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Discussion Starter · #1 · (Edited by Moderator)
Hey guys. New to this forum so forgive me if anything posted is out of order, etc.
Anyways, like the title states I have a 99' Malibu with a 2.4L 4 cylinder. Car has about 88k miles. So the issue I'm having is that the car has trouble taking off and starts sputtering/losing power when giving acceleration from a stop. When in park the engine idles around 800 to 900 rpms, and you can feel a noticeable shake/stutter in the engine. When revved over 1100 rpms the shaking goes away and sounds/feels normal. When removing gas and letting drop back down to idle, it repeats said issue. It also does the same thing when out driving. Will drive fine at higher speeds, but anytime I go from a stop to acceleration, it bogs down and shakes and has trouble climbing in rpms till about 1200, where it appears to just switch to normal operation. I have a Foxwell NT301 OBDII scanner I picked up on Amazon. It's lower end but has always worked well. Upon hooking it up, it shows no error codes whatsoever. I run it through the I/M test and it says that everything appears to be working good. No CEL, no other indicators on the dash showing something may be wrong. I started with replacing all 4 plugs and boots, and same issue. Then went ahead and replaced the IAC valve as well, and same issue. I'm assuming its some kind of misfire or fuel related issue. Don't have the money to take to the mechanic at the moment, but also don't have the money to continue dumping parts into it. Was wondering if anyone would have any advice or can offer any solutions with similar problems they have had. Or maybe some advice for any kind of test to run on my scanner. Can also post stats of the live test on the scanner if need be. Appreciate any and all feedback!!


I should mention, it still is not throwing a code(although I believe since I unhooked the negative terminal, I have to let it power cycle) and all electrical including headlights dim off and on when it dips in power.
 

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I think you are on the right track


I think you are experiencinga harmonic convergence ( lol stole that term from the movie Thor.....for lack of a better term)
It when the motor is all wonky from being down a cylinder .....then becomes kinda normal at a different rpm . Making it sound (harmonically ) normal

Could be a few things....like injectpr being stuck.... some spark isssue, etc. A proper scan is in order to possibly look at duty cycle on the injectors and other things
 

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Scan tool should be able to report misfires, long term fuel trim, short term fuel trim, MAF sensor report in grams/second, system voltage and pending trouble codes
Misfires more from one cylinder to the next can help pinpoint the problem.
Long term and short term fuel trims should tell a story, short term fuel trim should shift more quickly and I would watch them under various conditions while driving. Under %5 is best but %10 is acceptable. These readings are from warmed up engine.

MAF sensor at idle [600 or less rpm] should report around 2.4 grams/sec. Far less would suggest an air leak.
System voltage, does the voltage spike low after the engine has dropped to almost stall speed or is the voltage spiking low and the engine drops in power as a result?

A bottle of fuel injector cleaner placed into half a tank of gas might improve things.
 

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MAP sensor is also a possibility. I chased a loss of fuel mileage in my Tahoe and it was the MAP sensor. I did not set any codes either. Got 3-4 mpg back after changing it. It was off enough but not bad enough to hit the code trigger point.
System uses the MAF and the MAP to help calculate fuel mixtures. MAP comes into play more when throttle is not open very far at low engine speeds or idle as it's the primary sensor to sense low speed engine vacuum.
There is no ''cleaning'' a MAP sensor.
Just another suggestion when chasing a problem that is not setting any codes.
 

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Dam,some great suggestions above . I love hearing about these issues and their sollutions.
 
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