Overheating 3.3 (long)

Yesterday, while on the expressway my 2001 caravan "bonged". I checked that my seat and tray were in the fully upright position, but that wasn't it.

I checked the cluster to find out what happened, but before I could register what was wrong, it stared chiming steadily. I then noticed that the temp gauge was pegged. I got to the right and got off the road, also turned on the heater, it was blowing hot, really hot, but the gauge started to come down a little.

I shut the engine off, but left the key on to check which way the temp would go. By this time it was down to about 75%. It shot up a little, and then started to drop. I let the car rest for about 5 minutes and it had come down below half.

We were only 2 or three miles from home, and since I didn't seem to have a leak, we headed home. I kept it under 70 and it seemed to stay pretty cool. After letting the car cool off at home I started it up and let it run, the fans came on when they should and after 45 minutes of idling the temp was still normal.

I decided to bring the car to work today and watch to see when the problems started happening. Until we hit the x-way the temp stayed below normal. After about 3 miles it shot from normal to about 75%. I flipped on the heat and it seemed like it "pulsated". It varied from really hot air to lukewarm for a few seconds then back to hot again. I kept the speed down and the heater on to keep from having it burn up. Varying the fan speed of the heater seemed to control the engine temp somewhat.

I am scratching my head at this point. The coolant level is above the minimum line (cold). The only thing I can come up with is that sometimes the serp belt will squeal. Usually when it gets wet, and only for a second or two at that. Could the belt be slipping at highway speed letting the water pump slow down?

The van has less than 50,000 miles and I have never even thought of doing anything with the cooling system. Until yesterday the gauge never went past the halfway point.

Any input is appreciated.

Assuming no leaks anywhere, your thermostat is most probably stuck closed. It's a $4.00 item and located in a housing at the end of the top hose.

The thermostat controls the water flow into the radiator and keeps the engine at a constant temperature. Your temperature gauge is on the engine. There is a seperate temperature sensor on the radiator for control of the cooling fans. It takes about 3 minutes for the water in the engine to get up to temperature and then the thermostat should open. Once open, it lets hot water into the radiator and is replaced by colder water from the radiator. It takes another 3-5 minutes for the radiator to reach the temperature for its cooling fans to come one. If the thermostat sticks, the engine just gets hotter and hotter and no cooling fans ever come on.

Watch your temperature gauge after starting the car. The needle should rise up to the normal setting, and pause (thermostat opened at this point). If the needle just rises and rises (without pausing, it is a bad thermostat. If it does pause, then climb higher after 4 minutes, cooling fans did not come on.

Excellent advise. Consider buying a failsafe thermostat, they will fail in the open position. The cost more but could save your engine if the thermostat fails again. Also consider changing the belt. If it is squealing it is probably worn out and may break stranding you alongside the expressway. They cost about $25 -$50 depending on where you buy them and take about 30 minutes to replace with a couple simple hand tools. Good Luck!

Turned out to be the thermostat. I've never seen one stick closed without having a serious amount of corrosion. This one was clean as a whistle.

I also changed the belt (what a PITA that is).

Thanks for your quick response.

Concerning thermostats failing: At one time, the failure rate of brand new in the package thermostats was one in ten. I've also seen thermostats fail after a mechanic has done a water pump and liberally used a blue silicone form-a-gasket sealant. When that stuff gets into the water, it's like bubble gum. The thermostat spring release tension at proper temperate, but the valve is stuck in the gum and won't open.

I had this very same problem last winter.. same symptoms everything.. it was a royal pain. I ended up changing thermostate twice.. (got a bad one), flushing out radiator.. new radiator hoses, new water pump, new heat sensor, new radiator cap.. and it kept right on doing it. Then just as sudden as it started it stopped.. I can't tell you why it stopped or if it was just a winter thing. I still to this day don't know what it was. I wonder maybe if the heater core was plugged up or something.. I don't know.. wish I could tell you more other than good luck. It really didn't cost much to replace all that, and in the long run it probably needed changed anyway.