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Old 06-02-2018, 07:48 AM   #1
urimashe
 
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Default V6 idles at 250rpm, sluggish to raise rpm.

Little background. I got my second Nissan truck from my neighbor. its been sitting for 2 years because it was not running great, now before you say check plugs and check wires, cap and rotor and all the basic stuff, it has new plugs, wires, cap and rotor before it sat for 2 years in an attempt to fix this problem, it also has a new throttle position sensor, i am pretty sure ive set that correctly.

so in the interest of diagnosing this properly, i have tested the fuel pressure, its good, as well as the regulator, timing appears to be correct, though it is hard to tell with it idling so low and unable to judge rpm by getting what throttle i can, the maf sensor is also putting out the correct voltages.

so i checked vacuum to see if i was getting back pressure from a clogged exhaust, low and behold, 5 inch of mercury. i was sure it was a plugged cat so i took the 02 sensor out, this did not resolve the issue.

now the vehicle has an exhaust leak on both banks, i thought this might be an issue, but with the o2 sensor out and seeing a bunch of oxygen with no change to how the vehicle is running, i doubt this is the cause of the problem.

the issue is when the vehicle is started, it runs normal, after it heats up, the vehicle drops down to 250 rpm, i believe the low vacuum is from the low rpm, and when you open the throttle, it tries to die just for a moment before the rpms start to clime, once it reaches around 700+ rpm, it takes off and runs normal. (i also used the battery to test the idle air control solenoid, it clicks and as far as i can tell is functioning, but no matter what i do to try and set idle speed, it stays at 250 rpm) when i let off the throttle she will backfire a little out of the exhaust, the vehicle then rolls down to 250 rpm again, and somtimes might stumble for a moment. funny part is, it sounds alright while it idles at 250 rpm, no miss fires.

so by chance when i bumped the harness that runs on top of the engine near the distributor that goes to the tpms, temp sensors, distributor, ect, the vehicle jumped up to idle at around 850rpm. i tried this several times, and it seemed hit or miss, so i stripped the harness back and found a few splices that could use some repair, but are still viable and would not cause this issue i am having, and the vehicle still idles at 250 if i play with them.

sorry i am going on with this but i want to give you guys as much info as possible.

now to the last and the weirdest part, if i unplug the temp sensor connector, the vehicle will go up to normal rpm and run fine. the first time i unplugged it, it did this but then after a bit seemed to idle back down, so i plugged it back in and then unplugged it again, no change, slow idle, played with the wires, nothing, played with the harness, nothing, then unplugged it again, it went back up to 800 rpm and stayed there, so i left it unplugged and wiggled the harness around and tried to see if it would drop in idle again, but it stays at 800.

my question is, has anyone seen this before? is there some fault in the harness that normally gets repaired? i am a mechanic by trade, and this is getting me to scratch my head a little bit. i will be taking it in to the shop in the morning, if i can get it there. to try and do some more testing. but in the mean time, i would love your guys input, as you have more nissan experience then i do.
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Old 06-02-2018, 07:49 AM   #2
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oh, this is a 1993, with the vg30e, 4x4, king cab, se
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Old 06-02-2018, 12:02 PM   #3
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Have you checked the ignition timing and checked the ecm for codes?
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Old 06-02-2018, 06:00 PM   #4
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Humm...my pathfinder ran like crap because the temp sensor (on the front of the block that is a pain to get to) failed. It sputtered and drove like crap. Replaced the sensor with a resistor and it ran great but after it got warm had a high idle, around 900-1100 rpm. I might try the same trick just to see if the temp sensor is flakey (or finally died). Moving the harness around and having it come and go is just frustrating to find.

I do not remember the resistor size unfortunately but kinda split the middle what the temp sensor would put out. Living in FL at the time I went more towards the warm side (living in FL is probably why it started fine).
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Old 06-02-2018, 06:12 PM   #5
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Quote:
Originally Posted by urimashe View Post
oh, this is a 1993, with the vg30e, 4x4, king cab, se
as i stated in my story, the timing appears to be correct via timing light. no codes.
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Old 06-02-2018, 08:50 PM   #6
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I missed that part lol

My VG30i idled all over the place before I changed that cylinder head sensor. There is a part of that harness that is bad. There is a replacement harness off a Z car that is a little longer. The part that goes bad is where it plugs in to the sensor behind the timing cover, and the top part is around the top water neck on the block.
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Old 06-03-2018, 05:36 AM   #7
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Quote:
Originally Posted by cadillacdude1975 View Post
I missed that part lol

My VG30i idled all over the place before I changed that cylinder head sensor. There is a part of that harness that is bad. There is a replacement harness off a Z car that is a little longer. The part that goes bad is where it plugs in to the sensor behind the timing cover, and the top part is around the top water neck on the block.
that might explain the part where if i pull on my harness the rpm changes, i went ahead and replaced the coolant temp sensor, but what is the one wire sensor beside it?
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Old 06-03-2018, 06:32 AM   #8
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If it also goes into the coolant passage the sensor with one wire is for your temperature gauge on the instrument cluster.
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Old 06-03-2018, 01:51 PM   #9
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You can get the little connector that goes to the temperature sensor on eBay. I think mine had broken wires inside or something, because it was making random issues. I just snipped off the old one and soldered the new one in.
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Old 06-03-2018, 03:13 PM   #10
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The actual cylinder head temp sensor is on the driver side head behind the rear timing cover plate. It is at a 45 degree angle. You have to shine a light above the water pump pulley about the one o’clock position behind the cover to see that sensor. I gently bent my cover enough with a crowbar so I could slide a deep well socket in there to get the sensor out. And don’t worry, it is not in a water jacket.
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