and raise the idle back up. This is not the only job of the IAC, though. When the throttle is open, the IAC will open completely. Then, when you let the throttle snap back shut, the IAC will close off slowly, and prevent stalling due to excess vacuum in the intake. There's also some other tricks that the EEC does with timing and fuel when you take your foot off the throttle and let the vehicle coast to help slow it down. It isn't as noticeable with automatics, but you can feel the timing being changed when you gear down a manual tranny when slowing down. This helps provide braking with the engine.

Since the IAC is in close proximity to the EGR (Exhaust Gas Recirculation) valve, sometimes the back port will gum up with carbon deposits from the exhaust. The IAC can be removed and cleaned without removing the Throttle body, but I don't recommend it. If the IAC's got stuff in it, so does the throttle body, and you're only going to knock it down in the intake if you try to clean it with the TB in place. It only takes about an hour to strip it down to the lower intake, which I would recommend doing so if the plenum's never been off the engine, and it has any significant mileage on it. The gasket set will cost about $8 at Advance Auto, and trust me, it will make a HUGE difference in the idle if there are leaks between the plenum and intake.

Have a peek at my gallery, and you'll get the gist of what I'm talking about. There's one bolt that's a little bit tough to get to, but it's a T40 Torx. You'll need one that fits in a 1/4" drive socket, because a 3/8" drive socket's shoulders are too big to get the bit in the bolt. I did it without pulling the distributor, meaning I didn't have to re-time the engine. The bottom half of the housing on the dizzy came off with two bolts, and gave me plenty access. While you're in there, you should probably consider o-rings on the injectors, minimum, and if you have time, get the injectors cleaned. Don't even think about new injectors. They're about $80/injector, so you're looking at $640 for a set. You can get injector rebuild kits from mrinjector on Ebay, which include the o-rings, filters, and pintle caps. I don't remember the price, but it wasn't very expensive.

Jump in with both feet, 'cause there's lots of great help in here, and you'll learn a lot about your truck while you're working on it. Best of all, you won't be paying a bunch of monkeys a ton of money to screw it up worse than it already is. Study the job, and ask questions. That's what we're all here for.

 

Ok now i remember what t is called.

ECT - EngineCoolant temperture Near where the top Radiator hose comes out of the engine.

 

Thanks alot guys, sounds like I have a few things to check out this weekend. Il let yall know how it pans out. Thanks agin.

 

Ok, so I cleanned the IAC and the throttle body, put it all back together and it still ran way to fast. so i took the actuator off the IAC and it runs fine. Does this mean the actuator is bad?

 

I would recomend getting the truck scanned. Mine was reving high and I took the lazy approach and just replaced the IAC because it was about the onlything I hadn't replaced.

Anyway my problems came back so I scanned and it said TPS. TPS was only a year old so I looked at the voltage and it was 1.1v. The nice people on this site said .8-.99v is where you want to be. I reset it to about .95 and so far so good. I guess it kept thinking I was hitting the gas a little...

Good luck

 

By scanned do you mean run a computer diagnostics?

 

yes, scan the computer for trouble codes.

 

Ok, thanks. Il give that a try.