First of all, I've been visiting the site for years with my last Dodge. I totalled it, had a Chevy S10 for a while (yuck), and now I have my Dodge back. It's a 2000 Dodge Dakota V8 4x4 Quad Cab. It has 207k miles. Couldn't love it any more than I did my old Dodge.

Anyway, that evil Check Engine Light is on and I'm getting the codes P0138, P0171, P1294, P1596 (Have to double check that number, but it's the Speed Control Switch), and P0123. I've replaced the TPS and the downstream O2 Sensor, but the codes won't go away. I've read that I may have to have it cleared, but I may do that today actually.

The real problem is that the idle is erratic. While idling, it will drop RPMs by a few hundred, but then come back. It will do it occasionally parked. While running, it feels like it's bogging down a bit, and does the same thing. While either accelerating or braking, it drops a few hundred RPMs for a few seconds, then goes back to normal, and will do it several times in a row. It REALLY bogs down sometimes when it's below a quarter tank of gas which leads me to believe that the severe part is caused by the fuel pump. It will even stall at times. It does not do this with over a quarter tank. But the less severe drop remains indefinitely even with a full tank of gas.

Can anyone shed some light on some of this? I've ran SeaFoam through the TB, but I've been considering cleaning it. I may not be a seasoned DIY mechanic, but I learn very quickly.

Thanks to any and all who answer. Will be here the majority of the day.

EDIT: Oh yeah, and I wanted to ask if that all the "high input" and "high voltage" or whatever could be the result of a bad PCM? I've had link errors at AutoZones, and a mechanic couldn't adjust the idle through the computer. He said it had been replaced. Could it have been done wrong?

 

What mechanic couldn't adjust the idle?

P0123 - TPS voltage high
P0138 - 1/2 O2 sensor shorted to voltage
P0171 - 1/1 Fuel system lean
P1294 - Target idle not reached
P1596 - Speed control switch always high

What brand 02 sensor did you put in? Do you have 2 or 4?

Start by taking the throttle body off and cleaning it thoroughly. It's not hard to do. Remove the IAC carefully, you'll likely find lots of crap in there. A dirty IAC can cause an erratic idle.

Did you have the code before you replaced the TPS? It sounds like it's not the correct one of was damaged.

You may have a problem with the fuel pressure, put a gauge on it and see what you have. You may be able to rent one.

The speed control switch may be due to a bad switch on the steering wheel. You can access the switches by removing two bolts from the back of the steering wheel and removing the airbag - BE SURE to disconnect the battery or remove the airbag fuses for at least two minutes before doing so.

 

Pretty sure I have two sensors. Used a Bosch. Yes, the code appeared before I replaced the TPS. And now that you mention pressure, I just went to the gas station, and I thought it was odd that when I removed the gas cap, it released a fair amount of pressure, like the amount from a pop bottle. This is the first time I noticed it, and I don't recall if any vehicle I had had ever done it.

Should I have went with the Denso O2 sensor? And I have the Haynes manual, and have been checking out the codes for a few weeks now.

Oh, and the mechanic was one of my dad's friends. He hooked up something to it, but couldn't adjust the idle. He said something about the computer not being original. Forgot to mention the idle is at nearly 1500 parked, 1250ish in gear. The truck wasn't in my hands at that time, but now it is.

And this is also a 5.9l. I'm slightly forgetful.

 

Start by replacing the O2 sensor, NTK or Denso are preferred. Bosch can cause a lot of problems.

The pressure when removing the gas cap is normal. The system is sealed for emissions purposes and it can build up some pressure. The truck actually pressurizes the system to check for "evap leaks."

I was talking fuel pressure and you'd need to hook up a gauge to the fuel rail to see what's going on with that.

The computer being "original" won't matter. The PCM would have to be set up (flashed) for the specific application or you'd have all sorts of problems, other than a high idle.

A vacuum leak can also cause a high idle. It could be a vacuum line, a loose fitting, plenum leak, or a loose throttle body. Open the throttle body and look down with a flashlight. If you see oil then it's leaking.

Start with the basics and eliminate them as well.

If you want a factory manual for your truck, much better than Haynes, follow this link:

https://dodgeforum.com/forum/2nd-gen...-download.html

You need to convert it to a pdf but the utilities to do so are free.

 

Thought I had left a post earlier, but I must not have hit submit.

I will look into all this and report back. Thank you for all the info. Definitely going to clean all that crap out of the TB.