Automatic acceleration - scary!



Yesterday I was driving around in the heat of a Texas summer when my 1996 Mazda B2300 pickup started accelerating all by itself. I was just driving down the road when it put the pedal to the metal (the pedal didn’t actually FLOOR itself, but it accelerated at full throttle). This was really scary but I was able to get home by clutching/braking my way down the street. Any idea what this could be?


Stuck throttle cable, or throttle plate, or possibly something else. Hard to say.


I was thinking the throttle position sensor went flakey. That tells the ECU how much the throttle has been opened, and it may be giving an erroneous signal.


If you have cruise control, that might be malfunctioning.


[b]A defective Idle Air Control motor can cause the engine speed to go out of control.



I don’t think that tha idle air control can pass THAT much air. The engine was under LOAD. The ias COULD cause the engine to race with no load on the engine. I would check for a stuck throttle plate or throttle linkage.


[b]You don’t think so?

Just ask a coworker of mine who took out a gas pump while pulling into a gas station when his IAC motor malfuntioned. The engine RPM’s went to max and before he had time to react, the pump and the front of his vehicle were history.



The IAC can allow only enough air to pass through the throtle plates to increase the rpm’s to 2000. That is the maximum limit of its control. I have seen a couple of instances where a faulty IAC motor caused uncontrolled acceleration but 2000 RPM is the maximum it can allow the engine to reach.


A faulty TPS will not cause this issue. The reading from the TPS is used to monitor user demand. It is one of many controls the ECM uses to fine tune the fuel air mixture. The engine speed is still limited to the amount of air entering the motor (Mass Air flow, MAP, etc.) and the injector pulse width which is adjusted based on incoming air, engine temp, O2 readings, etc. The reading from the TPS has little effect on engine acceleration. Hope this helps.


How fast was the truck going when this situation occured?
Do you have cruise control?
Did the pedal move at all or did the engine speed just increase?