I am having some issues with morning start up on my 95 F150. My most recent problem is this;

Early AM, as I go to work...truck bogs and hesitates. Sometimes it misfires as if it was fuel starved. The minute I floor it, it will bog/misfire a little and then it'll be fine. It seems to happen at low RPM and I would say 15-25mph speeds... Once it does "clear itself", the rest of the day the truck runs fine.

It does however trip the CEL. I put a code reader on it and that's where I get the 121 code. I just recently replaced the TPS and the O2 sensor.

It has me a little frustrated... Any help would be appreciated.

Thanks,
~Larry

 

Code 121 is TPS Sensor out of self-test range. (what ever that means)

Ford Computer Codes

 

Sounds like your TPS is misadjusted or defective.

Should read about .96 volts from ground with throttle fully closed.

 

Sounds like something in the TPS is either getting stuck or shorted out some how and by depressing the accelerator all the way you clear what is hanging it up. Not sure what the heck that would be, but it sure sounds like an issue with the TPS. Why did you replace it in the first place? Was it for the same problem?

 

I did replace the sensor after some troubleshooting and it worked fine for a short time. I cleaned out all the crap that was built up around the throttle body as well. I'll check it with a meter in the AM...

Larry

 

most ford tps's will fail at less 1/4 pedal.

 

Is this an inherent problem with Fords? I've never had this happen before... Is there a reason for this?

I mentioned my problem to a friend of mine and he told me to check the closed voltage as well... But he also told me that if it wasn't the TPS, then there might be more or possibly not enough voltage going to the TPS.

 

I wouldn't be making any drastic changes since it sounds like the problem goes away once you floor it and it stops sputtering a few moments later. The 121 self test error indicates the the computer doesn't find the default reading of the TPS to be in the right range when it's closed. This is a test the computer does on many sensors with engine not running but the key in the on position. It's not a code that would be thrown while the engine is running. Since it doesn't sound like you are getting any run time error codes, it doesn't sound like the TPS is causing any the stalling and stumbling problem, especially since it goes away with one good flooring of the accelerator. It sounds almost more mechanical than electrical. Perhaps fuel filter is getting a little clogged, injectors are getting a little clogged. I would give your thought about the engine starving for fuel a little more thought. Look at what happens when the engine runs hard and strong by flooring it. Vaccum drops, fuel pressure in fuel rails increase, exhaust pressure increases along with intake air flow.

 

OK...it went away for a little while and now is back.

I get the same problems in the AM... It runs a little while and then, it starts to sputter, backfire and hesitate until I floor it. I changed the fuel filter about 3000 miles ago. I also get a 172/173 or 171/172 code (whichever it is) for a O2 sensor running rich/lean.

So what would cause that since I changed out the O2 sensor about 4000 miles ago? The TPS was changed less that 4000 miles ago... I am baffled and frustrated right now.

Thanks.