Crazy idle - no cause found

For about 6 months my 95 ford ranger 2WD 4 cylinder pick up has occasionally gone into “over drive” when it is not in gear the idle will get crazy high, like one of those party toys that yo blow into and it spins an makes noise. Once put into gear it calms down but it has more forward motion without gas than before. If it gets really bad at stop lights I will turn it off and turn it back on. It may still do it when I turn it back on but by then I will be putting it in gear. When it is in this condition and I turn it off it takes longer to turn off, like it is winding down. I would say that this happens every 4th time it is drive, sometimes less sometimes more. There seems to be no rhythm. I took it to an electrical specialist and they could not find anything, and last week the check engine light came on so I took it to our normal mechanic. They did a test and found that the engine speed limiter had been reached which was why the check engine light came on and also pretty scary. The diagnostic codes were P0500 and P1270. I am worried 1. that this is making the care unsafe to drive even though it seems like we can manage it to a degree and 2. that is is doing damage to the car by putting it in such a hyper work mode. Any advice would be great, thanks.

The first thing is to use terminology that helps–rather than hinders-- a diagnosis when you take it to the next mechanic.

If you say that your truck “went into overdrive”, the mechanic will be looking at transmission-related issues, rather than engine-related issues, so it is not a good idea to invent a description of the problem that will only lead a mechanic astray. Instead of saying that it “went into overdrive”, you should tell the mechanic that you experienced extremely high idle speed before you even engaged the transmission.

If you say that “it went into overdrive” a mechanic will logically assume that the transmission was engaged, and that the vehicle was moving. Also, if you noted the actual RPMS that the tachometer was registering at the time of the high idle speed, that might be helpful to the mechanic, but the most important thing is to tell him that you had not yet engaged the transmission, and that the truck was not moving when you experienced the high idle speed problem.

All of that being said, the P1270 code (relating to the engine speed limiter) is most likely the result of the P0500 code.

P0500 indicates a problem (in this case, a severe problem) with Vehicle Speed Sensor A (VSS-A). Hopefully, either replacing that VSS (or repairing the wiring leading to the VSS) will remedy the problem.

It is also possible that there is a problem related to the Idle Air Control valve (IAC), and there is even a possibility that the problem lies in the Engine Control Module (ECM). However, a good mechanic should be able to diagnose the source of the problem as long as you don’t use totally non-relevant and misleading terminology like “it went into overdrive”.

The problem might be with Idle Air Control valve.

The IAC valve is mounted on the throttle body. The next time the idle goes crazy, open the hood and with the handle of a screwdriver rap on the IAC valve. If this causes the idle speed to change replace the IAC valve.