I have a 2001 F150 Supercrew with the 5.4 L engine with 48,500 miles on it. Just today I started my truck up and the idle kicked up to 1000 rpms-normally normal because it is a little cool today. However, then the rpms just keeped increase the truck got up to 2,500 rpms then I turned it off. It keeps doing this. I drove the truck to see if that helped and I noticed on the slower road that I had to ride the brake just to keep it under the speed limit. Can anyone help me? Would like to try to fix it myself so I dont have to waste a ton of cash at a dealership.

 

throwing any codes?

if not... maybe your IAC is sticking, or a possible vaccuum leak in the upper intake?

 

No I do not have any lights on. Not sure about codes - did not see any.

 

Did it run rough when it is cold. If rough when first started then smooths out, just very high idle, you would have a very large vacuum leak. It would be rough when fueling from the MAF because the MAF wouldn't know about the leaked air and smooth out in closed loop because the O2 sensors would fuel correctly regardless of where the air came in. If it's this big a leak, try looking at the PCV valve and hose or the vacuum brake booster and hose. Unplug both hoses and close the opening with your thumb to see if it quits.

The only other real possibility is through the air intake. This means the throttle plate is sticking open (pull the air hose off of it and make sure it closes completely) or the IAC is retracting. If it is the IAC and it will start without overspeeding at first, just start it and unplug it at some modest high (say 1000 RPM) level until you can replace it. At least if you unplug it and it stops changing you would know that's what it is.

 

No, it is not rough when it starts out. It starts up just fine and idles at around 650 then the idle starts to increase to 1000 then up to 2000 then I turn the truck off. Idle sounds like it will just keep getting higher if I let it. I think I will try the IAC maybe that is what it is. Can I get this at any auto part store? Sorry for my ignorance, don't work on vehicles much. Thanks for your help.

 

To see if it's the IAC, when you start it and it's idling fine, unplug the IAC electrical connection. The IAC should stay where it is. The idle won't correct for small stuff but should run just fine. If that fixes it, the IAC may be bad, or dirty, or maybe the computer is commanding it to do this. The IAC isn't expensive (maybe $70), but I'd hate for you to replace one if that isn't it.

Maybe a Ford tech like bigbronc can help here. Long shot, will this engine run off the CMP? If so, could the IAC get it's RPM from the CKP and the CKP be going out? Causing the computer to order the IAC open?

 

Well, I replaced the IAC and sorry to say that apparently wasn't the problem. It may be my imagination but it did seem to help just a little. However, the idle is still starting out at about 1,000 then increasing to 1,500 and sometimes as high as 2,000. Anyone else have any ideas or do I just need to break down and take it to the dealer?

 

Sorry man, that's why I suggested unplugging it as a diagnosis method. To idle faster, the engine has to have more air. When in closed loop, no matter where the air comes from, the PCM will use the O2 sensor feedback to give the right mixture for it.

Three categories of air for the fast idle:

1. Through the throttle body. It could be stuck open or the cruise control could be haunted. Very long shot, I really don't think this is it but just to CYA, pull the cruise control fuse. If the idle goes up by itself, it isn't a dirty throttle body, sticky throttle cable, etc., but if it just doesn't go back down after giving it gas it could be. If you have throttle by wire (no throttle cable, just a wire from the accellerator pedal, look under the dash and see), it could be a APP (name on GM vehicles) that measures the acellerator pedal position or the throttle actuator. Worth looking into.

2. Through the IAC passage. There are three possibilities here, you've eliminated one of them, (1) dirty IAC passage causing sticking, (2) bad IAC, (3) computer improperly commanding the IAC. This truck won't run without a crank (CKP) signal which is where it gets its RPM signal so that isn't it. But it could be that the TPS is indicating that the throttle is not at idle and then following it's own mistaken RPM. Look in the throttle body to see if it is dirty enough to hold the butterfly valve just barely open. Or the TPS could be bad.

3. Pirate air. Air that isn't coming through the throttle body or being measured by the MAF, AKA vacuum leaks. Hard to explain a vacuum leak that gets worse as time goes on but I have heard of them. Here is a list of where to look:

1) Vacuum hoses loose, cracked, broken
2) Brake vacuum booster diaphram broken
3) intake manifold leak
4) PCV valve/hose

Vacuum hoses go into the cab for the HVAC controls, to the EVAP and EGR systems, to the brake booster, basically all over. Instead of tracing each one, pinch them off. If pinching them shut doesn't slow it down, tracing it back to the far end won't help.

Carb cleaner or propane won't help find a vacuum leak in closed loop (after O2 sensors warm up). The PCM is already commanding the 14.7:1 A/F based on the O2 sensors, spray propane and the engine will correct immediately and the engine speed will stay the same. Disconnect one of the pre cat (between the exhaust manifold and cat) O2 sensors and it will stay in open loop and carb cleaner or propane will work again. Waft some propane (cleaner and safer) around the intake (front and back more likely than the sides) and where the throttle body bolts to the intake manifold and if the RPMs go up, you have a leak.

