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Old 08-31-2021, 10:38 PM #1
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Minor fluctuations to RPMS while idling. Is this normal? Tune up? please help

I have a 94 4Runner its new to me, 241K on the clock, the air filter has been replaced the oil just changed by PO. I noticed while in line for at a drive-thru that about every 45 seconds there is a slight fluctuation in the idle, by looking at the gauge, i have the idle set at 750 (per tach), it will drop to as low as 500rpms and then go back to 750. The AC was on so i thought it was that, but when the AC was off it would do the same thing but wound drop as much. is this a normal thing? or should i be looking at other things? I figured it was too like clockwork to be fuel, maybe spark? Timing? Or maybe clogged injectors? The truck runs fine, never feels like its gonna die, just a little shaky when it drops RPMs, but then evens out. I wanted to get suggestions before just throwing parts at it. My idle is set at 750, is that too low? any suggestions would be greatly appreciated, I am about 5300 feet above sea level and i only put the cheapest fuel as this is my around town vehicle. Thanks in advance!
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Old 08-31-2021, 10:44 PM #2
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Mine has very small "jumps" up and down but maybe like less than 25 rpm I guess. I think that is normal.

Dropping to 500 doesn't sound right to me... Have you cleaned the mass airflow sensor? What kind of fuel economy do you get? Has it changed over time?
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Old 09-01-2021, 12:16 PM #3
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The RPM fluctuation was just an estimate when looking at the tach, I havenít done anything to the 4Runner since I got it. I will look at the MAF this evening, I put some seafoam in the tank today on the way to work to see if it was just a clogged up injector, havent noticed anything tho but only drove like 15 miles.
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Old 09-01-2021, 12:48 PM #4
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Are you running the A/C? Or perhaps defog/defrost is on and the A/C compressor is cycling? If so, it's supposed to idle up when the compressor kicks in. Not sure what Gen3s have, but my 1988 had an "Idle-up VSV" or Vacuum Switching Valve. Its job is to bring engine speed up a little at idle to prevent stalls when the A/C clutch energizes and provide enough RPMs to the compressor. That failed once and caused similar symptoms.
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Old 09-01-2021, 01:25 PM #5
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Mine idles around 1000, and it will drop to 900 when the AC turns on. This is a 3RZ. The year you gave makes it a 2nd gen, those came with 2 engines in the US: 3vz V6 and 22RE l4.

Have you checked the air filter. If its super dirty it could be restricting air.
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Old 09-01-2021, 05:21 PM #6
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First thing that should be asked, is it just the gauge moving or do you hear the engine dropping to a lower RPM and then raising back up to a faster one?

The 3VZ-E and 22-RE engines operate on the OBD-I system which behaves much differently than the 'newer' OBD-II. There is very little data you can view beyond reading voltages on a multimeter. Since there's basically no data we can look at, I would suggest a full tune-up and replace any maintenance item than has not been done. Pull the plugs, pull the wires and the cap and rotor off the distributor. Looks new and sound? Keep it. Test the components you can with a multi-meter or ohm-meter using the resistance setting. Anything out of line? Replace it. See if that clears it up. Definitely sounds like a misfire.
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Old 09-02-2021, 03:48 PM #7
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TPS is the culprit. My 92 corolla did it too! Get an OEM Throttle Position Sensor.
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Old 09-04-2021, 04:17 PM #8
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The air filter is new, the gauge moves and u can feel the car shake, I think itís only when the compressor cycles tho. My idle is set at 750, is that factory? The gauge shows a drop to the line below thatís then kicks back up. It has kinda evened out a bit, Iím thinking there prob was a clogged injector, Iím now at about half a tank and it seems to be a little smoother and not dipping that low just a little bit but not enough to ďshakeĒ the truck like before. Iím gonna run another fuel treatment after this one and make sure they are clean. Iím gonna check the TPS tho, bc I still get minor fluctuations when the AC isnít on a well but they are really minor donít even notice it unless Iím idling for long periods of time.
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Old 09-04-2021, 05:04 PM #9
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Try cleaning your throttle body bore. Pay attention to the area were the throttle body butterfly seats against the throttle body. Make sure that area is free from carbon and oil that's blocking the gap inbetween the throttle body and butterfly.
Pull the ecu fuse, or disconnect the battery for a minute to reset the computer and force the ecu to learn how to idle again.
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Old 09-05-2021, 01:42 PM #10
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Quote:
Originally Posted by jasindude1 View Post
The air filter is new, the gauge moves and u can feel the car shake, I think itís only when the compressor cycles tho. My idle is set at 750, is that factory? The gauge shows a drop to the line below thatís then kicks back up. It has kinda evened out a bit, Iím thinking there prob was a clogged injector, Iím now at about half a tank and it seems to be a little smoother and not dipping that low just a little bit but not enough to ďshakeĒ the truck like before. Iím gonna run another fuel treatment after this one and make sure they are clean. Iím gonna check the TPS tho, bc I still get minor fluctuations when the AC isnít on a well but they are really minor donít even notice it unless Iím idling for long periods of time.
Cleaning the maf or replacing an old maf with a denso unit wouldn't hurt. Throw in a a new pcv valve while you are at it.
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Old 09-05-2021, 02:09 PM #11
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Cleaning the maf or replacing an old maf with a denso unit wouldn't hurt. Throw in a a new pcv valve while you are at it.
My 2nd Gen didn't have a traditional MAF sensor. The OP is in the wrong forum unless he made a typo
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Old 09-05-2021, 03:09 PM #12
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My 2nd Gen didn't have a traditional MAF sensor. The OP is in the wrong forum unless he made a typo
Thanks, yeah my suggestion is for 3rd gens.
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