Questions and Answers

929 High Idle And Surging Q. Dear Vince, I came across your site and thought that perhaps you could shed some light on a problem I am having with my Mazda 929 S, V-6, double overhead cam, 24 valve engine with 103,000 miles on it.

The problem I have been trying to solve relates to a high idle speed at startup when cold. Engine high idles at 2,000 RPM and when I shift the transmission to Park, the RPM surges up and down from between 1,600 to 2,000. Once the engine warms to normal temperature, is shut down and restarted, idle speed is stable at approx 650 RPM.

Also if I drive five or so miles, shut down the engine and let it sit a while, the problem does not repeat on restart. It seems that the residual heat of the warm engine travels to whatever part is causing my problem and changes something for the better. I have been seeking a solution but have not been successful. Do you have an opinion on the most likely cause of the problem and what part I need to check and/or change to fix it?

Thanks,
John

A. Well it's a DOHC V-6 so that narrows it down to five model years.

The one thing that would cause this problem in all these years is the Engine Coolant Temperature (ECT) sensor. If it is telling the PCM the engine is warm all the time, then the cold start would be as you describe and warm operation would be closer to normal. A scan tool will reveal instantly what temperature the ECT is telling the PCM it is.

The ECT should read, when cold, the same as ambient temperature within a degree or two.

Additional Information provided courtesy of ALLDATA