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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 Sep 21 '11 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 Jun 9 '13 at 10:17
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2 Answers 2

up vote 9 down vote accepted

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 Sep 21 '11 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 Sep 21 '11 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 Sep 26 '11 at 17:37
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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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