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

Wouldn’t any of these throw a code on ob2? Because I’m not getting any codes.

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

After I change the TPC should it work automatically or is there anything I have to do first? Thanks

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.

I changed the TPS, and IAC. Still same.

I also took a multi-meter and hooked it to the TPS wire adapter and it read 8.7 volts. is this normal?

Sorry, I did that wrong. whenever I test the positive on the TPS main wire I’m only getting 0.05 volts. isn’t this suppose to be 5.00 volts?

Void the multi-meter stuff I can’t do it right.

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.

I’m not sure I did it right. just let me know how to do it and I’ll check it tomorrow. Thanks

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

Okay, I did change the TPS and the IAC with new ones but same thing. So what should I do now?

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.

Do I take the electrical connected off of it, because I don’t see a way to test it with it on?

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

I have not gotten there yet but I have question, on my truck when I bought it I noticed the middle PCM Harness was cut and replaced so I took all the tape off to make sure they were all connected good. It all seem okay but when I got down to the last wires they was two different colors is there something not right.

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.

Only thing I know it’s the middle harness and one wire in pink and the other looks white with black stripe.

I will look and let you know what I find. That may be enough to tell you. Sometimes they use the same color twice but I will send you what I find.

Can you find me the PCM pin out diagram? Thanks

Sorry for the delay. I have been traveling all week for my full time job. Here is the document that you need. Looks like the middle connector (B - White connector) has a pink wiring that is for the governor pressure control. The pink and black is the transmission relay control. Hopefully this helps. Let me know what you find. Thanks

If some reason if the injector driver wire was hooked up to the wrong wire would it cause the problems I’m having ?

Well I can't say I have ever tried that to see how the engine runs but if the driver wire was not correct, I suspect the injector wouldn't operate. This is a ground side control. That means the PCM grounds that driver wire and that opens the injector. The other wire is a constant power wire when the key is on coming from the ASD relay. When the PCM wants the injector to fire, it grounds that driver wire, the injector opens and then it removes the ground creating an open circuit and the injector closes. Assuming the wire that is there rather than the driver is a wire with voltage on it, I suspect the injector will just remain closed or possibly fail. If the wire that is there rather than the driver wire is a ground wire then the injector would be open all time. Either way, I would imagine you would have a dead misfire on that cylinder.

Plus, you would have a DTC for an injector circuit issue.

Are your injector wires a different color? Remember this lists all 8 injectors so your engine does not have 7 or 8.

I was looking at the white PCM connector there that has been replaced. there are 2 different wires coming from from the connector that are different colors.then the 2 wires coming from the wire harness. I’m not sure if they are hooked up right.

Ok. So the way to test if a wire is hooked up to where it is supposed to be is to use a multimeter and do what is called a continuity test. Set it on resistance and unhook the PCM connector and then the other end of the circuit. In this scenario lets just say it is injector number 1. So you have the PCM connector unhooked and the number one injector unplugged.

Get a small safety pin and put in the front of the connector on the PCM terminal in the pin that goes to the injector. Then go to the injector connector and disconnect it and put another safety pin in the terminal of the wire coming from the PCM. Just make sure you use a small pin. If it is too large you can spread the terminal and it will not make contact any more and cause an open circuit.

Then with your meter set on resistance, touch one lead to the PCM safety pin and the other side to the injector safety pin. You should have a small amount of resistance like less than half an ohm. This is just the normal resistance of a copper wire. If the meter is reading OL, zero, or is blank then this is called an open circuit which means the wire from the PCM is not going to the place it is supposed to.

I went and bought a PCM, and after changing the car doesn’t rev up real high no more and come down slow. So did fix that problem? But also my car when parked will idle up and down back and fourth this did not fix.

Have we done anything about checking for a vacuum leak? If the IAC and TPS are good, the PCM is replaced then we may have a vacuum leak.

Here is a guide that will help determine if you have a leak. Let me know what you find. Thanks

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge