Rough Idle

[JacksonH]

So I have a 2000 Jeep with the 4L I6 with 104000 miles on it. I'm currently having a rough idling issue. I took it in to a local shop to see what the issue was because my jeep would stall out in reverse or whenever stopped. They told me that it was the TPS (P1294) and all that needed to be done was to reteach the sensor. After that, I was idling around 700 RPMs and saw that the idle would fluctuate. The other day I was driving to work and my jeep's RPMs dropped all of a sudden and then it stalled. I took it back into the shop and they told me that the thermostat that was in my jeep was not the correct one (they could only get it up to 170). The code they got was for the engine not getting up to temp (P1281) and that my TPS voltage was high (P0123). They also found that my clock spring is bad and that there might be a short. Now when I cold start my jeep, it starts at around 900 RPMs, but as soon as the engine starts to warm up it goes down to around 600 RPMs and drops down to around 300RPMs when I stop.
I'm planning on replacing my TPS and clock spring.
Any ideas of what might be the problem?

 

[Jerry Bransford]

That shop doesn't know much about TPS sensors. What goes wrong with them is, as a variable resistor, they get dirty inside which causes them to send out erroneous signals. Resetting the computer of course can't fix that. Unlike the IAC the TPS cannot be cleaned. Once it goes bad, and they do, it has to be replaced.

Yes your thermostat could be the wrong temperature, it needs to be a 195 degree model like from Stant. Some ignorant types think by installing a lower temperature thermostat into an engine the engine will run cooler when in hot conditions.... not going to happen. Do NOT install any thermostat that claims to be "fail-safe" as they are a different type of design that actually tends to fail in the open position. Any standard 195 degree thermostat should be fine.

A bad clockspring can definitely send a noisy signal to the computer bus which can cause the TPS to look bad. But since the TPS is inexpensive and very easily replaced, I'd replace that first before spending the $$$ and significant time required to replace the clockspring.

Finally, a bad upstream O2 sensor will definitely cause engine problems like a rough idle plus reduced mpg and power. The O2 sensor only starts telling the computer what to do once the engine has started to warm up so if a problem only occurs after the engine has warmed up, odds are good it's related to a bad upstream (but not downstream) O2 sensor.

 

[JacksonH]

That shop doesn't know much about TPS sensors. What goes wrong with them is, as a variable resistor, they get dirty inside which causes them to send out erroneous signals. Resetting the computer of course can't fix that. Unlike the IAC the TPS cannot be cleaned. Once it goes bad, and they do, it has to be replaced.

Yes your thermostat could be the wrong temperature, it needs to be a 195 degree model like from Stant. Some ignorant types think by installing a lower temperature thermostat into an engine the engine will run cooler when in hot conditions.... not going to happen. Do NOT install any thermostat that claims to be "fail-safe" as they are a different type of design that actually tends to fail in the open position. Any standard 195 degree thermostat should be fine.

A bad clockspring can definitely send a noisy signal to the computer bus which can cause the TPS to look bad. But since the TPS is inexpensive and very easily replaced, I'd replace that first before spending the $$$ and significant time required to replace the clockspring.

Finally, a bad upstream O2 sensor will definitely cause engine problems like a rough idle plus reduced mpg and power. The O2 sensor only starts telling the computer what to do once the engine has started to warm up so if a problem only occurs after the engine has warmed up, odds are good it's related to a bad upstream (but not downstream) O2 sensor.

Thanks for pointing me in the right direction. I normally would try to fix the Jeep myself but it's starting to get cold and I don't have the time or place to try and troubleshoot. I'm gonna replace the TPS and Clockspring this weekend and see if that fixes it and then try and replace the Upstream O2 sensor.

 

[Jerry Bransford]

Replace the clockspring dead last.