Need some help. I have the local mechanics stumped. I have a '91 4.9L F-150. Just gave it a tune-up to include, plugs, wires, cap and rotor, pcv valve, fuel - air and breather filters, and oil change. I have to pat the gas a little upon starting to get it going. Then hesitates a little at every exceleration. After the hesitation it runs fine, until you excelerate again. No "check engine light". According to the mechanics: no vaccuum leaks, good gas pressure/flow, no code readings on the computer. They changed the throttle body sensor, no luck. The only thing they didn't check was the EGR Valve, but said they didn't think it was that. Also said it was putting out a little black smoke from the exhaust. I've never noticed this, so it must be minor. I back the truck out of a 250 foot driveway. By the way, they didn't charge me for looking at the truck after having it for two days. I have no reason to suspect that they are hosing me. Particulary since they have the contract for the business I work for.... Any suggestions, other than buying a new truck?

 

Sounds like the TPS is worn out.

 

Thanks, but my bad. Instead of Throttle Body Sensor, I meant Throttle Position Sensor. They replaced that and it didn't have any effect so they put the old one back. At least thats what they told me.

 

Then I would suggest getting your injectors cleaned. Actually, buy a few bottles of injector cleaner, and add it (all of it) to a quarter tank of gas, and drive around for a day or two. If you notice any improvement, have your injectors cleaned or replaced.

 

what does it take to replace the TPS? im assuming the 4.9 is somewhat similar to the 5.0?

 

It's mounted to one end of the throttle shaft on your throttle body, and is held in place usually by two screws. Word of warning, though, the best way to remove the screws is with a hand held (non powered) impact driver. You know, the kind you hit with a hammer? The reason why is those screws are in very tight, and using a screwdriver will almost always strip the heads.

Before replacing it, test it. Unplug it from the harness, and attach the leads from a DMM with a bargraph, set to ohms. Pos lead to the gray/white wire, neg lead to the brown/white wire, and then rotate the throttle by hand, slowly, while watching the bargraph. It should move forward smoothly. If it jumps around at all, replace it.

Take care,
~Chris

 

well i dont have a fancy DMM so I just tested mine for volts with the throttle closed like my chiltons says, and it was at 0.63 instead of the 1.0 it should be, so i just replaced it and set the new one to 1.0. i didnt have any trouble with the screws, just used a plain screwdriver.

I also cleaned my throttle body and intake out, there was hella carbon in there, what causes that? i havent driven it much, but my funky hesitation thing seems to be gone, nice smooth throttle again

 

The voltage testing method works too, it's just not as accurate because it relies on a voltage source.

The carbon is from the EGR valve.

I'm glad you got it fixed!

Take care,
~Chris