By February 14, 20078 Comments Read More →

1991 Mitsubishi Montero Runs Bad

Reader Question: Hello,

I am not sure how this works but I did fill out the insurance quote info. Anyhow, I have a 1991 Mitsubishi Montero LS 3.0L with 130,000 miles on it.

For about 4 years, I have been trying to fix it or have it fixed and so far no luck and I am running out of patience. It used to be a good reliable car but now I cannot even sell it to pay my son’s $1,600 orthodontic bill. At first, it idle real rough and sometimes died when I accelerated.

I have had it at two mechanics for timing and replacement of the Mass Air Flow (MAF) sensor (ouch). It didn’t change anything. Now I can start it with the MAF unplugged and it will start right up, idle well, and respond when I give it gas.

With the MAF plugged in, it idles rough and fluctuates eventually dies. I thought it might be a clogged cat so I unhooked the cat and still no difference.

What the heck do I do? No one seems to be able to fix it and I am running out of options.

Please, please help.


Hey Jack,

I feel your pain! I am not sure how much help I can really be over email. Usually, usually when you can unplug the MAF sensor and the engine runs better the MAF sensor is faulty. Since you already replaced it and nothing happened…hummmmmm.

I would first make sure you do not have a vacuum leak under the hood somewhere. Make sure to look under the hard plastic hose between the air cleaner and the throttle body of the engine. I would also clean the throttle body and idle control motor.

If you do not see anything wrong there, next step would be to manually test the fuel pump pressure at the engine with a manual pressure gauge. I want to rule out the possibility of a weak fuel pump.

Last thing to do is hook up a hand scanner to your onboard computer and see what the computer is seeing. I would start with checking the oxygen sensor readings and go from there.

Austin Davis

Posted in: Running Problems

8 Comments on "1991 Mitsubishi Montero Runs Bad"

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  1. Russell Tebbs says:

    My 91 MM does almost the same thing. Could an exhaust leak make it run this way? If you pump the gas pedal it will rev up and after it gets hot it runs a little better.

    • By Austin Davis says:

      Probably not, although I have seen an exhaust leak at the engine before (upstream) the oxygen sensors in the exhaust pipe cause some running issues. I would probably start with checking the fuel pump pressure at the engine with a manual hand held gauge. Then scan the computer codes to see if anything is in memory.

  2. Justin in Utah says:

    I am in the same situation with my ’91 montero RS. I’ve replaced the fuel filter and spark plugs/cables.

    It idles steady but high when I start it in park, then after a few minutes the RPMs start to fluctuate. The needle jumps up and down and doesn’t stop. It is like this when I am driving in a low gear sometimes. When it is in gear and idling, it idles a little high, but it does not sound like someone is slowly tapping the gas like it does in neutral.

    Someone help out here!

    • Austin says:

      I would first start with cleaning the throttle body, which just about any fast lube mechanic can do for you while you wait.

      second, I would read your computer codes and test the idle air control valve

      third, I would double check the base engine idle speed and make sure it is not too high

      forth, I would check for a vacuum leak under the hood, listen for a hissing noise with the engine running and look for any loose or broken vacuum lines.

  3. Dave says:

    i’m trying to find “jack” he’s the person that left the original post i’m looking at about a 1991 montero. Mine is doing the same thing and I’m wondering if he ever came up with a solution. I’ve done all he did…just put a new air flow sensor in and nothing has changed. I know it was 3 years ago but I’m hoping this gets back to him some how! Help Jack!!!!!!!

  4. Kei says:

    Hi… I own a Mitsubishi Montero Sport 99, lately i feel that my suv is losing power when in driving between 35 to 45 miles. What is wrong. I went to a mechanic, he put it into a computer and he said the suv need a new oil pan seal because it is leaking… Is somthing else going on? or it could be just the oil pan leak?? Thanks!

    • admin says:

      I would find another mechanic as the oil leak is not the cause of your lack of power problem. This could be caused by a number of things, restriction in the exhaust system, weak fuel pump, an engine missfire due to worn out spark plugs and wires etc etc. I would first start with checking fuel pressure and inspecting plugs and wires for an engine miss and go from there. The oil leak can wait most likely

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