Q: I have a 2000 Mazda B2500 truck with standard 5-speed transmission. I have a throttle control or stuck accelerator problem. I normally shift at 2000-2200 RPM. When up shifting, the accelerator sticks. That is, after depressing the clutch and taking my foot off the accelerator pedal, the engine RPMs do not drop, but rises. When I depress the clutch and put the car into neutral before coming to a stop, the engine RPMs could rise to 3000+ before dropping to idle speed. At times when cruising at a steady speed, I could take my foot off the accelerator and the car will maintain its speed. The local mechanic replaced the Idle Air Control valve (IAC) but the problem persists. Any ideas?
A: There are several possible reasons for an engine to increase in or hold speed on its own, and this needs to be fixed now, for obvious safety reasons. During an episode of inappropriately high idle speed (ease to a stop without touching the throttle?), I'd check the cable linkage at the throttle for mechanical binding that's preventing the throttle from fully closing. Binding could be caused by a flawed throttle or cruise control cable, incorrect cable adjustment, or pedal mechanism interference. Next, during a high RPM episode, try tapping on the IAC (idle air control) valve with a blunt tool such as a screwdriver handle. If RPM drops, the IAC is sticking. If there's no change in speed unplug the two wire connector attached to the IAC. If engine speed falls dramatically, this indicates the IAC is the henchman, but not the root cause of the problem.
An IAC is an electrically operated valve that allows air to sneak around the closed throttle, and is found next to the throttle valve on the engine. This gadget is necessary for cold fast idle and maintenance of the specified idle speed during a variety of accessory load conditions. Yours is an easy one to understand and check; when its turned on fully, you have very fast idle, off results in very slow idle speed, and a varying electrical pulse can provide a variety of speeds in between. IACs are controlled by the engine control computer, based on a handful of sensor inputs and programmed operating strategy. They sometimes become sticky and erratic due to gummy carbon buildup.
If the IAC is found to be the cause of the fast idle, a scan tool check of engine data will determine if the IAC is being told to go fast- by mistake, or perhaps is sticking in a fast position regardless of commands. Incorrect IAC commands may be caused by erroneous information from the engine coolant temperature sensor, power steering pressure sensor, clutch or transmission range switch, or the air conditioning request switch. At normal idle speed, a typical IAC command is approximately 15-20% (on). A higher percentage indicates a faster than normal command.
Based on the severity of your symptoms, it sure sounds like a sticky IAC valve. New parts aren't always good parts! If the high engine speed should only occur while the truck is moving, looking at scan tool commands will be the only way to assess the IAC - tapping on it would be difficult!
Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at email@example.com; he cannot make personal replies.