Hi, this is my first post. First, a gloat: I bought an '88 F150 Ext. cab XLT Lariat, 351 auto last weekend for $700, and drove it 350 miles home. It's in overall good shape, and runs great down the highway. The cruise even works. My father-in-law stumbled on the deal, and told me about it. I quickly made my way to his town and snagged it.

My problem is that it has a rough idle and hesitation on acceleration from a stop. Past that it runs like a top. I have been researching the problem here, thanks to a friend's recommendation of this board. So far, I've tried cleaning the butterfly valves in the front of the throttle body, replaced the O2 sensor and the IAC valve, replaced a weak positive battery cable (and reset the codes at the same time).

None of this solved the problem, so I checked the TPS voltage. The idle voltage was about 1.1V, and the high was about 4.65V. My question: is the low value too high? Could that be causing my idle problems? If so, should I replace it, or try to adjust it? I've checked around for vacuum leaks, and haven't found any yet, but I haven't been very methodical in my search. Also, it could probably use a tuneup, I'm not sure when that was last done. Thanks for any help.

 

I believe the high/low should be .34volts to 4.84 volts per my chilton. Do you have an adjustable sensor? If so it should have slotted screw holes. You could try temporarily adjusting it to .34 volts idle, and checking out how it drives. Of course this is gonna make it run like when you get much past idle. Still it should let you know if that's you're problem for sure before replacing it.

By the way, save yourself some grief and get the #'s off of it before you head to the parts store. My 92 has 3 possible choices and you can't read the factory numbers with it installed.

 

Thanks for the info.

> Do you have an adjustable sensor?

I'm not sure if it's adjustable, and I've been trying to hold off finding out, since I need to remove the throttle body to check it. I guess I'll bite the bullet and pull it tonight. According to my Haynes manual, and several posts here, the low reading for this one should be about 0.95 - 0.99V. I don't know if mine being 0.1V higher would have that much of an effect, but I'll see if I can adjust it lower. If not, I'll buy a new one and see if that fixes the problem. If it doesn't I guess it's back to the drawing board.

 

You have to set the TPS voltage at idle,even on a new one.When you changed the IAC do you clean the ports in the TB.There are 2 small holes on either side of the IAC that lets the air bypass the throtle blades.If these holes are plugged the IAC won't work properly.Also check the EGR.If it is leaking through it will cause a stumbling idle.
A good tune up would'nt hurt and could be the whole problem.

Billy

 

> According to my Haynes manual, and several posts
>here, the low reading for this one should be about 0.95 -
>0.99V.


Can you be more specific as to where you got this info? Please don't think I'm doubting you, because I'm not. It's just that the chilton I've got says .34 and my truck is having a similar problem with a .91 volts at idle reading. I think both trucks should be the same even though I'm a 92.

If anybody else knows or has experience please share. It'd be greatly appreciated.

 

Here are several earlier threads that contain supporting info:

https://www.ford-trucks.com/dcforum/DCForumID39/5384.html
https://www.ford-trucks.com/dcforum/DCForumID39/5177.html
https://www.ford-trucks.com/dcforum/DCForumID39/5085.html
https://www.ford-trucks.com/dcforum/DCForumID39/5151.html
https://www.ford-trucks.com/dcforum/DCForumID39/4712.html
https://www.ford-trucks.com/dcforum/DCForumID39/3850.html

I'll double check my Haynes manual tonight or tomorrow, and post my findings.

 

[updated:LAST EDITED ON 06-Sep-02 AT 04:19 PM (EST)]I've heard .98 from every credible source I've ever trusted. Obviously .01 volt in either direction is not gonna be a big deal, but IIRC it needs to be below 1.0 volts, at that point something happens, I think the EEC thinks you're no longer at idle. Hopefully someone can elaborate further.

I've never trusted Chilton's, they've gotta be the world's worst repair manual IMO.

For Project MPG Four Wheeler set theirs somewhere between .95 and .98, I think it was .98.

Zuiko, you give the max and min voltages, but was it smooth throughout it's range? If it jumps around your TPS probably has issues. Otherwise, it should trip a code.

 

It seemed very smooth from min to max. I cycled it slowly several times, and didn't see any jumps. I didn't test resistance though. I'll try to adjust it to below 1.0 and see if it improves. If I do that and it lowers my Max value, what is the lowest acceptable 'high' value?

 

Thanks for the voltage info and links. I'm real glad I got it before buying that 20 dollar sensor. If the chilton ain't careful it's gonna make a short unhappy trip to the trash can.

 

As a followup, I checked my Haynes manual, and it says the idle TPS value should be between .50 and 1.0 volts. Mine is above 1.0 (about 1.1), but I haven't found time to remove the throttle body to try to adjust it. Will try that this weekend. If adjusting it down to the correct range doesn't work, I'm going to do a tuneup (will probably do that anyway) and check the EGR valve. It was supposedly recently replaced by the previous owner. Maybe it's a vacuum leak.

 

Following up on my original post. Last night, I adjusted the TPS down to .985 V at idle position, replaced the distributor cap and rotor, and replaced the spark plug wires. I also gave the throttle body a more thorough cleaning while I had it removed, and replaced its gasket. The truck now idles pretty well. It still has a very minor miss at idle, and the throttle seems to barely stick just slightly above idle(maybe 3-400 rpm too high). I can lightly tap the accelerator, and it idles down. It does it every time I accelerate, then come back to a stop. Seems like something is hanging up -- I'll check that tonight.

I took it in this morning and had it inspected, and it passed emissions (Dallas county, TX). I'm finally legal.
Thanks for everyone's input and information on this.