Help with idle issues

[Dmenz929]

Looking for some help with diagnosing my 02 pathfinder. I have been a long time lurker of this forum but this is my first time posting. My pathfinder had a problem with the idle jumping from 1500 rpms to 2k. Switched the iacv now it is stuck in the fully open position and idles at 400 rpms and dies. I had recently replaced the injectors with fbjc101 injectors changed all the intake manifold gaskets. After it drove great for over 6 hours. Then the idle problem started. I have taken out the pcm and the idle air circuit isnt burned looking or smelling. My main questions are could the pcm be bad even without evidence of it being burnt , or is my problem a vacuum leak from a problem with the intake. I cannot hear any hissing nor seen any smoke when I did the youtube cigar smoke test. Thanks for any help.

[VStar650CL]

Unless it's huge, a vacuum leak will typically increase the idle and not decrease it. That's the principle the IACV works on, so one that's stuck open should cause a high-and-uneven idle and not a stall. Stuck shut might cause the symptoms you're seeing.

The IACV itself is a unipolar stepper motor that only draws about half an amp per phase, so yes, it's entirely possible to blow up the driver chip with no smoke or external evidence. Shorting either of the two power poles to any of the four phase wires will blow that phase in the IC as soon as it tries to operate. IIRC the chips used employed darlington BJT's and not FET's, and overloaded BJT's usually blow open and not short. That means you won't generally get blisters in the epoxy like you would from a melted MOSFET.

[Dmenz929]

Thanks for the reply I have the mec14-346 pcm I have heard the mec14-345 pcm is compaitable with my pathfinder. Do you know if that is true?

[VStar650CL]

No idea, sorry. Those are Hitachi part numbers, and Nissan has never published anything to cross reference the Hitachi numbers with Nissan p/n's.

[Dmenz929]

OK i got a new ecu I tried the new iacv on my old ecu before so would it be burnt up now to or would it be ok to use with the new ecu. Im leary of using it becuase I really dont want to burn up this new ecu

[VStar650CL]

You can check the stepper with an ohmmeter. You can see the pinout on EC-446 here:
https://www.nicoclub.com/service-manual ... 2%2Fec.pdf

Each place you see a coil in the diagram, take a measurement (1~2, 2~3, 4~5, 5~6). They should all read 20~24 ohms. Anything lower than 20 means an insulation failure, 0 means a dead short. Anything above 24 means a weak coil, OL (infinity) means an open coil. Weak or open coils aren't dangerous, they'll just make the stepper non-functional. Partial or dead shorts are dangerous, they can overload or blow the driver IC in the ECM.

[Dmenz929]

OK, so if the ohm test works out there is no worries about it burning up the ecu. Thanks for your help seems like ur doing your part to keep this forum alive.

[VStar650CL]

OK, so if the ohm test works out there is no worries about it burning up the ecu. Thanks for your help seems like ur doing your part to keep this forum alive.

You're most welcome, I try. As long as the ohms don't show low or zero then there's no risk to the ECM. If they show high or OL then your stepper is shot, but that won't hurt anything. Only low readings are dangerous.