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Mazda 6 engine revving really really fast in neutral

I own a 2005 4-cylinder Mazda 6 sedan with 105,000 miles. In the past 2 weeks, I have encountered a problem where the engine seems to rev at very high RPMs. The first time it happened was on a very hot day. I had just returned home in the car from a 6 hour drive. About 3 hours later, I again drove the car. I noticed that, while driving on the expressway, I could take my foot completely off the accelerator and the car maintained its speed of 65 mph for about 1 minute. When I got off the expressway, I had to apply much firmer pressure than normal to the brakes to stop the car, and the RPMs remained high (1500-2000 RPMs—normally reduces to less than 1000). I shifted the car into neutral while stopped, and the engine started to race between 2500 and 3000 RPMs, cycling about every second. It sounded like I was getting ready to start a drag race (vroom…vroom…vroom… you get the idea).

The second time it happened, it was again a very hot day, but I had not been driving the car long. Same thing happened when I shifted the car in neutral at a stop light. I got a really nasty look from the woman stopped next to me because she clearly thought I was challenging her to a race! I have the car at the Mazda dealer today, but their initial inspection revealed no codes and they haven’t seen anything like this before. There are a few other points I can make:

--I was running the A/C, but turning it off had no effect.

--I checked to make sure the accelerator wasn’t stuck on the floor mat (it wasn’t)

--stepping on the accelerator in neutral did not reduce the RPMs

--both days, the temperature was above 90 degrees

--the check engine light has not been on

Does anyone have any suggestions about what to do about this? It is a much cooler day today, and the mechanic cannot replicate the effect. No codes come up when they check the car with the computer.

Ask the mechanic to show you where the idle air control (IAC) valve is - normally easy to find once you know what/where one is.

Keep a hefty screwdriver, small block of wood, or rubber mallet in the car. The next time it does this, stop safely off the road, put in park & apply the parking brake, pop the hood and give the IAC a good whack or two with the butt end of the screwdriver - or whatever. See what happens.

You could also ask what they want to pull the IAC and clean it - it could help. At the very least the wiring for it should be checked.

Cigroller this is EXACTLY what is happening to my car in the “self-revving engine” discussion. It must have something to do with the heat, mine only does it when I have been in a traffic jam and my engine is hot hot hot. I did not have luck with the IAC but I have not fully cleaned it yet. The difficulty is that I am in traffic so rarely that I would not be able to immediately tell if this is the problem. Cigroller what do you think about the heatsoak to the cruise control cable theory that I posted in my string?

rjb did you check the IAC? Have you fixed your problem? PLEASE let me know what you did, I actually had to pull over the other day because my engine was overheating. Not ok.