2002 QX4 idle issues P0506

[heelsrtops]

Hello all,

I'm at my wits end chasing down a problem with my 2002 qx4. Two weeks ago my wife was driving the car and it died while she was at a stoplight. Long story short the vehicle would no longer maintain idle but she was able to limp the vehicle home. I checked the car out when I came home and the car still would not idle and would not start unless I gave it gas and held the gas pedal down. I checked the codes and pulled P1102 MAF. I checked the harness and voltages and everything looked good so I replaced the MAF. The MAF code went away but the car stil wouldn't idle. I' searched and read every post I could find on similar issues. A lot of the posts centered around the IACV but I had no codes indicating that was the problem. As a precaution I went ahead and changed the IACV (new hitachi from Amazon) and thoroughly cleaned the throttle body while I had it off the QX4. After I put everything back together I cleared the codes and tried to start the car. The car still wouldn't start but after the engine reached normal operating temperature it would idle at a very low RPM (250). The car will drive and rev to any needed RPM and while moving the car runs completely normal. As soon as you stop though the idle drops to 250 and the car will sometimes stall out. I've tried the idle air relearn procedure 3x but nothing changes. The car now shows P0506-idle control system rpm lower than expected. I've also changed the fuel filter and checked all the vacuum lines and fuses but no dice. Has anyone else had this issue? Any ideas for what else I might check? This is very frustrating. Any help would be greatly appreciated.

[Buzzman]

This is a known problem and has been discussed on here many times. There are lots of threads on this subject. (try searching again).
A bad IACV can and will take out the ECM. It doesn't always happen, if the IACV is replaced right away.
Driving around with a bad IACV for any length of time will damage the ECM.
The resulting codes are P0500, 0505 or 0506.
I suspect you'll have to pull the ECM and have it repaired.
Circuit board medics is one place that does them.
The usual price is $250.00.
You can also try and take your truck to the dealer and ask them to do an idle relearn via their diagnostic reader.
The relearn procedure in the service manual is not always effective.
I tried it numerous times with no luck. The tech at the dealership did it for me in 3 minutes, for free.
Keep us posted.

[heelsrtops]

Thanks Buzzman. While I agree that the code is consistent with the ECU problem the sequence of events doesn't seem to be. I never had a code for the IACV or anything else related until after I changed the IACV. As far as I know the old IACV was working properly before I changed it. I changed it out as a precaution and because I already had the throttle body off. The P0506 code didn't appear until after I replaced the IACV with a new one. I would also tend to think that if the ECU were damaged there would be symptoms at other modes of operation not just at idle. Im not saying that the ECU can't be the problem just that I don't see the chain of events leading up to the IACV failing and taking out the ECU. I'm a novice at this so I'm certainly willing to be corrected if I'm in error here. Thanks for the heads up on the idle relearn procedure. I've done it exactly as described in the manual but it never seemed to take. I think I'll follow your lead and let the dealer have a shot at it.

[Buzzman]

Just as an FYI, when the IACV fails and "takes out" the ECM, it actually doesn't affect the rest of the operation on the truck.
The IACV only takes out the one chip that controls the valve, nothing else.
I have heard of some people being able to source the electronic part that fails, and actually repairing the ECM themselves.
Mine failed about two years ago, and at that time, I was not aware of what chip it was.
When I took out my ECM and opened it up, I looked for a burned component, but never found anything.
I sent it out for repair and then had the dealer do the relearn after I reinstalled it.
Anytime you replace an IACV, you need to do a relearn, so maybe that's all that's wrong. Good luck.

[TheReplay]

Dollars to donuts its your ECM, even if you dont think it is...

I also had similar symptoms, including the "die in the middle of the road" first symptom. My IAC codes didnt show up til much later, and by much later i mean not until my idle started being erratic.

Pulling your ECM isnt hard, and you can open it up and look for damage. The chip in mine was cracked down the middle and burnt.

[heelsrtops]

I'm going to pull the ECU this weekend and see if there are any visible signs of damage. I unplugged the IACV and the truck behaves exactly the same as when it's connected. That would lead me to believe that the ECU isn't doing anything to adjust the IACV. I located a business in GA that can rebuild the ECU for $119 plus shipping so I'll send it there early next week. Will update this thread when I know more.

[TheReplay]

FYI --- You can see a couple of pics I posted of my burnt component in this thread

post6697874.html#p6697874

I work on electronics all day long so my confidence in soldering is high. Luckily, it appeared visually that only the component was damaged and none of the traces seemed damaged on my printed board.

I was able to buy two parts for 15 bucks shipped off ebay.

If you have even a halfway decent iron and can fashion a small chisel tip, buy some solder wick, and watch a couple youtube videos on "through hole" soldering, you can do this repair this yourself.

granted if there are other issues with the board i am sure that the company you are using will ensure function, if repair is needed beyond just simple remove/replace of the component.

[sroberts]

If you've not done so already, I suggest testing the IACV with an ohmmeter to confirm it is in spec. I had idle issues and found the IACV resistance to be about 50% of spec. My guess is that the lower the IACV resistance, the greater chance of damage to the ECM. For specifics, look at the P0505 diagnosis guidelines in the "EC" manual at http://www.nicoclub.com/archives/nissan ... nuals.html.

[heelsrtops]

Thank You TheReplay and Buzzman.

I pulled the ECU and just as you described IC17 has a nice big hole in it. I don't see any danger traces or other issues so I may try replacing it myself. Actually this is encouraging to me as it's the first of all the issues I've had to repair on this truck where the source of the problem was relatively obvious. I'll update the thread after I give it a shot.



Just to share the info, before I pulled the ECU I decided to hook up the old IACV just to see whether the ECU was sending any commands. I noticed when I pulled the original IACV that the plunger was completely extended thereby closing off any air from the throttle body except when the gas pedal is compressed. When I connected the IACV back up and then turned on the ignition nothing happened. That explains why the truck wouldn't idle. I pulled the stepper motor off the IACV and turned the ignition on again. The stepper motor then advanced the plunger further out. I turned off the ignition switch but the stepper motor did not retract the plunger. I cycled the switch several times and the stepper motor walked the plunger completely out of the motor!

I failed to mention that before this I tested the old IACV resistance and oddly enough everything was within spec which makes me question which is failing first the IACV or the ECU?

After all that I reconnected the plug to the IACV on the throttle body and cycled the ignition switch. No movement detectable in the stepper motor (as expected) and the truck acts exactly as before.

[heelsrtops]

If you've not done so already, I suggest testing the IACV with an ohmmeter to confirm it is in spec. I had idle issues and found the IACV resistance to be about 50% of spec. My guess is that the lower the IACV resistance, the greater chance of damage to the ECM. For specifics, look at the P0505 diagnosis guidelines in the "EC" manual at http://www.nicoclub.com/archives/nissan ... nuals.html.

Thanks! Good advice. You must have posted this while I was composing my latest post. I tested the resistance at the terminals and everything was in spec (around 23 ohms).

[heelsrtops]

Ok. Problem solved. I desoldered the failed IC this morning and replaced it with the new one. After I reconnected the computer and restarted the QX4 the truck behaved exactly the same as before. I let the truck come up to normal operating temperature and restarted the truck twice more. On the third start it started idling correctly and has run fine ever since. Oddly enough I have not completed the idle relearn procedure but the truck is running exactly as it should. Strange that but I'll take it.

If you are considering doing this for yourself, it is doable if you are patient and wiling to move slowly. I spent maybe $40 total on a soldering iron kit with a wedge tip, some tin-lead solder, and two replacement parts. Google eevblog soldering tutorial for a very good YouTube primer on soldering.