Been having an ittermittent problem that is getting worse. At what is apparently random intervals, my '97 F150 (5.4, E4OD) will go into a high idle ~900rpm, in gear. It started about a year ago and would only do it about once every 3-6 months. It's getting to the point of doing it every other day. It just doesn't do it when I have my code scanner in hand.

Has anyone heard of this before? Anyone know of fixes to look for?

Oh, yea: It happens only when the engine is warmed up, and will clear when I restart. I have been able to restart & clear it sitting at a light, or rolling down the street.

I can't really leave the scanner in the truck, apll the time, as the summers in phx get a little hot. (105-115)

TIA,
Tony

 

maybe throttle position sensor, you can pull it off pretty easy and check it with a multi meter that is the first thing i would check, hope this helps

 

Thanks! I'll give it a look this afternoon.

 

So, I hooked up my scanner to it last night; No codes; And, according to the absolute throttle indicator, the TPS is normal. I'll have to wait until it fails & keep my scan tool with the truck.

Any other ideas?

Tony

 

Well, it actually failed on the way to work yesterday. Stopped, left it idle & looked at the tps with the scan tool. Appears normal: 18% at idle when running normal and in the 'bad' way. I just haven't had time to pull it for a bench test.

As usual, turn it off & restart to a normal running truck.

Anyone expereince this problem?

Tony

 

Have you tried cleaning the IAC ? These things get gummed up and cause all kinds of idle problems.
I have to clean the one on my 97 5.4 about every 20 thousand miles.

 

864Lightning,
Thanks! I'll have to look at the manuals tonight for location. Refresh my memory...IAC = ???

Tony

 

IAC I believe stands for "Idle Air Control" - its a little stepper-motor type module that sits behind the throttle body and controls the engine's idle speed by routing a varying amount of air (controlled by the computer of course) around the throttle butterfly which is mostly closed at idle.

 

OK, so I replaced the IAC. Since the high idle was intermittent, I won't know for a while if it is fixed. Now I have a truly intermittent idle. In park, idle is a consistent 750. In gear, it will vary from 750 to 500, rarely down to 350, and even stalled once. Could this be a bad IAC? Oh, yeah, I forgot to reset the computer. Will do that tonight.

Any thoughts?

TIA

Tony

 

That sounds like a vacuum leak. Check the PCVfor being clogged then the grommet and the hose for cracks.

Regards

Jean Marc Chartier

 

JMC, Thanks. I also had that thought shortly after I posted the update. I'll recheck vac lines tonight, and spray wd40 around the IAC gasket & see if that improves idle. Old trick; WD temporarily plugs the vac leak.


Thanks!

Tony

 

OK, so I didn't check for a vacuum leak leak yet (no time, gotta fish!). I did notice this morning, that the AC compressor cycling affects idle, ranging from 500-750. I know that the idle speed will change to accomodate for the sudden load change. This would seem to support JMC's contention that there is a vacuum leak, as the computer is unable to (transparently) compensate for the load change.

Just a thought.

Tony

 

Don't know if you tried this or not but reset the computer by disconnecting the negative battery cable for a few mins. Since you got a new IAC valve it may need to recalibrate itself. Or if you did this it may take a little bit of driving to recalibrate with the new IAC valve which may be why it's fluctuating more. Just a thought.

 

sloshua,
I keep forgetting to reset the PCM. Will do this weekend.

Thnaks!

Tony