Completely, utterly stumped

[rogerontherun]

I have been working on a 2001 QX4 with 197,xxx miles. It has been sitting for a few months, and now the owner has decided to sell it. A week or so ago he went to fire it up and it stumbled and chugged, but made it back home. Eventually it stopped running altogether. I read the codes for it, P0505 and P1140. I told him what they meant, and the 0505 code was more likely the culprit. I went ahead and did my research, then replaced the IAC. Here are where the problems truly begin. It definitely ran again, but had an idle of 1500-2500 rpm. We attempted an idle relearn to no avail. I checked and double checked all wires and hoses I disconnected, then I cleaned out the vacuum hoses for the SCV, sprayed the MAF (with MAF cleaner) and applied dielectric grease. I have checked voltages and resistance in the IAC, TPS and MAF. I am still getting P0505 plus 1130, 1131 and 0171 codes plus dramatically reduced performance and a completely erroneous idle. The idle seems to have a pattern though.. I will start at 1,000 then climb to 2,000 and then bounce from 2,000 to 1,500 6-8 times then go back down to 1,000. All that happens every 45-60 seconds. Since I do not completely understand the whole SCV system (or Nissan engineering for that matter), it's easy for me to point my finger and blame that. The only things I have in mind are a stuck/corroded valve in the SCV system or a plugged fuel filter. I beg of you.. please help me!

[AZhitman]

Check those things, but also look at the temp sensor. That'll cause a "hunting" idle, as well as a bad MAF.

Do you have the FSM for it?

[rogerontherun]

I did download a .pdf file from this site today about the Engine Control System, not sure if that's what you're referring to, but has a lot of info.. 650 pages of it.

As far as the MAF sensor goes, I'm thinking the owner won't want to put another few hundred dollars into a sensor that may or may not be the fix. I'm not wild about such limitations, but understand them fully. Is there a sure fire way to test it? I went as far as to disconnect it while the vehicle was running and nothing happened beside the fact it ran a code for it. I also did the same thing with the swirl control valve control solenoid valve (what a f***ing name), and again nothing changed but the vehicle was also at operating temperature.

The temp sensor appears to work..? I have a reading on the instrument cluster.

[4xq]

Well, high idle, running lean, air fuel mixture codes. I would go looking for a vacuum leak - sounds like too much air. Brake booster hose / valve, PCV valve etc.

Should be a vacuum hose diagram under the hood.

[AZhitman]

The temp sensor appears to work..? I have a reading on the instrument cluster.

Check the voltage and make sure it's within the parameters. Vac leak makes sense as well (esp with the high mileage) and is easy to test for.

[stylesjas]

Rogerontherun, did you ever find the fix for your issue?

[Buzzman]

P0505 code is a bad IACV.
Unfortunately, the IACV failure usually takes out the ECM as well.
I had this problem recently and sent out the ECM for repair.
The repair is a flat rate of $250.00, as opposed to a new ECM from the dealer for over a thousand.
There's a whole thread on this subject:

2002-pathfinder-wont-idle-any-suggestio ... 79-30.html

[Chuck Tribolet]

The temp sensor appears to work..? I have a reading on the instrument cluster.

I believe the instrument panel has one temperature sensor and the electronics have their own temperature sensor.

Also, there are two FSMs for the '01s. One for the early ones, and one for early and late, usually with separate subsections for the two. I don't know where the dividing line is for the QX4.

Chuck

[Towncivilian]

The temp sensor appears to work..? I have a reading on the instrument cluster.

I believe the instrument panel has one temperature sensor and the electronics have their own temperature sensor.

Also, there are two FSMs for the '01s. One for the early ones, and one for early and late, usually with separate subsections for the two. I don't know where the dividing line is for the QX4.

Chuck

I think there used to be two sensors for WD21s, but I don't believe R50s use two sensors. Both read from the ECM's engine coolant temperature sensor.

CanuckQX4 was generous enough to give us a copy of the 2001.5 QX4 FSM: http://www.nicoclub.com/FSM/QX4/2001.5_QX4/
SECU differences based on VIN serial number (at least I think that's the serial number it refers to) are noted at the top of the EL section. Not sure if that is also the cutoff date for the .5 model altogether, though. IIRC I have a 2001.5 but with the "old" SECU.

[asnorton44]

As far as the MAF sensor goes, I'm thinking the owner won't want to put another few hundred dollars into a sensor that may or may not be the fix.

If you do end up needing a MAF you can use a Nissan Maxima MAF its the exact same and a fraction of the price. The part # is 22680-2y001. I've had one in my 01 QX4 for a few years and its working great.

[Chuck Tribolet]

I think there used to be two sensors for WD21s, but I don't believe R50s use two sensors. Both read from the ECM's engine coolant temperature sensor.

My '01 PF FSM shows two different coolant temperature sensors, one for the ECU (F114) and one for the gauge (F117).

Chuck

[Towncivilian]

Right, I see F117 (thermal transmitter connector). Thanks for clarifying that! I figured the gauge would just be a buffered version of the ECU's ECTS.

[Chuck Tribolet]

It's not uncommon to have separate sensors for the temp gauge and for the ECU. My Pathfinder does, my '87 Corvette does, my '01 Camry daily driver does. Maybe it's for redundancy, maybe they want to measure it in different places.

Chuck

[stylesjas]

Broke a valve cover bolt, can I get away with not extracting it?