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I have a 2007 Kia Spectra5. Car has been surging when coasting in neutral for close to a year. Mostly 1400 to 2200 rpms. Rhythmic, up, down, up, down. Only after the engine has reached operating temperature. If you coast in neutral before that, the revs drop right down to 750 like they should. When warm, proper idle would settle at 750 rpm after car has come to a stop. Figured there was a problem with the IAC as I had cleaned earlier I thought possibly some sensor kleen had gotten into the motor and buggered it. Replaced IAC and gasket. Didn’t help. There weren't any vacuum leaks and the throttle body was cleaned. Replaced upstream oxygen sensor, throttle position sensor, thermostat and engine coolant temperature sensor. Cleaned MAF sensor. As the car only surged in neutral while coasting and stopped surging once I had come to a stop, I was wondering if it could be speed sensor related. So, along with the TPS, I replaced the VSS. Also bought a new radiator cap. Burped the rad and used new cap. Took throttle body out and cleaned again. After engine is warm, can stop the surging when coasting in neutral by turning on the air conditioning. Could this be a problem with the ecu?
 

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My, a lot of parts and work for no gain! Is the CEL illuminated? and how many miles(klm's) are on the car? Have you checked motor mounts allowing engine rock that could play "games" with the air intake tube from the air cleaner/MAF? A crack in it or the rubber connecting boots can do that.
 

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No cel ever. Have checked motor and transmission mounts, Looked at MAF sensor readings on OBDII scanner, they were fine. Car has 268,000 kms. Had about 10,000 less when this started.
 

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You might want to connect an OBDII scanner and look at the short and long term fuel trims...
It sounds like you have some air getting by the MAF (mass air flow sensor) causing the engine RPMs to increase...
Of course this is a "guess"

Look closely at the rubber air intake tube as it may have a crack in the folds... this tube is between the throttle body and the MAF...
Illegal air slipping by can cause the idle to increase...

I can't remember when/if an ECM/ECU in a Spectra (2004.5~2009) has ever been bad here on these forums and I have been here since 2010...
Dave
 

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Discussion Starter #5
Thanks Dave. I don't think it is the ecu either, it's just that I can't think of anything else. I'll take a really close look in the folds of the tube between the maf sensor and throttle body tomorrow.
Thanks,
Tim
 

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You also might want to REMOVE the throttle body and clean it on the other side of the throttle plate... Carbon build up on the back side can restrict the air flow through the IAC port into the throttle body and also cause problems with throttle plate movement...
Dave
 
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