Engine revving up and down on its own

Hi,

This could be a number of different items. I attached some documents that explain what is happening during idle. This could be the idle air control motor but if this happens at different times and the throttle does not return quickly I suspect the throttle position sensor has failed. These are really easy to replace and cheap enough that if it were me, I would just go ahead and replace it based on experience. If that is not the issue then we can dig further.

Review this material and let me know what you think. Thanks

Only if they are outside of a predetermined range. However, if the throttle sensor stays with in this range, the PCM will just think the throttle is opening and closing. It was for these reasons and others that OEMs starting developing checks and balances in more modern cars.

Basically, they just added a second sensor that the PCM needs to see them in half of the other. This way the PCM can see if one sensor is out of range and it will set a rationality fault. Your truck does not have this. You have a throttle cable that attaches from the pedal to the throttle body and this sensor reads the movement of the throttle plate on the throttle body.

As you can see under the operation, the PCM will adjust fuel and ignition based on this sensor so if it is ranging up and down, the PCM will add and remove fuel which will bring the RPM's up and down. Stepping on the accelerator changes it but if the sensor sticks there, the RPM will remain high as you are describing.

Here are a couple videos of similar issues. These are a not the same year as your vehicle but the similar complaint. I don't think they called it the correct name. It is not the idle position sensor. It is the throttle position sensor as shown in the diagram.

https://youtu.be/6tKhTSWJCzA

https://youtu.be/QtdJvuKHCoE

Good question. There is no programming for this sensor. It is plug and play. Here is the instructions from the manual. Just see step 3 on the installation. That is just checking the mechanical operation. However, there is not programming required.

Okay. Then you may have a wiring issue if those numbers are accurate. However, we need to make sure we have the meter set on the right scale. You are correct that there should be 5 volts but this is just the supplied voltage. We want to see what the orange with dark blue wire is saying while this is acting up.

This wire should be between 1 volt and 4 volts. The lower the voltage the closer to idle the engine is. 4 volts is wide open throttle. If this voltage is accurate then the PCM is most likely the issue.

Also, the violet with white wire is the 5 volt supply and the black with light blue is the sensor ground.

Put your red lead of your meter on the orange wire and then the black lead of the meter on a good ground like the negative battery post. Start the engine and when the RPM is ranging, watch the meter and see if it is moving or not. If the engine is idling, the throttle plate should not be open very much so you should have about one volt. If the sensor is faulty I would expect to see the voltage ranging up and down and hence the RPM is going as well. Since you just replaced the sensor I suspect it is okay. If this is the case, then we need to find what is causing the RPM to range which is most likely the PCM. However, we have not gotten to that point so don't just replace the PCM because I would hate to just make a couple hundred dollar guess.

Let me know what you find and we can go from there. Thanks

We need to monitor the voltage on the TPS as I said on the previous post. This way we know the TPS is functioning and sending a good signal to the PCM. Then we will need to look at the PCM itself.

So just refer to the previous post for where to put your leads and tell me what the voltage is when the vehicle is acting up.

You need to back probe the connector and then plug it in to the TPS to test it. Here is a link to how to back probe a connector. If you don't have those types of probes, you can use a small safety pin.

https://youtu.be/op6Djd9PbIc

The wire color is not as concerning because the wire colors can change. As long as that circuit goes to that sensor or correct location.

What wire is it and pin location in the PCM? I can tell you what it goes to and you can check continuity to make sure it is the same wire.