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'87 MJ Idling at 300RPM? Wacky idle/dying at idle


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1987 MJ 4.0 AW4 - NP231

 

 

If I'm in gear idling at say a stoplight, it will settle into a 250-400 RPM idle, will die when at operating temps.

 

In Park/Neut will sit around 500-700rpm

 

If by chance it doesn't die, as I accel from the stoplight, it bogs. Rich fuel smell from exhaust.

 

Unknown age of O2 sensor. Where should I start looking? The IAC was my first complaint but I'd like a 2nd opinion.

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I might be wrong, but it could possibly be the CPS sensor (located on the top of the transmission). They're known for going out in our jeeps. Does it ever just die randomly while you're driving down the road?

 

 

No random deaths nor will it die if I keep working the gas pedal

 

 

 

Pete, all of them are nice and clean save for the O2. THats my next step

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Try un plugging the MAP sensor then starting the truck. It should idle better, but it'll start running like crap around 1500rpm.

 

My frontier had an idling problem that turned out to be a very dirty MAP. I cleaned the hell out of it, and now it purrs like pissed off kitten!!

 

Definitely try cleaning it first before you just buy a new one.

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The oxygen sensor could account for the rich smell. If it isn't reading accurately, the ECU may be "overclocking" the injectors because it thinks the mix is too lean. The last time I replaced the O2 sensor on my '88 XJ, I could hear it. It's a 4.0L 5-speed, and I began to notice muffled backfiring either in the downpipe or in the catalytic converter when I'd upshift or when transitioning from acceleration to coast.

 

I don't know if an over-rich mix could cause dying at idle, but I suppose if it's VERY rich it might essentially flood out.

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I am in the same boat. :hijack:

 

Tried IAC (2 of them), MAP, Both CPSs (Cam/distributor and crank), spark plugs, EGR, TPS, compression tested the engine, wires, O2 sensor (2 of them), checked all the intake and TB bolts for leaks and changed the air filter :rotf:

 

Next on the list is the checking power to the injectors and then maybe swapping them out with some spares.

 

Funny thing about this is it went bad and went bad QUICK.

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I am in the same boat. :hijack:

 

Tried IAC (2 of them), MAP, Both CPSs (Cam/distributor and crank), spark plugs, EGR, TPS, compression tested the engine, wires, O2 sensor (2 of them), checked all the intake and TB bolts for leaks and changed the air filter :rotf:

 

Next on the list is the checking power to the injectors and then maybe swapping them out with some spares.

 

Funny thing about this is it went bad and went bad QUICK.

 

I know eh, this dropped itself on me when it went from driving 15 feet TOPS to carting my @$$ to work each day.

 

Its still running super rich as fawk. I swapped out a known good O2 sensor lastnight. I'm wondering if my coil is weak, as its began to putt-putt while at speed (unless I'm accelerating)

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Checked the engine block ground? Just a thought.

 

I also had a super rich condition in my 86 (carbed, so I don't know if it really correlates) due to a weak connection at the coil (the wire that feeds 12volts to the coil). The weak spark wasn't burnign all the fuel in every cylinder. Eventually I blew up my muffler while trying to diagnose it. :D

 

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I ran an additional ground so that wasn't it. I checked the power to the fuel injectors and they all checked out. We pulled spark plugs while running and it is cylinder 6 that makes no difference. I changed the fuel injector on cylinder 6 and that didn't work. Did a pressure test on it and it lost some (50%). Cylinder 6 also was the lowest on the compression test too (125 when the others were 130-150).

 

Some buddies think it might be the head gasket. I find it hard to beleive that I potentially damaged a head gasket in the driveway moving it 40 feet once a day. :doh:

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