What I would do, not having seen it, is remove the cruise fuse and: (1) disconnect the IAC electrical connector to verify it isn't the IAC increasing the idle, (2) pinch off the vacuum lines including the brake booster line for 10 seconds each to see if the idle comes back, (3) pull an O2 sensor electrical connector and waft propane around the intake manifold and pull the breather and waft some propane into the block (I know the danger, you shouldn't do it but I probably would) to check for leaks at the bottom of the intake manifold gasket, (4) watch the throttle actuator (if throttle by wire) or the throttle cable to see if the throttle is opening.

I think I could do all 4 of these in 15 minutes. Good luck.

 

Okay, I removed cruise fuse and here is what happened.
1. Disconnected IAC connector and the truck died. Tryed restarting and died again.
2. Pinched off vacuum hoses and idle did not come down. Not sure I got all vacuum hoses but I think I did.
3. looked at cable between gas pedal and engine did not notice it moving and i pulled back on it to see if idle would slow and it didnt.
4. Did not have any propane so I did not do this but it doesnt appear to be a leak anywhere that I can feel or hear.
So does this confirm my suspicion - computer problem?

 

Wow, you really did do this. We need a Ford mechanic. On GM IACs, if you pull the connection to them, they stay at the current position. Ford IACs may be different. I'll go google that and see what I can find. That's what I'm thinking now. Better yet, I'll go down and pull mine now. Back in a minute.

 

check the throttle body for dammage or sticking open, check the voltage at tps, key on engine off must be less than one volt. also after installing IAC did you clear the computer memory, any time components are changed yo have to clear the computer. also you ma y want to moniter tps voltage at idle.
Also check pcv valve can be stuck open check pcv vacume line and block off other possable vac sources.

 

OK, I was wrong, I think. I cranked, pulled the IAC connection, and the RPM fell to about 500RPM. I assume this has the equivalent of the OBD-I MAR (minimum air rate) bypass on it. Sort of a bypass around the IAC bypass. I put it back on and got a strong, maybe 1400RPM flare, assuming the PCM had been commanding it open for the low idle.

I shut the truck off, disconnected it again, started it and got a low 500RPM idle but now it varied from 250-750 RPM. Assuming the PCM was playing with spark timing to stabilize idle and got hunting instead.

But I had time to think. If you have help, start truck, have someone crack the throttle to 1000 RPM and hold it while and after you unplug the IAC. Unplug the IAC and see if the idle still climbs. If it normally does and then doesn't, it would be the IAC creating the high idle. It won't tell you if it's being commanded or not, but it's a start.

 

chucks,
The throttle body is in pretty good shape and the idle increases itself so it doesnt appear to just be sticking. I don't have the equipment to check voltage (don't normally have to work on vehicles).
Yes, I cleared the memory when installing the new IAC. I unhooked the negative battery cable and left it off for about 30 minutes and then turned the head lights on to make sure all charge was gone.
Could the tps cause a problem like this?

yawr,
I will try that tomorrow. I will have wife hold truck rpms up while i unplug IAC and see what happens. I just have a bag feeling that it might be a computer problem for some reason it might be telling the IAC to stay open. Or could the tps cause something of this nature to happen?

 

danman, computers are often blamed but rarely faulty. They do go out very infrequently but this would be a baffling symptom of a bad computer.

You do bring up an excellent point. There are two modes the IAC operates in. At closed throttle it controls the RPMs selected by the computer for idle under those conditions (cold engine, right after startup, etc.). At open throttle, it acts as a throttle or RPM follower, opening more and more as the throttle is depressed or the RPMs climb. The purpose here is to catch the engine gently if the throttle is snapped closed. Some vehicles follow the throttle, some the RPMs. Maybe a Ford tech can tell us which it does on this application.

The TPS is the sensor used to detect whether the throttle is closed or not, and if this IAC uses throttle following the sensor it is following. So, theoretically, assuming I don't have my head up my *ss, it could follow an errant TPS wider and wider open causing this problem.

Even though I didn't understand Ford IACs earlier, here I go into the unknown. Try unplugging the TPS. The computer will notice and set the check engine light and throw a TPS code. It may go into limp home mode or it may just ignore the TPS and leave the IAC in idle RPM control mode, which would work it just wouldn't follow the throttle. Anyway, if the idle doesn't run away without the TPS connected, then the TPS probably needs replaced.

Any Ford techs out there to second guess this. We have someone who is actually trying to work on his truck.

 

Okay after reading a post a few down by bad he indicated that many people have complained about new IAC from autozone. That has me wondering if maybe I just got a bad part from them. He indicated that he noticed that the new part did not have a spring in it so he took the IAC back and got another one. I notice right off when I replace the IAC the other day that my new IAC did not have the spring in it like the old one did but figured "oh well". Guess maybe I should take IAC back to autozone and show them this and see if the replacement IAC they give me does the trick. Long shot - maybe but probably worth the try since other people are indicating they received bad IACs from them.