hello... I have a 1994 ford f150 4wd with 351w. the truck has had several incidents of sudden acceleration. I disconnected the speed control and replaced the IAC the first time it happened. getting ready to change TPS this time. any idea what can cause this problem? it is like a rodeo ride every time it happens.it is not throwing a code and it doesn't seem to matter if it is hot or cold. any ideas would be greatly appreciated.

 

Lets help viperbowstrings! this sounds dangerous. up to the top!

 

Well, I guess we can rule out the possibility this might be a Toyota.

Seriously, Toyota's very similar problem may have been caused by an electronically controlled "drive by wire" throttle. The F150 does not have such a system. The throttle control is manual and no electronic fault will cause sudden acceleration. That is reinforced by the absence of any codes.

So, the detective work begins. First, make sure your throttle linkage and throttle body are in top shape. No broken return springs or damaged cables. Make sure there is no unwanted movement of any brackets.

Next, document some details about what the conditions are when this undesired acceleration occurs. Is it a slight extra pull? Is it like flooring the pedal? Does it happen at/above/below a certain speed? Do you feel movement in the accelerator pedal? is the engine racing? Every investigation needs information.

 

I'd give my left ******** to experience sudden acceleration with a stock 351.


pull the codes before you go replacing parts.

 

How about cruise control like accel botton being pushed but it's doing it electronically. The Toyota issue I guessed was cruise control issued but the drive by wire is more likely the issue. How about fuel delivery, could that be possible. Maybe computer?

 

Codes?......

 

Yeah I'm not buying this sudden acceleration thing either, the only way I see one of these trucks doing anything that could be confused with this is if the brakes are dragging and suddenly release while you're trying to accelerate anyway, or if there is something seriously wrong with the transmission where it's not disengaging a band when shifting to another gear which again causes a heavy drag that suddenly releases and produces more acceleration than you're expecting.

 

That should answer that, and no, it's not the computer.

I don't think there are any codes.

 

Koer? Koeo? M?

 

so far the truck hasn't thrown any codes. it just all of a sudden tachs up over 3k, no jerking. has happened 3 times, usually cold weather. I have disconnected the cruise cable and plug, cant be that. replaced brakes also. the first time the vehicle wouldn't stop after cruising at 55mph. just kept revving. brakes were smoking. second time just revved up after being put into park. the 3rd time shutting off the engine stopped it. haven't driven it since. no one seems to have an answer. no pedal mix up here. not crazy. this is our first ford. now my girl is scared to drive it. it is a nice truck, wish we could find a for sure fix. cant be sure with inconsistent activity. replace everything is no good if not sure it is fixed. I had a throttle spring snap on my old jeep, wild ride, new motor, fixed, no question. I appreciate any help, not looking argue about the fact that my girl feels like this truck has tried to get her, 3 x. this phantom problem is not duplicatable. thanks guys.

 

Let's go back to the Code question again....please define your first statement of "it is not throwing a code". Did you you actually run the Key On Engine Off (KOEO) tests and Continuous Memory display? Or do you assume there are no codes because the Check Engine Light is not on? You can have codes without having an active CEL.

I had a 1991 GMC S-15 that would exhibit very similar symptoms. It was scary as hell when it would take off as if it were possessed, I too smoked the brakes once trying to get that beast to slow down. The root cause turned out to be an intermittently bad Engine Coolant Temperature (ECT) sensor. When it was acting up the PCM thought the coolant temperature was -100 degrees.

I have never had a Ford do that or heard of anyone experiencing this issue. Not once did the computer trigger a Check Engine Light....I found the ECT issue by checking the codes.

 

thanks guys for the help. I think I have found the cause of the acceleration problem. the throttle is not closing all the way. I thought tps, while I was changing it, I noticed a ton of gunk on the intake side. I felt like maybe a little careful cleaning would benefit. I was careful of the coating, but the butterflies still wont close completely. I cant afford a new one and wonder if there are any ways to "rework" this one to free it up. maybe an aftermarket return spring? need to get this rig back on the road. thanks again for all the suggestions.

 

OK I just had a couple ideas about this, when you mentioned that it happens mostly when it's cold I had a flashback to driving on the 401 in a snow storm in an old carbed caprice(belonged to a coworker of mine) and when we came to our exit the throttle was stuck.. or more correctly frozen.
So the question is are the coolant lines to the throttlebody still there on your truck? If not then it can ice up in cold weather so you need to reinstall those lines.

The other thing is you said the throttle plates are not closing completely. You need to disconnect the throttle cable and figure out if you have a binding cable or if there is something wrong with the TB. If a PO had the TB off and took it apart they may not have reassembled it correctly. The trick with these things when reassembling is to leave the plate screws a little lose and let the butterflies self center and then snug down the screws. That should produce smooth movement and complete closure, but also note the throttle stop screw will hold the plates open slightly, and if that has been messed with there is a procedure to go through to set it correctly. Also inspect the shaft holding the throttle plates for play or binding in the linkage, if your assembly is worn badly then go to the junkyard and get another.

 

By any chance do you know that procedure?

 

I seen something online about starting up engine, disconnecting IAC, Turning off and restarting and setting base idle with screw but im sure that's prob not correct.