I might need a new wiring harness, or do I? (1 Viewer)

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May 22, 2018
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Okayyy, this is driving me crazy. 97 FZJ80

One peaceful afternoon I started the truck, the motor is shaking and I was like that is a misfire, so I drove it in the neighborhood until the check engine light comes up.
Turns out it is p0304, number 4 cylinder misfire.
The symptoms when I drive it was interesting, I rev it up couple times, then the misfire go away, when it rest back to idle speed then I try to speed up like 2-3k rpm, the misfire comes back. Basically, the misfire happens under load or when the truck is moving.
So far the spark plug is good, plug is good, I thought it was the injector so I change all of them to Bosch 4 hole injector, still no luck, idle good, when I started driving, the misfire comes back.
Same symptom happens.
I have an another ECU just laying around I got from a parts car, plug in, no luck.
I check the wiring harness behind the glove box all look good.
I peel open the wiring harness at the egr which I deleted about a 2 years ago, all look good.

What is wrong with this truck?
 
Okayyy, this is driving me crazy. 97 FZJ80

One peaceful afternoon I started the truck, the motor is shaking and I was like that is a misfire, so I drove it in the neighborhood until the check engine light comes up.
Turns out it is p0304, number 4 cylinder misfire.
The symptoms when I drive it was interesting, I rev it up couple times, then the misfire go away, when it rest back to idle speed then I try to speed up like 2-3k rpm, the misfire comes back. Basically, the misfire happens under load or when the truck is moving.
So far the spark plug is good, plug is good, I thought it was the injector so I change all of them to Bosch 4 hole injector, still no luck, idle good, when I started driving, the misfire comes back.
Same symptom happens.
I have an another ECU just laying around I got from a parts car, plug in, no luck.
I check the wiring harness behind the glove box all look good.
I peel open the wiring harness at the egr which I deleted about a 2 years ago, all look good.

What is wrong with this truck?
Try switching your coil/ spark plug that has the misfire to a different cylinder. I'm not familiar with the 97 so I'm not sure if you still have a cap and rotor/distributor or individual coils but either way that's a place to start.
 
Okayyy, this is driving me crazy. 97 FZJ80

One peaceful afternoon I started the truck, the motor is shaking and I was like that is a misfire, so I drove it in the neighborhood until the check engine light comes up.
Turns out it is p0304, number 4 cylinder misfire.
The symptoms when I drive it was interesting, I rev it up couple times, then the misfire go away, when it rest back to idle speed then I try to speed up like 2-3k rpm, the misfire comes back. Basically, the misfire happens under load or when the truck is moving.
So far the spark plug is good, plug is good, I thought it was the injector so I change all of them to Bosch 4 hole injector, still no luck, idle good, when I started driving, the misfire comes back.
Same symptom happens.
I have an another ECU just laying around I got from a parts car, plug in, no luck.
I check the wiring harness behind the glove box all look good.
I peel open the wiring harness at the egr which I deleted about a 2 years ago, all look good.

What is wrong with this truck?
Yeah if the spark plug gap is becoming excessive anytime you accelerate uphill or from a stop a puts a lot more load on the spark plug and with an excess of Gap you can cause it to stop firing. You also might have a short within the coil itself better. If you have a voltmeter do a ohm check on the primary and secondary coil
 
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Plugs and wires are cheap. I would replace them even though they test good. Just don't use the 4 prong bosch spark plugs.
 
With the miss, is it smoking at all? Does the exhaust smell like gas?

The typical failure point for the injector wiring is by the EGR or by the glove box crossmember, leading the wire to ground out and leave the injector on.
 
How much salt has your 80 been around?

Any chance you can power-wash the motor while running, and start from there?

I’m wondering if you have a engine harness ground that needs the bolt pulled, dielectric grease & torqued back down

-Have you wiggled the fusible links when idling / if you have a hand throttle run it to about 1500rpm & see if you get misses & smooth outs in it?

Wiggle the harness while running (esp the EGR pipe area, despite removing) - see if that does anything.

At minimum, pull the battery terminal for the 2-5mins (IDK the number, so I just go 10+ mins) - let it reset whichever ECU you’re on (saw you swapped, so slim/none prob on that one).



———— Just some off the cuff Ideas I would check out if it was my 80 acting up.

FWIW - I greased all exterior sockets / most inner too - with either white lithium or dielectric over the yrs when working in an area - really smoothes out any ‘quirks’ & keeps the 80 running consistent even in monsoon rail, deep puddle crossing, etc.
 
Try switching your coil/ spark plug that has the misfire to a different cylinder. I'm not familiar with the 97 so I'm not sure if you still have a cap and rotor/distributor or individual coils but either way that's a place to start.
thanks, I got a new set of wires ill put them on and see what happens tomorrow.
 
Yeah if the spark plug gap is becoming excessive anytime you accelerate uphill or from a stop a puts a lot more load on the spark plug and with an excess of Gap you can cause it to stop firing. You also might have a short within the coil itself better. If you have a voltmeter do a ohm check on the primary and secondary coil
my spark plug is quite new, they were only like 2 years old. how ever ill try swap them to different cylinder and see what happens.
 
With the miss, is it smoking at all? Does the exhaust smell like gas?

The typical failure point for the injector wiring is by the EGR or by the glove box crossmember, leading the wire to ground out and leave the injector on.
no smoking, haven't smelled the exhaust yet
 

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