I have a 2.5L 4cyl dakota with a P0505 code for IAC system issue. Idle is just a tad high at a stop, rpm's surge from 1000-1500 or hold at 2000 while clutch is dumped and approaching a stop. Once stopped, idle drops back to just over 1000. I have removed the throttle body and thoroughly cleaned, replaced the throttle body gasket, replaced the TPS (I know it's not included in the fault code criteria, just worn and figured I'd do it while I was in there), inspected for vacuum leaks, inspected the electrical lines, and tried replacing the IAC valve. After installing the new IAC, the rpm's spike up past 3000 and continue to climb, So I put the old one back on. Took it to a shop and they said they did the exact same process that I did but couldn't figure it out and suggested a potential problem with the PCM. Before I go and drop $400 on a new PCM, I thought I would ask for some input on any bases that haven't already been covered. Thanks

 

How did the MAP sensor look and did you inspect the little rubber elbow that connects it to the throttle body?

I believe both IAC and TPS need an O ring do they have them?

Did you inspect the intake manifold for air leaks at all 4 ports ?

Also on the manifold their is a port that has a rubber plug over it this can rot off look for it.

The middle Pic may hold the answer that's the rubber port plug is it missing?

 

A pic of it missing the rubber plug

 

thanks for the reply. i checked the plug you posted pics of, and it is intact. also checked the tube leading to the MAP sensor and it looks ok. the seals on the TPS as well as the IAC are ok and in place. not sure as to how to test the MAP or the intake ports.
i noticed that the seal around the PCV valve (just a breather? all i see for parts is a PCV elbow) is loose and has some oil around it. i assume that may be contributing to the problem but it doesnt explain why the code would stay cleared after installing the new IAC. Perhaps there is an error in my process of installing the IAC. Is clearing the code with an OBD-II sufficient or is there a different method to resetting the PCM.

 

Not sure why I didn't think of it until now. I had this problem intermittently and the code didn't come on until a couple days after I ran a can I sea foam through. The vacuum line that I pulled for the seafoam was the line that runs from the round box top of the drivers side engine compartment and runs right below the map sensor in the center of the vacuum line cluster.

 

I'm not sure this is part of the problem or not but worth checking.

Pull the air temp sensor and clean it or replace it they usually are very dirty covered in oil.

The air temp sensor is next to that rubber plug I told you about they are cheap so worth replacing.

I don't like seafoam I tried it and didn't get any results from it I also think its dangerous dumping it down the throttle body, as a gas additive yes but not dumped down the throttle body.

To reset the PCM just unhook the neg battery terminal about 5 minutes then reconnect.

The seafoam may have gotten into the IAC and messed it up some an IAC should never be sprayed with any cleaner it can damage it


A new air temp sensor 15$ to 25$

 

IAT should not affect idle RPM, even closed loop. All it does it control the timing. You tapped into the brake booster line which is the correct place. Are you sure that you do not have a split vacuum line when you put back on the fitting?

0505 is a very generic error- it can be that the IAC is not behaving well, or that it's simply not able to compensate enough to put the idle RPM on the target. Check and make sure the throttle body is completely closing- the butterflies should seal 100% to the bore such that 100% of intake air comes through the IAC port.

 

i replaced the pcv elbow and the gromet. was just shooting to replace the gromet but the pcv snapped in the process. i inspected the vaccuum line that i removed to run the seafoam and it is fine. getting a little frustrated, dont wanna throw too many parts at this thing and not be able to afford getting it fixed at a shop if need be. the throttle plate does appear to be a bit off so im going to go at it with a toothbrush ans come cleaner again tomorrow and see if it helps.

 

I agree dumping the seafoam down the vacuum line is correct but its about the same as dumping it down the throttle body.

Also did you check that rubber plug to see if it is on good it may be sitting their cracked open sucking a lot of air it may look ok but did you actually pull it off and inspect it.

Everything says you are sucking excessive air in.

I had this plug come off and it causes high and erratic idle

 

How tight/hard did you tighten the throttle body bolts?

These should be tight but not excessive tight these parts are aluminum over tightening them can warp the throttle body leaking in air.