Sudden Spike in RPM


#1

I have a 05 Hyundai Sonata in good condition at just over 31,000 miles. Recently, though, I have noticed that some times when I am getting off the highway and slow down that as I sit idle at a stop sign or light that my RPMs will suddenly spike for no apparent reason; as if the engine is being reved but no pressure is being applied to the gas.



It just will have an odd vibration in the wheel and I will catch a glimpse of it shooting from around 1rpm really quickly up to 2-2.4rpm and then back down to 1 and I cannot make any sense of it. I just had its 30,000 mile maintenance done where every thing was changed and inspected at Jiffy Lube and they said it looked good.



The only thing I can think of is perhaps a gear issue as it occurs when I am either slowing to a complete stop, I am at a complete stop or I am accelerating from a complete stop. The engine hasn’t failed or stalled or anything and so I am not sure what it is.



The gas attendant (since the lovely state of Oregon has it outlawed to pump your own gas; ridiculous) has made comments about their being a lot of air in my gas tank - causing the continual clicking off of the pump far from the tank being full. So could that be a potential culprit? Perhaps the air in the tank interupts the fuel and so causes little bursts of fuel causing the engine to race a little?



Any ideas or suggestions would be great and much appreciated! Thanks.


#2

more likely a compute speed control module needs to be replaced if engine idles at high RPMs. have it checked out


#3

First of all, forget anything a gas station attendant tells you. Of COURSE there’s air in the gas tank. What else would occupy the space as the gas is pumped out? If there were no air there would be a vacuum and the tank would collapse.

Does the engine actually rev up, or is the tachometer just jumping? If the engine is revving from idle to nearly 4,000 rpm you should hear and feel it.


#4

Something is not right. First, forget Jiffy Lube. They change oil but aren’t really mechanics. The gas station guy is even less of a mechanic. Take this to a real mechanic and have it checked out. I’m guessing a bad Idle Air Control valve, or something along those lines. The gas pump clicking off could be related, but maybe not. I think the evaporative control system is usually the culprit for that problem.