My '94 E350 feels like the TPS is bad - runs rough at idle and feels like its misfiring driving down the road. The check engine light is not on. Was having these issues on a road trip and I didn't have the means to test the TPS so I just replaced it and still having the issue. With the TPS unplugged though it runs WAY better. It idles high but doesn't cut out at idle or when driving down the road. When I got home I tested the new TPS and it appears that the sensor itself is working fine. Any thoughts on what would cause this and not throw a CEL code?

 

If you have an ohm meter, you can check your TPS. What goes wrong with them is they will get a dead spot in them . You can easily see it using an ohm meter and slowly working the throttle. This works best with an analog meter but a digital one will work too. I don't think yours is bad but the computer is making an adjustment to compensate fot the absence of the TPS. I would put the original TPS back on it.TPS and O2 sensors have to be the most replaced item when there was actually nothing wrong with them. I never had a TPS go bad in any vehicle I have owned.

I would want to know if your fuel pressure is good which should be 30-45 psi and a good working ignition system.

 

Thanks Raski, I checked the TPS with a multimeter and seems to have a consistent voltage change across the range of travel. I have also checked fuel pressure and it is ~30 psi. I think it must be some component affecting the PCM fuel trim system because last night out of curiosity I tried unplugging the MAP sensor and it does the same thing as unplugging the TPS sensor - the engine runs great but idles high - like it's bypassing the PCM's fuel trim and going into a default mode. Without a check engine light code I am wondering which components I should be testing. Thinking EGR and EGR position sensor, MAP sensor, IACV, and O2 sensors. What am I missing? And is there anything that would cause this and not throw a CEL?

 

You might want to check for that 5 volt reference voltage at either the MAF sensor or the TPS..
There are a few components that share that reference voltage and if any one of them is shorted or has a problem ,it can affect the function of the other sensors.

 

Something that extreme I would expect a code(s) to be set but we are dealing with an OBD 1 system which "ain't too bright"....

 

Because that problem did not always set a code, that was what separated the men from the boys, lol

 

The O2's signal is ignored during open loop operation, so if the condition happens on a cold engine, you probably can rule out the O2 sensor. You mentioned idling high. I would begin looking for a vacuum leak causing a lean miss. PCV, brake booster hoses and intake leaks are what I would be looking at.

 

Hanky, I checked the TPS while it was connected so definitely got 5 volts there. And good point about open/closed loop Raski - It is present when the engine is cold so I guess we can rule out O2 sensors.

 

What engine is in your vehicle?

It is difficult to obtain values from an OBD l system, When we remove an input from the system, like disconnecting the TPS, the ECM substitutes good values for the bad ones. There could be a few reasons for the increased idle speed like low system voltage so the alt output is increased to compensate.via idle speed.

If there is no check eng lamp you are going to have to chase this down one component at a time.
If your vehicle still has the original ign coil on it , they were known for internal problems from difficult starts to poor engine performance especially under load.

 

Its a 351 Windsor
To clarify - the high idle speed only happens when I unplug a sensor. It runs fantastic with a sensor unplugged. No hesitation or anything, just idles too high. Which makes me think the ignition coil is probably fine. When The sensors are plugged in it idles rough and driving down the road feels like its cutting out every few seconds.
I will see if I can test the MAP sensor, EGR and EGR position sensor, and IACV. Are there any other components that the PCM would be getting info from that I should investigate? Thanks!