I have a Mazda B3000, 6 cyl auto, 72k miles. When driving the motor never falls below around 1300 rpm. I can either apply the brakes while in gear or put the truck in neutral and coast to a stop. When I slow to about walking speed or below the idle will fall into the normal range, 8-900 rpm. I have replaced the idle air control, throttle position sensor and the speed sensor (located on the rear differential). Does anyone have any ideas as to a solution? This problem is somehow connected to the speed of the vehicle. Any assistance will be much appreciated.

 

I'm having the same problem, but with a 2.3L and a manual transmission, but mine is staying up above 2000 rpm.
https://www.ranger-forums.com/dohc-2...ing-ac-170042/

To some extent, a higher idle when moving is normal. I don't know what the specifications are for an automatic, but when shifting between gears a manual should be about 1300 rpm to 1700 rpm at 55 mph. This is for reducing NOx emissions and assists with a smoother shift in a manual.

In your case, check for vacuum leaks first.

 

You may be right about a vacuum leak, a friend has suggested the same idea. I just don't understand how a vacuum leak can be connected to an issue with the vehicle speed. As I stated before, the idle drops into a normal range when I slow to approx. a walking speed and the truck idles and revs just fine when it is parked.

 

The speed sensor sends a signal to the PCM that the vehicle is no longer moving. The PCM then sends a signal to the idle air control valve to reduce the idle speed to whatever is is for that vehicle when at a stop.

 

So does that make the idle air control valve faulty? Or is that still an indication of a vacuum leak? Thanks for the replies by the way.

 

For pollution control, that's the way it is any time you are moving over about 5mph.

There is nothing wrong with your truck.

 

You know, I've thought about this but I have owned this truck for 5 years and I swear that when I first bought it I could take my foot off the gas and slow down without tapping the brakes. As it is running today, I take my foot off the gas and I will travel along at about 25 mph nonstop.

 

No. The idle air control valve is doing what it is told to do. You can think of it as a simple program with an IF, THEN statement.

IF vehicle speed = 0
THEN idle speed = 800
ELSE idle speed = 1200

Or, the other way around
IF vehicle speed ≥ 5 mph
THEN idle speed > 800
ELSE idle speed = 0

So, when you aren't moving the PCM adjusts the idle air control valve until the idle is at 800 rpm.

If the speed sensor were to continue sending a signal that the vehicle is moving then then the PCM would continue to signal the idle air control valve to maintain a higher rpm. It's that old principle of garbage in, garbage out. The end result is only as good as the data input. In my case, I can't find a vacuum leak and I think a sensor is bad. So, potentially bad data is resulting in the problem I have. It is possible that the idle air control valve can have a problem and mostly work right. I just don't like replacing parts until it starts working right. In my case, fuel trims are telling me there's additional fuel added to the mixture. The idle air control valve shouldn't do that, but another sensor could send bad data to the PCM and cause that problem.

 

You will have to look up the specifications. I found them by accident when looking through the Chilton's online website. There are several tables that specify the values for the sensors, what the rpm should be at under certain conditions, etc. Mine stated at operating temperature the idle at less than 30 mph should be 1300-1350 rpm and 1750 rpm over 55 mph. I don't know what it does for an automatic. It also states what the idle should be at certain coolant temperatures. So, if your coolant temperature sender is reading low the PCM will take that bad data and tell the idle air control valve to increase the rpm and enrichen the fuel mixture to warm up the engine.

Check your local library for online repair manuals. A lot of them have Chilton's online. Some have others. The information I mentioned is under the diagnosis tab in Chilton's, then go to specifications.

 

Geographer, great info. I'll certainly try to track that info down. Thanks for going the extra mile.