Hesitates and power loss when hot; now trouble starting when cold

I have a 1995 Plymouth Voyager with 120k miles on it. This summer, while returning to California from a long road trip to Alaska, the car suddenly started losing power and hesitating. We had been driving for several hours at freeway speed and the engine just seemed to lose power and we slowed way down. Pressing the gas pedal further down didn’t do anything at first, then the van would gain a little power and speed up again, but only to about 35mph. This happened repeatedly, so we got off the freeway to find a shop. I’d also have to put the van into neutral and step on the gas to keep it from stalling out at stop lights. The check engine light came on, btw.

A mechanic checked out the car and said the computer error code (code 22) indicated the coolant temperature was too high, but he didn’t trust that because the computer claimed it was 265 degrees, which he said wasn’t possible. He tried a new sensor - same results. He even disconnected the sensor from the computer entirely, and the computer still claimed coolant was at 265 degrees!

It was too late in the day for him to do anything else, so we spent the night in town and brought it back the next morning. Now everything was fine. He replaced the plugs, rotor and cap (because there was a small crack in the cap), test drove it, and it was fine.

We drove off and got about 250 miles before the power loss thing happened again. Remembering the high coolant temperature thing, we pulled off the road and waited 2 hours for the engine to cool while we ate dinner. After that we were good for about another 200 miles or so. We repeated this driving/engine cooling cycle all the way back to California.

Back home I told my regular mechanic what happened. Since the check engine light wasn’t currently on he said he couldn’t check the engine code. But a few days later - after just a little driving, a stop, and a hot engine start - the light came on and the van started losing power again. Brought it right to the mechanic and the computer code indicated a faulty O2 sensor, which he replaced.

Well, the power loss problem still happens, usually after you drive for a while, turn off the engine, and turn it on again (a hot start). But since my son usually just drives it 15 miles to school in the morning, is there all day, and just drives home again in the afternoon, it doesn’t show up much (since he doesn’t make many hot starts).

Now that it’s getting colder outside, the van sometimes is difficult to start in the morning (when it’s cold- 30-40 degrees or so).

Could all this be related to the computer thinking the engine coolant is hotter than it really is (making it hard to start in the cold weather, and hesitate and lose power when it really is hot)?

I’m guessing the coolant sensor is broken (I know the dealer replaced, but could he have put the old one back in by mistake or could the replacement one be defective?). The other thought is perhaps you are running really hot (e.g. clog in the radiator) I’d hate for you to replace a radiator on a hunch, but maybe it’s time for a flush and fill if you haven’t done so in the past few years.