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I have a 1997 Caravan (base model) and normally on my drive to work and my highway drives in rush-hour, the engine warms up just fine.

I took a road trip for work, about an hour and fifteen minutes of driving at basically 60-80mph the whole way. The engine never warmed up. In fact it went from being warmed up (about halfway up the gauge) to just barely above the Cold marker.

My blower only works on high, (a problem for another day) so turning on the heat for that chilly drive made the problem even worse (lost like a bar and a half of engine temp for running the heater at 75% temp, full blower, for maybe 10-15 minutes.) I turned off the heat for the last 30-45 minutes of the drive and it still never returned to normal temperature.

It's the Mitsubishi 3.0L V6, which I believe is a cast-iron block, if that helps.

It's also a 3-spd, so the engine works pretty hard to maintain highway speeds above 60, it should be producing plenty of heat. What's the best way to go about solving this problem?

(For comparison sake, in stop and go traffic, on my way into work, it takes about 20 minutes for that engine to warm up, not using the heater.)

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  • Also could any serious damage have been done with the engine below operating temp at highway speeds for an hour? The car seems to be running fine (well, running as well as it was before the trip anyways.)
    – Robbie
    Commented Sep 21, 2011 at 15:32
  • I haven't seen a temperature gauge that didn't begin cold at the lowest end of range. You say it starts cold in the middle of range and drops? Sounds like a temperature sending unit or gauge wiring issue. Maybe the gauge itself. You also don't mention any performance changes from before the issue to now, does the Caravan drive unusually?
    – geoO
    Commented Jun 9, 2013 at 10:17
  • Im putting cardboard or insulation in front of my radiator today. I figure if it heats up while idling or in town its not a thermostat. Could be faulty Engine Temperature Sensor. But, im no mechanic.
    – David Weum
    Commented Dec 8, 2019 at 19:11
  • I've got a ford 4.9l inline six and i've gone through 3 thermostats and have all new cooling parts except for the water pump. I let it idle for an hour and it still barely comes off the "C". I know it's not a faulty sensor because when I had a freeze plug leak it used to overheat. New radiator, new fan clutch, no fan shroud, new hoses. Heater core was deleted. I'm just going to try blocking off my radiator with a thin metal sheet and see if it helps. I hear some people use cardboard for the same purpose in a pinch if it helps.
    – Wade Wilson
    Commented May 27, 2021 at 3:51

3 Answers 3

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It sounds like the engine thermostat has failed in an open state. The thermostat regulates the flow of coolant between the radiator and the engine. When the engine needs more heat, it closes and cuts-off flow through the radiator. When the engine needs less heat, it opens and allows flow to the radiator. With the thermostat stuck open, the flow through the radiator is constant and so your engine will have a hard time keeping it's heat.

This sort of problem can happen anytime, but it's not until the weather starts cooling that it gets noticed. Colder outside air means more cooling from the radiator.

As your your other question, "could any serious damage have been done with the engine below operating temp at highway speeds..." The answer is probably not. You may have lost some fuel economy, but cooling an engine too much shouldn't be too much of a problem. The other extreme, having too much heat, is where engines start to have big problems.

In either case, a replacement thermostat is typically very cheap, usually < $25. Changing it depends on how easy it is to get to and how comfortable you are with draining some radiator fluid and refilling it.

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  • Thanks for the feedback. Is there anything "related" to the thermostat that I should be worried about inspecting? (For example, the resistor pack that failed for my blower motor is actually caused by the blower going bad, is there anything I should be on the lookout for that could cause the thermostat to go bad? Or is it just dying of old age?)
    – Robbie
    Commented Sep 21, 2011 at 16:23
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    @Robbie Thermostats usually die because of old age, so there shouldn't be anything else to look at.
    – jwernerny
    Commented Sep 21, 2011 at 18:30
  • Possibly time for a coolant flush. In my own observations (no scientific controls applied) it seems like people that keep up on the coolant flush schedule have a lot less issues with thermostats failing.
    – Brian Knoblauch
    Commented Sep 26, 2011 at 17:37
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    "20 minutes to warm up" is way too long, and consistent with the thermostat being stuck open. You would normally expect the gauge to show that it was warming up in 2 or 3 minutes at most.
    – alephzero
    Commented Dec 15, 2016 at 16:51
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In case replacing the thermostat doesn't work, it might just be a broken Coolant Temperature Sensor, which is what sends the info to the gauge.

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Though I defer to the two previous explanations a third one is also possible because I've experienced this in one of my own past vehicles.

I had a small mid-engined sports car that had the radiator in the front and long pipes that carried the to and from coolant round-trip between the front radiator and the engine in the back of the car. During winters, regardless of thermostat, if the system was too low on coolant, the engine block temp sensor would not get enough hot water contact to ever get the temp gauge off the cold mark for the entire winter trip, most noticeable around & below freezing 32 degrees F. Even though the car never overheated it definitely wouldn't get the cabin heater hot until I added more coolant to the car. Put more coolant into the system and the temp would pick up and the cabin heater would heat.

Ironically the cooling system was so big for such a small engine that even with the a/c on sitting in Dallas, Tx traffic during the hottest of summers in August in Texas, that car never overheated as long as coolant was at least 2/3 of full, thus adequate.

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