I have an 01 Harley and have had experience with it reving very high when sitting at a traffic light. When this happens the tach and odometer both shoot to the max positions and the engine revs so high that if I'm not on the brake, i would actually rear end the person in front of me. This happens maybe 2 or three times in a year and started maybe a year after I bought the truck new in 01. I've taken it to the dealership where nothing has been found. Is there anyone else out there that has had this problem that could possibly give me a clue as to what's happening.

 

possible iac issue.

 

Thanks.....is there any way of testing the idle air control valve? You would think that the dealership would have suggested this, but now-a-days, if the light isn't on, they assume there's not a problem. My dad used to tune a 68 Camaro by ear, now a computer tells us everything.....technology, yikes!

By the way, i should have specified that the high revs only lasts for a split second, then the engine's back to normal.

 

I would go ahead and clean the throttle body and replace the IAC. Might be something else though. Being a guy who has spent many hours working on old cars myself I can say yes old cars are easier to fix but the bad thing is you gotta fix them a hell of allot more then you do todays cars. Clean the throttle body replace the IAC no mater if its the problem or not your truck will run better. Might be a vacum leak to. I'd inspect the vacum lines while your at it.

 

My 1999 F150 5.4 engine was having high idle up to 1800 or 2000 for a while.

It threw a P1120 code one day. I tied the wiring harness away from the accumulator and my high idle went away and no more P1120 codes.
Just some food for thought.

Larry

https://www.f150online.com/forums/sh...cumulator+tank

 

I don't like this. A very intermittent problem like he describes doesn't sound too much like a sensor or something equivalent.

If he is "drive by wire", the engine RPMs are controlled by the PCM. If it "thinks" the trrottle is being depressed, it will command high engine revs. So, is the problem a fault in the PCM, or is it in the throttle position sensor? And, which of these is likely to be intermittent?

Please understand, I am not familiar with a 2001 truck. I just know if I had a similar problem with my 2005, I'd suspect the PCM right away.

- Jack

 

JackandJanet, the 97-03 F-150 is NOT drive by wire. there is/was a physical throttle cable.

 

Thanks, minimonster - I'm happy to be corrected! :o

So, what else can apply a "throttle open" pull to the cable on your truck? This is the ONLY way the engine speed can increase if it's not computer commanded.

(I admit a woeful ignorance when it comes to trucks before my model year)! And, I'm still learning about mine, since I've only had it for about a year.

These forums have been a big help to me!

I need to add: The reason I didn't like the sensor diagnosis, is because I don't think they fail intermittently. My experience with sensors is that they are all good or all bad.

- Jack

 

The IAC can get stuck its part of the air intake system and can get fouled. The throttle body's need to be cleaned as maintenance anyway. The IAC is only about $45 at autozone it could be cleaned by when I did mine I just replaced it and mine wasn't faulty either but my truck did run better after.
Could be a vacuo leak or some other issue.

 

And, you're saying this could cause the throttle to "open" after coming to a stop at a liight? Can you elaborate?

No argument whatsoever about throttle bodys though. I know dirty ones can screw up your idle, badly. Don't vacuum leaks just make things run rough?

And, wouldn't the IAC just cause a "strolling" idle? Can they really cause the rpms to shoot way up?

Honestly, I'm not trying to argue here, I'm just trying to understand.

- Jack

 

Jack,

The engine can run rough or die if the air is added past the MAF quicker than the PCM and oxygen sensors can react.

The air to the IAC is routed around the main throttle and to the IAC ( Think of the IAC as an Idle throttle ). The PCM controls the air/fuel minute to the correct flow rates to achieve the idle rpm.

I experimented added some extra air after the Maf but not through the IAC one time and the rpms increase because the PCM sees the air/fuel ratio is out of whack and adds more fuel to correct it which increases rpms.

Larry.

 

Thanks, Larry and jethat. I guess what really threw me was the OP's statement that, "the tach and odometer shoot to the max postiions". I can accept that "odometer" was probably a typo - maybe he meant "temp guage"? But, saying the tach went to max - wow! On my truck, that would be 7000 rpm! This just seemed more than some Idle Air Control fault.

And, I thought IACs mostly got carbon fouled, which would lead to surging and stalling, probably happening all the time at idle. Not some 2-3 times a year event, but otherwise everything is fine.

What I was getting from his description was a random electrical spike that "commanded" open throttle. And Larry, your comment about the wiring harness seemed as good a possibility as any.

Like I said though, I'm a real newbe when it comes to f150s. (But not to cars - I've been driving and working on them for about 50 years now. Do the math and you can tell I'm a real old fart).

- Jack

 

Guys, I've read all of the threads and really appreciate what I've read. Jack, it was a typo. I meant the speedometer and the tach both shoot to the max positions, not the odometer. This happens within a second or so then all is back to normal. Other guauges may spike as well, but it happens so fast to where i can only focus on the bigger guages. I also thought it was a sensor, but like mentioned earlier, a sensor is nine times out of ten either all bad or all good. I'll get the IAC replaced and see if that makes a difference. It's hard to diagnose these 'blue moon' situations.

 

Ok, Im awake now Jack. I am not sure the IAC can let in enough air to cause RPM to go to 7000. Its sure not possible to have the speedometer go to max while sitting unless the tires are spinning on ice. LOL

The only other thing that could open the throttle is the cruise control.

Maybe the computer is turning on cruise control at 120 mph setting. That would really suck.

I had a Mustang in my earlier years and wore boots. I pulled up to my garage and got both the brake and accelerator at the same time. I drove into the garage door and broke it before I got the car stopped.

Did your truck get recalled for the fire hazard on the cruise control, kakings?

I think I would disconnect the cruise control and see if it happens again.
I would als find out how the cruise control works on your truck and check for a reason it might go to maximum.

Larry.

 

The IAC by its self will not cause the engine to surge. The engine needs to have fuel added for the rpm to climb. The PCM commands the rpm in gear at idle and adjusts the air and fuel to make the engine rev to the commanded rpm. A faulty tach signal could cause PCM to see a lower than actual rpm and the engine to surge because the PCM is adding fuel and at the same time increasing the duty cycle of the IAC. One way for a faulty signal to be produced is a momentary interruption of power to the PCM. This will cause a stumble and when the PCM wakes up again it will see very low rpm and quickly try to get the revs up to avoid a stall. This will also cause the gauge sweep that you see. Time to check fuses and wires. I will get back with the circuits to test. I need to find the schematics for an 01.

Regards

Jean Marc Chartier