1. #1
    Metalmorphoses's Avatar
    Metalmorphoses is offline Junior Member
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    1987 XJ running too lean

    Jeep is at my dad's, he's been trying to figure this thing out.
    It backfires, engine temp skyrockets.
    All sensors are supposedly working fine.
    He thinks it's a computer/electrical/"high impedance" issue?
    He can't narrow down what's causing this.
    He says he can't find a main wiring harness for that year, can't find a code reader for that year, as I guess generic code readers won't work on an old jeep.
    He stuck a new 89/90 computer in it, didn't solve the issue.

    Sent from my [device_name] using Jeepz.com mobile app

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    Re: 1987 XJ running too lean

    Quote Originally Posted by Metalmorphoses View Post
    Jeep is at my dad's, he's been trying to figure this thing out.
    It backfires, engine temp skyrockets.
    All sensors are supposedly working fine.
    He thinks it's a computer/electrical/"high impedance" issue?
    He can't narrow down what's causing this.
    He says he can't find a main wiring harness for that year, can't find a code reader for that year, as I guess generic code readers won't work on an old jeep.
    He stuck a new 89/90 computer in it, didn't solve the issue.

    Sent from my [device_name] using Jeepz.com mobile app
    Why do you believe that the engine is running too lean...? If the answer is: "Because lean engines run hot and tend to backfire", I would not argue that point with you. However, if I knew that the engine was running hot and backfiring, I wouldn't necessarily assume it was a mixture issue. I know it seems like a subtle difference, but it is an important distinction to be made here...

    I really need more recent history on this vehicle. Significant mechanical work or electrical work done...?

    When did this problem start? Was it sudden and recent? Was it a progressing problem going on for some time?

    Waiting for your reply...
    Restoration Rides

    Sent from my Z981 using Tapatalk

  3. #3
    Metalmorphoses's Avatar
    Metalmorphoses is offline Junior Member
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    Re: 1987 XJ running too lean

    Quote Originally Posted by RestorationRides View Post
    Why do you believe that the engine is running too lean...? If the answer is: "Because lean engines run hot and tend to backfire", I would not argue that point with you. However, if I knew that the engine was running hot and backfiring, I wouldn't necessarily assume it was a mixture issue. I know it seems like a subtle difference, but it is an important distinction to be made here...

    I really need more recent history on this vehicle. Significant mechanical work or electrical work done...?

    When did this problem start? Was it sudden and recent? Was it a progressing problem going on for some time?

    Waiting for your reply...
    Restoration Rides

    Sent from my Z981 using Tapatalk
    After talking to my dad, here's what I gathered:

    - It was sudden, not progressing.

    - The Jeep was running fine, though the last few times it was running okay *before* it started acting wonky, it smelled faintly of gas, and when it was shut off, there was a wisp of smoke coming from under the hood.

    - Upon inspection, my dad found that there was some puddled gas on the aluminum tray around the intake, by the 5/6 injectors, but it was still running fine at that point. Because of that unsafe condition, my dad started swapping out the injectors with newish ones from a Ford V8.

    - It started running funky *before* the injectors were swapped out, but they were swapped out because of that unsafe condition of gas dripping from them, not because of the funky running.

    The things that have been checked/replaced:

    - The crank shaft position sensor was changed out when the engine was changed out a few years ago -- my dad replaced it with a new one before the new engine was put in.

    - The intake manifold gasket was replaced and sealed with copper spray because he thought there was a slight leak.
    After eliminating all exterior vacuum sources, it ran a little better.

    - The manifold air pressure sensor/iac motor/throttle position sensors were checked and found to be working properly.

    - Fuel pressure gauge 39 to 49 PSI, which is what is recommended and standard for that year of Jeep.

    - Went through all electrical sensor grounds

    - Replaced cracked vacuum lines and checked the engine electrical harness

    - On a whim, he pinned (literally, with a straight pin) the 5V input to the output on the MAP sensor, which changed the air/fuel ratio to "something the engine could survive on". This caused the Jeep to run normally, idle normally, no backfiring, no lumpy running.

    - Without the pin/output being linked to input, it cranks but doesn't want to start. You have to pump the gas, do several things to trick it into starting up.

    - It starts up, but only runs for about 8 seconds, which he explained to me as the computer running a "default" program for about 8 seconds before all the sensors and whatnot start doing their individual jobs. After 8 seconds, it backfires, the injectors make popping noises under the hood, and it dies.

    In the last couple days:

    - The oxygen sensor was replaced with a new one, used contact cleaner on the connector

    - With the pin/input to output, he let it warm uo for 10+ minutes, engine temp was 190 degrees, it was running fine.

    - Took the pin out, within 7 - 8 seconds, backfired and died.

    - Put the pin back in, drove it around, engine temp was still 190 degrees, it ran fine, drove it around for about 10 mins, noticed the engine was running a little rich.


    Hope that helps. I'm just reiterating what I could write down that he was telling me, so I hope it makes sense.

    Thanks!

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