I have a 1997 Toyota Avalon with 168K miles on it. This car was purchased used with 47K miles on the odometer. Ever since we purchased it, there has been an intermittent surge problem with it.

After the car has been driven for a while, it doesn’t want to idle properly. While idling it surges between 500 to 750 rpm and runs rough. When driving at highway speed, the surge goes away until you let off the accelerator. This problem seems lessen during cooler weather and is worse in the summer months. As the mileage has increased, the problem has worsened. Sometimes you can do a hard acceleration and the problem will cease.

We took the car to the Toyota dealership and they could not do anything until the car actually surged and then they could make a diagnosis. As for engine maintenance, the car has been on a regular routine with tune-ups, filter changes and using fuel additives such as Techron.

Does anyone have an idea what this Gremlin might be?

Thank you in advance!

Update, this problem was solved by replacing the EGR valve.

Consider changing your PCV valve. They're about $5, and joins your valve cover over pressure to your air-intake.