Hello everybody.
I am a first time poster, but a long time lurker.
Before I get to my IAC / PCM question, I'd like to thank the board as a collective.
Over the years, this boards wealth of knowledge, and my D providing me the opportunity (aka breaking down a lot) has made me into a semi decent mechanic. THANKS.

Over the last couple of months, my D has been throwing a "Catalytic Converter not running efficiently" code. It was intermittent, so I would read it, then clear it knowing I'd have to replace the Cat before my next smog cert came up (June 15). I have 233,000 on the vehicle, so probably due for the new Cat anyway.

So last Friday, had a local guy put the new Cat in. Now my D is throwing the P0505 code. No sweat, I get a new IAC and pop it in. The D idle goes up to over 2000. I reset the PCM, and restart the car. Idle is now at around 1200, but when I push the accelerator, it stays over 2000 again.
Note: P0505 still gets thrown after I clear it.

I figured I had a bad IAC, so I take it back and get another one...same problem. Just to be on safe side, I replaced the TPS as well. Reset PCM.
Same problem. With the throttle assembly hat off, I can hear the loud sucking at the IAC. I put my palm over it (thought it was going to suck my skin off) and the idle dropped. So just for sh*ts and grins, I pulled the connector off the sensor. Absolutely no difference. High idle and P0505.

Just to test one more thing, I put the original IAC back in and start it. (Note: I never had a really high idle with old IAC). No "really" high idle (around 900), but still throwing the P0505. I pull the connector off, and no difference.

I checked all the electrical lines and connections and everything seems to be fine.

So after that long and detailed description and after reading everything on the board I can find regarding the IAC and PCM, here is my question.

Am I going to have to break down and buy a new PCM?
If this was your D, what would you do?

Thanks in advance.

 

I'd start by picking up a can of throttle body cleaner, and cleaning out the throttle body and IAC port since you're having the issue with the weird idle RPM's. I'd also go through the wiring and make sure you don't have an open circuit or it's shorted to the block.

 

Thanks for the reply.
Forgot to mention that I did a pretty extensive cleaning of the IAC port...3 times. Once for every time I swapped the IAC. I have checked the wiring visually, but don't know how/where to check to see if it is shorted.
Do I check this right off the PCM?

 

IACs usually come with a gasket and a rubber o-ring. You didn't use both did you? On the 5.9L, you only use the o-ring.

 

Just the O-Ring.
Actually the IAC didn't come with the gasket, just the O-Ring which I used.

The TPS came with both gasket (which I used) and the O-Ring.

Thanks.

Because I need to get this smogged in the next 2/3 weeks, I went ahead and ordered the PCM.

I'll keep you updated when it comes, and I swap it in.

Thanks again.

 

I didn't use both on my TPS. I just used the o-ring. I tossed the paper gasket... doubt that has anything to do with your issue though.

 

I wasn't very clear on my response.
My TPS came with both. There was no "groove" for the O-Ring (same with the TPS I took off), so I used the gasket, and ONLY the gasket.

 

That is okay. I always was told with the TPS you use what your old TPS came with, just so happened that on my 2000 2WD 5.9 Durango used the gasket, so I used just the gasket also. Basically the TPS is a multi-use sensor and can be used from anything from a Grand Cherokee to a Ram 2500.

As far as your problem, I'd check anything and everything where that wire travels from the sensor to the PCM because there could be a small kink maybe where it rubbed against the intake manifold while it was running, got hot and melted the wire coating.

 

Just wanted give an update.
The new PCM arrived, and I dropped her in (that was easy).
I also re-installed the new IAC.

She started up and ran like a champ. No codes and running smooth.
So now I have a new PCM, new IAC, new TPS and a new Catalytic Converter.

I drove her for about 150 miles then went to get her smogged.
Emissions were so low, the guy running the test said it was like a brand new car.

Not bad for 232,500 miles on the beast.

Thanks again everybody.