With carbureted engines a vacuum leak causes the engine to run slow and lope. Just opposite of fuel injected engine.
The vacuum leak may be in another area of the vacuum system or intake manifold area, but if it happens when it rains I would check heater controls first.
Franks answer is a good possibility and easy to check---I think. Check the wiring diagram for the car and pull the fuse for the cruise control. That should turn it off even though the brains of the control are in the computer. Drive it for a while in that condition and see what happens.
Sounds like malfunctioning cruise control that isn't deactivating.
It could be several things. Is the "Check Engine Light" on? Probably not since you didn't mention it.
My first "guess" is the MAP (Manifold Absolute Pressure) sensor is faulty. This sensor measure the air pressure inside the intake manifold (air inside the engine just before it goes into the combustion chamber) and compares it to the atmospheric pressure. If the pressures are the same or very similar, then the computer revs up the engine to increase the manifold pressure.
My second "guess" would be that you have a vacuum leak. You could buy a can of starter fluid (ether) or other highly flammable spray and with the engine on, lightly spray it in short burts around the vacuum hoses. If the engine revs up, then it is confirmed you have a vacuum leak and then you just look around the area where you spray and look for a corroded rubber vacuum line.
My third "guess" would be that you have a problem with fuel pressure. You could have a leaking fuel injector(s), faulty fuel pressure regulator or a faulty fuel pump. The best way to confirm that is to have someone conduct a fuel pressure test.
My last "guess" would be a fautly TPS (Throttle Position Sensor). The computer senses where the throttle is and adjust the rpms accordingly. If it can't find it or gives a false reading, it will start to "hunt" for it by reving up the engine or throttling the engine down until it finds it.
I have a pontiac montana. It has nearly 200,000 miles but has had regular maintenance, great shape for such high miles.
When I take off from, say, a stop sign, I don't have to give it gas and it accelerates on its own. If give it enough time, it would accelerate on its own up to 50 or 60+ mph. It doesn't cruise like a normal vehicle. When I let off the gas, it continues to speed up rather than gradually slow down as you would normally lose momentum.
It doesn't do this all the time. Another main driver of the vehicle says that he notices it happens more when it has rained. Sometimes it will drive perfectly normal, but then it will randomly start doing this.
The other day, the other main driver said that he put it in park, but the van still wanted to go forward and it spun out on its own. He said even with the brake pedal, it still didn't want to stop.
On a more rare occasion, I will stop and place it in park. The van will immediately rev up to 2000 rpm and just stay there. I will turn off the vehicle and restart the ignition and it struggles to start. The ignition turns over, then it, like, coughs, then just doesn't start. I have to keep the key turned a little longer for a successful startup.
My car knowledge is limited. The first thought is the transmission.
A friend told me that he has this happen with his van (a different kind) and he had some sort of filter changed or something like that, and it fixed it.
What is the problem with the van?
