I have a 1999 Ford F-150 4.6L AT 4x2 Supercab, LB with approximately 172k miles. The truck is nearly perfect, except I notice that if I am driving at any speed, lets say 50mph, and I drop the shifter into neutral. The RPM act like they are going to drop to the sitting idle of 700 (which it does) but just as it starts to do that, the RPMs are moved to 2100 and sit there. The RPMs are normal when in drive at any speed. No lurching or odd behavior when the truck is in gear. I had a 2000 Excursions V-10 and it would drop to idle of 700, when coasting after dropping it into neutral, as all the AT cars/trucks I have had over the last 16 years.

If you keep the truck in neutral and slow down the rpms eventually reach 700. My first thought was the torque converter, but had it and the tranny tested by a tranny shop and all was normal. While the shop was testing it they did tell me the throttle position sensor and IAC valve were showing activity either reacting to/or causing the RPMs to move to 2100.

The problem has been there since I got the truck. I have since changed the spark plugs and wires (with OEM FORD), OEM Ford DPFE, and a Wells IAC valve. I have cleaned the EGR piping into the throttle body, and have no trouble codes.

My thought is a bad sensor telling the truck the wrong temperature, etc. causing the engine to adjust for a cold start or something. But that doesn't make sense, since the RPMs move towards 700 in relationship to the speed of the truck. This is why I thought it was the torque converter. But the 2100 happens regardless of speed say 40 vs. 80mph. It just takes longer to go from 2100 to 700 when coasting to a stop.

Any help would be greatly appreciated. Thanks in Advance!

Jeremy

EDIT: I searched and found someone with a similar problem, and they had a reply from a similar truck that did what was expected. (i.e. drop to idle) So the question may be where the computer controls the engine and tranny speed matching. I would bet that there is a sensor/chip that controls getting the engine RPMs up to match the tranny prior to locking in the converter, after being at idle with the truck moving at X speed. Mine might be bad and not allowing the truck to return to idle, thinking the transmission is still engaged (maybe a neutral sensor, etc) although with no load.

 

Do you have a manual transmission? Just curious as to why you are coasting in neutral... Not something that I ever find myself doing...

And if you are, say, driving down a grade, then coasting in neutral will use more gas, bacause you PCM will shut-down your fuel injectors when coasting down a grade in gear.

 

A faulty IAC will cause the high idle.

Steve

 

Ditto for me...

But since he's saying Torque converter that is only in an automatic... Soooo ... again ditto. Either way in a standard or automatic tranny, save your gas and your brakes by useing your gears. That is how it is designed to be driven.

 

Thanks for the replies, with all do respect and with the chances of sounding ungrateful, none of you read or answered the question. But provided opinions about a driving style of which I did not ask about. Which is fine, but not what I need.

1) I assume that the universal "AT" in a vehicle description means Automatic, my fault I will spell that out next time.
2) I mentioned that I just installed a new IAC, it is working.
3) Not arguing the reason or thought about coasting in neutral, but noting that the engine is doing something that is not normal. The reason for concern is not MPG, engine wear, or whether I should put my tranny in neutral at a stoplight, but trying to identify if I have a potential tranny or engine problem before it turns into something major.

Like I said no disrespect to your opinions, I appreciate the insight, but I am strictly trying to identify a potential problem or lack there of. With a engine and tranny with 172K miles, any unusual occurences are noted and an attempt to understand them is begun.

Thanks for the help, if I find the cause I will post.

Jeremy

 

No disrespect here...
I'm just saying that nobody is likely to know if it is normal or not because it is not good nor a common practice.

Edited to add...
I admit... I have kicked vehicles in netural while they were rolling before....But the engine died or other problem causing me to shut the engine down and I was coasting to the shoulder. So the RPM's did indeed drop those paticular times...

I see this is your first and second post so let me welcome you to the board... Anything you can ever want to know can be found here.


Ohh yea I almost forgot there is a lot of time and money saving stuff we share about our trucks too.

Also there is a lot worse charecters here that like to have fun, dont take life too seriously. It's way to short for that!

 

I also have to ask... Youve been told that your TPS is giving an erotic signal, Yet you make no mention of changing or considering this sensor that tells the truck where the throttle is VS speed etc.

At idle atleast for teh 5.4L the TPS should be putting out .99v
I'd do a search here and see if anybody knows what it should be for your 4.6 and start there if you think there is a problem. It can be adjusted so it may not even need replacing.

 

Yep. Check this first...takes about 5-10 mins

 

He said that he's already changed the IAC...

Edited to add:
Go to the transmission section and see if there is a neutral lock out that prevents the transmission from shifting in to neutral while the engine is running... It could be a safety thing to keep us from tearing up our trucks like that. Maybe BlueGrass wil see this thread and he might beable to provide some actuall input on how the tranny works. He's probably one of the smartest guys we have here overall.

 

I would have said check IAC first, but youve already replaced it, so I agree with PSS, check the TPS next.

I would not think a tranny problem would cause these symptoms...

If you do a search, the method for testing the TPS has been posted here many times.

 

I want one of those TPS's that give an Erotic signal!!! ;-))

The TPS is 0.99v on a 4.6 as well. Mine was way out of tolerance from the factory. Slotted the holes, changed the bolts to allen heads, adjusted and all is well.

For grins, I went out and tried this - did not do the same thing, so there is something amiss.

Good luck and let us know.

Cheers

 

Thanks for the posts! I do have fun on these forums, the 99 F150 I have only had since August and I have just got it on the road this past Christmas (shattered the differential 2 months after getting it). I have been part of car/truck forums for 4+ years with all my cars and motorcycle. I enjoy the sharing of information and have found numerous help here. I was posting a reply just to get the discusion back on topic. Which I am thankful we have.

The mechanic that tested the vehicle noted that the throttle position sensor was reading that it was X% open, even though the truck was on coast in neutral and foot off the throttle. He noted that immediately afterwards the IAC reacted to the TPS position, which I expect would be normal for the IAC. I will check the voltage off the TPS and test/replace it based on a search through here.

If I find the issue I will be back to explain the solution.

Thanks again for the replies. Helps me tremendously.

Jeremy

EDIT: BTW the reason this came to light was after I did the differential I was coasting in neutral on a flat, dry, not well traveled road, listening for whine and trying to pinpoint a vibration. Which from the board, I was able to correct by switching to RoyalPurple diff fluid. That stuff is awesome. Took away the vibration and noise. And when I checked the gears, the patterns were looking good with no metal in the fluid or on the magnetic plug.

 

No disrespect taken at all, bro. Just trying to get a handle on the whole situation and why it would occur. Your re-edit cleared things up.

Good advice given above, always best to actually check the operation of parts before you go about replacng things.

Oh, and welcome to these forums!

 

Did you check for a vacuum leak?

 

Why did you bring this thread back up when it hasn't been posted to in 11 years?