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Thread: 90 MM Rough Idle And Bad Gas Milage

  1. #1

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    90 MM Rough Idle And Bad Gas Milage **** UPDATE Partial Solution ****

    UPDATE with Partial Solution in post #13

    I have a 90 MM with 145k and I have been having a problem for about 5 months. I noticed my idle was getting rough and then the gas mileage started to get bad fast. Gas Mileage Chart I was averaging 22-25MPG and now its down to 10-14MPG. My guess is bad AFM but I don't know how to check it, is a 90 with the big round filter and square wiring connector. I have access to a AFM from a 91 but the plug a different shape and there is an extra wire in the plug.



    A quick list of what I have done and if it made any difference:
    Made sure all sensors were plugged in and wires looked good - all good
    Cap, Rotor, Plugs - No change
    Checked for vacuum leaks with starter fluid - no leaks
    Unplugged air flow sensor - MPG went from 10 to 14 MPG still idles rough
    Swapped engines with known good engine from a 91 (old engine ate timing belt), kept existing ECU, AFM, O2 Sensor, fuel pressure regulator, wiring harness from the 90 - idles at 2k with AFM plugged, idles rough when not, MPG same as old engine.

    Any ideas on what else to try or how to test the AFM?
    Last edited by jbrockman; 07-30-2012 at 09:26 AM. Reason: Update

  2. #2

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    I would say check the fuel and air filters. I have only have had the carbureted version of that engine and I have never worked on a EFI, but the way the chart slowly goes down, I would suspect something like the air or fuel filter. just my .02

  3. #3



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    Replace the O2 sensor and see what happens. It's dumping in fuel for some reason.
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  4. #4

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    That chart is cool how did you set that up?

  5. #5

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    Just a thought but did you change your timing? If it's off, you can get the results you have, rough idle and poor mileage.

  6. #6

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    Quote Originally Posted by camoit View Post
    Replace the O2 sensor and see what happens. It's dumping in fuel for some reason.
    Ill pick one up and see what happens. I tried the o2 from the 91 but it wouldn't idle, it kept dieing

    Quote Originally Posted by abeemanator View Post
    That chart is cool how did you set that up?
    Using fuelly.com as long as you fill your tank every time it will keep track for you and its free.

    Quote Originally Posted by lush90 View Post
    Just a thought but did you change your timing? If it's off, you can get the results you have, rough idle and poor mileage.
    I don't think that's it, it got worse over a few tanks and same running problem with 2 different engines.

  7. #7

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    Will fuels with added ethanol effect performance this way? I know that if I fill my tank in my toyota with any kind of ethanol enhanced fuel it runs like crap, especially the 3 cents per gallon cheaper Maveric gas, my truck absolutely hates maveric gas, It runs best on Sinclair or Texaco/Cheveron gas with no added ethanol.

  8. #8


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    The big problem with ethanol is it evaporates faster then gas causing either a lean or rich condition.The other problem is Ethanol will eat the inside of older rubber lines. Also any rust in the steel lines or tank will start to flake off and clog the fuel filter and fill the float bowl with rusty sand like particles.That's why I stopped using E90 on older cars and trucks.

    Not sure if the truck has a Mass Air Meter, if is does clean it real good with a contact cleaner (the wire inside) as that's what reads the amount of air coming into the intake and tells the injectors how much fuel to feed the engine.

  9. #9



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    E 85 = BAD everything in these trucks.
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  10. #10

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    I put in a new and correct 2 wire o2 sensor (it had a single wire one spliced on) and plugged the AFM back in. Idle is still rough but no more high idle when the AFM is plugged in. I have driven it for about 6 miles and will update the gas mileage when I fill up. It almost seams like its missing at idle when I listen to the exhaust.

    In phoenix all the gas is 10% ethanol gas in the summer.

  11. #11



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    Get sta-bil marine ethanol treatment - it negates the problems that ethanol causes in all vehicles, especially our carbed and older FI vehicles. My 92 honda sounds completely different from when I got it and I added the treatment - and the mileage jumped 2-4 mpg. 1 oz to 10 gal of fuel - 32 oz bottle treats 320 gals of fuel and costs 24.00 at most part stores around me.
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  12. #12

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    check your coolant temp sensor

  13. #13

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    **** UPDATE Partial Solution ****

    Here is an update with a partial solution to the problem I was having.

    I have averaged a consistent 11-12 MPG for the past 10+ tanks, no change from cooler weather to hot Phoenix summer temps with the a/c on full time. My dad (Dave the dude on here) has always said if its a weird problem its probably the ECU. He got a capacitor replacement kit from Mach V Motorsports and I soldered them my ECU. One capacitor was noticeably leaking but I replaced them all anyway. Here is more detail on the general problem with the ECUs.

    I'm still on my first tank but have 220 miles and still over 1/4 tank left. I was topping out at 150 on a tank before.

    Problems I still have:
    High idle - idles between 1400-1800 rpm
    Low power under 2500 rpm - it runs smooth with no hiccups but is gutless when accelerating

  14. #14



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    I would check or replace the coolant temp sensor for the ecu, not for the gauge. It tell the ecu how cold or warm the engine is, if bad or not working ecu think engine is always cold and dumps a lot of fuel. Do you have a check engine light on?
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  15. #15

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    Ive had the same idle issue. It hiccups when idling, it runs fine when in gear. If I'm stuck in traffic, I pull the ebrake and leave it in gear. I hope u come up with something, I've replaced all the stuff u have except the o2 sencer. Only difference I'm not having the fuel issue.

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