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Thread: 1990 mighty max 2.4l rough idle runs rich

  1. #1

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    1990 mighty max 2.4l rough idle runs rich

    I have a 1990 mighty max I bought off my neighbor he was tired of sinking money into it and got it fairly cheap so figured why not.
    New parts
    plugs
    timing belt
    rotor button
    distributor cap
    pretty sure the plug wires are new as well(might just go buy some to see if thats possibly a issue
    fuel filter under the truck looks pretty new as well

    Symptoms
    runs rich so rich it left black on my carport and plugs black as can be when I took another look at them they are only three weeks old
    Idles rough,has a miss
    bucks at low rpms when accelerating I would say 1000-1200 rpms(no tach)

    I have taken a look at the vacuum lines they look like they are correct but the diagram was a tad bit confusing that I found anyone possibly have a link?

    No check engine light which is odd to me usually with my old 1991 s10 it would throw a cel when anything was wrong

    any help would be appreciated I just dont really know much about these trucks. I have looked around the forum but not really found much

  2. #2

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    I.D.K.?
    As for not throwing codes, have you hooked up with a meter and seen that its not throwing codes? You could have a burnt Check Engine Bulb or the PO could have taken it out. These truck are great when using the K.I.S.S. method.

  3. #3

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    that was actually my first thought so i turn the switch to on and saw that the light does work. I have been trying to keep it simple.
    The vacuum lines look like they might have been moved at some point so have been looking for a good diagram on the 2.4 routing`

  4. #4

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    I'm a big advocate of checking the throttle position sensor, even since I replaced mine. That is one of the #1 gadgets that cause rough idling and running rich, and so far as I know, one of the few ways to correct the problem.

    Try just loosening it up and twisting it counter-clockwise (engine off) a quarter inch. See if it affects anything.

    The other thing i noticed, before I replaced the old one, was that adjusting it made little difference, or only worked for a short time.

    I've yet to fine-tune the new one, but I got amazing results when I replaced it, and the truck has been extremely happy ever since.

    Another note, regarding CEL: I was one of those who removed mine, at first. I have since put it back: this is the trucks' "voice", and I need to listen to "him".
    The greatest gift you have to give to the world is that of your own self~transformation.

  5. #5

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    beware the problem could be the ecu my truck has had a problem for the past like 6 years(was grandpas) and until it died one day neither my dad nor i could figure out the problem so i would pull it out and just check the capasitors on the board and make sure nothing is going bad. i have pictures posted in my project log of the damage to the ecu

  6. #6

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    ill probably try testing the tps with a multimeter and/or trying to adjust it. and the capacitors im guessing broke open and leaked i have seen that on some old honda tcm's i think. other than the idle issue and the bucking the truck runs great at say 35 mph i couldnt ask for a better vehicle.

  7. #7

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    i tested the tps had around .49 which if i recall was in the area it should be and when i slowly pressed the throttle no rapid change in movement(the voltage went up slowly ) so it seems good. i accidentally left it unplugged when i started the truck and had no cel still ran with a horrible miss.

  8. #8

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    tested it had .49 which seems about right for the truck if i recall what i read online correctly and when i moved the throttle it moved smootly and not jerky or random my blazer had a bad tps once and it jumped randomly when opening the throttle. I think im gonna end up putting more money than what i paid for the truck i just bought it to have something to drive while getting my other car painted

  9. #9

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    Another suggestion is to check the timing belt installation...particularly if it's a new belt or the problems started when the new belt was installed. There is a notorious problem with getting them one tooth off on the timing. That problems won't cost you anthing but time, but admittedly, a chunk of time.

    The tps setting sounds correct. If you hooked it up to the ECU per instructions, then I guess you're good on that.

    And thanks for the reminder, RNMM: I need to pull my ECU and have a look at it...soon. It's just such a pain to get to, it's one of those things I've put off...hopefully not to my detriment.
    The greatest gift you have to give to the world is that of your own self~transformation.

  10. #10

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    Quote Originally Posted by chrism1791 View Post
    tested it had .49 which seems about right for the truck if i recall what i read online correctly and when i moved the throttle it moved smootly and not jerky or random
    Well, with an true RMS meter, the tps is not an issue.

    2 more questions Sir, more info needed for an 'web diagnosis'.

    1) coolant temp ?
    2) fuel pressure?

    Thanks!

  11. #11


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    Vacuum doesn't control the timing, the ECU does, the base timing has to be set with the ECU timing adjust pin grounded. pin is in this plug on the passenger side.

    Click image for larger version. 

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    base timing is 3-7 degrees BTDC

    procedure is in the Haynes Manual page 1-36

  12. #12

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    See, This is what makes this Forum AWESOME!!!!!!!!

  13. #13

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    it was late last night after it warms up to operating temperature it still misses horribly i would think iacv but its not really a fluctuation and ill have to check fuel pressure
    The timing I do remember something very odd its would only take the 122 teeth 2.0 belt not the 2.4 belt i sorda just figured maybe a odd year for these trucks bc it looks completely stock and no scratches or anything in the bay from someone dropping a different motor in.

    as for the timing is there a link in the forum to a pdf file ?

  14. #14

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    Ecu (mD144743 E2T34172 9725 A) on top of ecu side )4743 E2T34172) photos it looks like somethings gone a little bad ?
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  15. #15

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    under the 9249 black burnt spot Click image for larger version. 

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  16. #16



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    Ya I see some crusty circuits there on that board. Here is an enlargement of the area, looks charcoaled. Strange that a capacitor didn't pop it's lid on this short. Looks like it got very hot though.

    click image to expand size
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    Here is a pdf that may help if it fits your specs.
    ENGINE
    4G6 SERIES
    4G6 Engine.pdf
    Click image for larger version. 

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  17. #17

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    One of my friends had a old mighty max the same year as mine at his dads im gonna see if i can borrow or buy his ecu if its the correct spec and see how it runs and purchase a new one. thanks for that pdf I have not owned a mitsu before

  18. #18



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    We have Manuals also for download. Those ECU are not a cheap item. Seems everyone is robbing the wrecking yards for them and selling them at top dollar these days.

  19. #19

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    Quote Originally Posted by BradMph View Post
    These ECU's are not an cheap item. Seems everyone is robbing the wrecking yards for them and selling them at top dollar these days.
    Spoken for truth!!!. & still will have the caps issue...

    chrism1791,
    So much for an 'simple' fix, that being said, before you buy new ck http://avproecm.com/.
    I happened across the site doing research for the inevitable on the '92..

  20. #20


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    You will find the manuals at

    http://www.mightyram50.net/vbulletin...er-216-Manuals

    This is where I got one of my ECU's http://www.foreignecurepair.com/

    I have a regular and a socketed prom version. I really should sell them, since I'm using the AEM now...Hmmm

  21. #21

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    i finally got a anolog meter off a buddy to see if maybe the meter would throw the needle in a crazy way(i currently have no cel) but it showed normal state after downloading and looking at the manual. would a ecu still show normal state even with the damage?

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