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Thread: electrical problem leading to bad gas mileage?

  1. #1

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    electrical problem leading to bad gas mileage?

    I have a used 1990 mighty max with 46g4 with about 157,000 miles efi auto. I bought this truck from a mechanic said that the truck at one point dumped serious black exhaust and replaced the o2 sensor and ecm. The truck runs rich you can smell it. So i unplugged the o2 to see how it ran, got better but the exhaust got smokey. The check engine would come on go off come on go off and was recently stuck on for o2 sensor. I accepted this at 15-17mpg. So recently i went to put in a new stereo system. This is where things get messed up. Someone cut out the original wire harness and wired a new one for a tape deck. I pulled all wires. The check engine light went off, also my door alarm stopped working along with cigarette lighter. Now if i let the truck sit for a day or two when i start and go to reverse there's a loud clicking noise under the hood for a second or 2 then goes away and won't happen for the rest of the day. There is also a switch on the dash next to room dimmer that has a wire running to the fuse box and is also grounded to the frame. I flipped once and noticed nothing. when trying to plug in the new stereo i unplugged and can't remember where it went. Does anyone have an idea where to start.

    things i've done:
    new wires
    new plugs
    on 2nd tank of rxp
    checked all vacuum hoses nothing looks wrong
    checked fuses all are good
    temperature sending unit looks new
    new belts
    new air filter
    (not all of these were because i thought they would fix the problem but because they needed replacing)

    I want to replace the engine with a v6 but that is going to take some time so i need to figure this problem out.

    Thanks!

  2. #2

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    4g64*

  3. #3

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    My truck does the same. Gets about 17-18 in town and 22-24 hwy and has a CEL on for a lean code and runs a little rich. I have yet to change the fuel filter first off and another thing to check is the exhaust manifold. They like to crack on these trucks pre-O2 tricking the sensor to think the system is running lean and dumping fuel. I'm having trouble finding one.

  4. #4

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    I'm beginning to wonder if it's a stuck injector or even a clogged cat, just haven't had time to take it apart yet

  5. #5

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    Clogged cat you wouldn't be able to drive it very far. However a rich condition could clog it. A leaking injector you should see when starting the truck after sitting.

  6. #6

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    only getting roughly 10-12mpg. Extremely rich exhaust. I'm thinking stuck injector but when i changed plugs none of them were wet

  7. #7



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    The plugs will be black not wet unless it's really broken. But you need to clean the injectors. If you pull them out and hook them up to the system just turn on the key and look to see if it's shooting fuel out of one.
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  8. #8



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    "There is also a switch on the dash next to room dimmer that has a wire running to the fuse box and is also grounded to the frame" This line reading sounds scarry, Power, but grounded? typo?

    Also, messing with wires under dash by the cig lighter will often disconnect the wire from lighter. It's not a very sturdy connection used for the lighter, so you may of just pulled a little to hard and it came undone.

    My test for rich mixture is kind of insane, but it works, lol. bend down and take a couple seconds, (not inhaling) of exhaust, burning eyes from the exhaust is sure sign your too rich, but your well beyond to rich if your smelling raw fuel out exhaust. If injectors don't fix it, maybe a hotter plug could help burn the fuel more if your still chasing the problem.

  9. #9

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    I pulled the old stereo out and wired in a new one. Now the cigarette lighter is working again. As for the switch no typo had a wire running to one of the fuses(can't remember which one) and grounded to the frame. I just pulled it out. No idea what it was for. I've noticed that it is smoking especially when sitting in a fast food drive thru, but no loss of oil or coolant. Possibly egr valve?

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