Question about 1991 Lexus LS 400

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1993 lexus, replaced spark plugs washed engine ran fine, next day car revs up to 1500 , the idle oscillates between 1200 and 1400 RPM. If I disconnect the MAF, Oscillation stops but runs really rough... Plz help! Thanks!!!!

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  • davspublic Sep 14, 2010

    Hi, one expert suggested that I replace the MAF, I did so but unfortunately the same thing is happening. After I reset the ecu, the engine starts revs to about 1800 or so, stays there for about 30 seconds, then the idle drops to 1200 or so and the climbs to 1400 and the drops to 1200 and cycles like that continuously.... PLEASE HELP!!

  • davspublic Sep 14, 2010

    I have already replace the MAF with another one same exact problem....any ideas??

  • davspublic Sep 15, 2010

    Hi, one expert suggested that I replace the MAF, I did so but unfortunately the same thing is happening. After I reset the ecu, the engine starts revs to about 1800 or so, stays there for about 30 seconds, then the idle drops to 1200 or so and the climbs to 1400 and the drops to 1200 and cycles like that continuously.... PLEASE HELP!!

  • davspublic Sep 15, 2010

    sounds reasonable, let me check sir and I will get back to you...thx!

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  • 2,926 Answers

You have a vacuum leak that is causing the idle to fluctuate and remain high. Check the rubber intake boots on the intake system for cracks or leaks and also check ALL vacuum hoses for the same. I'm betting a hose has cracked and is causing the IAC motor to try and compensat for the vacuum leak.
It was a bad call for someone to suggest the MAF replacement.

Posted on Sep 15, 2010

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You already answered your own question. The MAF sensor is bad. You can double check by taking the car to Autozone and putting the diagnostic computer on it for free and they will confirm that the MAF sensor is bad. When you washed it, it must have shorted out.

Posted on Sep 14, 2010

  • Rodney Clark
    Rodney Clark Sep 15, 2010

    The rpm going up and down sounds like the throttle positioning sensor. The computer thinks that the engine is going to die when it drops rpm, and the throttle positioning sensor kicks it up. But it is kicking the rpm up too high.

  • Rodney Clark
    Rodney Clark Sep 15, 2010

    You are very welcome!

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You are right this is a classic MAF problem or rather the air mass meter AMM which is the MAF working with the closely associated intake air temperature sensor IAT(a little bead like thermistor on wire sitting close to the MAFF in the air stream). The ECU was getting poor information from the MAF. The ECU had to rely on slow input signals from the IAT and feedback from the oxygen sensor on the exhaust to keep the engine running. The ECU hunted for the right mix, first rich and then lean but never settling hence the 'surge and slow' of the engine revs. When you disconnected the MAF you also disconnected the IAT and now the ECU had just the one signal from the oxygen sensor to depend on and so now provides a slightly rich mix giving the rough idle. You have done the hard work just change the MAF and IAT assembly.

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