'94 Sundance overcooling at highway speeds

Your heater core is stopped up causing the no heat and the temp fluctuation. Try and back flush the heater core to see if that helps.

Glenn Beasley Chrysler Tech

Please clarify: Do you have any evidence the engine is "overcooling" aside from the gauge readings? Does your heater grow cold at highway speeds? Do you get a Code 17 when you check for flash codes?

Okay, sorry... yes, other than the guage reading cold, the heater will only blow lukewarm air on the highway... when I check for any stored codes, I only get 5-5 (end of codes) so the computer doesnt know of any problems. I was having a problem with the heater core not giving me any heat, but I clamped on a hose to the inlet and outlet (alternating) and blew anything out that would come out... when I was done, the water coming out was coming out at the same rate as if I had the hose end in my hand... When I am driving in the city, there is lots of hot hot air coming out of the vents...

I hope this info helps...

thanks for the help so far!

Im not sure I understand... what would the head gasket have to do with cooling the engine too much? I am not losing any coolant and the engine is running good right now... I do not know exactly what is involved in checking the head gasket, but if it means taking the engine apart, that seems a bit drastic... are there any other suggestions?

And is your coolant level up? Maybe you have air in the system. If you changed the thermostat and temp sensor and such, you have lost some coolant. Did you let the ngine warm up with the cap OFF! to let air escape? And top up as necessary.

I should have mentioned I had that problem with a jeep before. Would not heat up at all. And the jeep was brand new. Turned out to be a small pinhole leak in the rad, which would not build the heat and pressure to warm up engine, let alone me when it was -30 in Saskatchewan. A rad cap that is old can cause this also.

Yes, coolant is full and I used the bleeder screw to eliminate all air pockets in the system... as previously stated, I am not losing any coolant. The coolant gets hot as it should under city driving conditions, it is only at highway speeds that it will not heat up...

Is your cooling fan stuck on? Maybe a burned out relay?

Try pulling the power connection to the cooling fan or fans then doing highway driving and see what the temp does. At highway speeds the fans shouldn't be powered because there should be enough road draft through the radiator to cool it, also your not burning a lot of fuel at highway speeds and so the engine isn't generating that much heat then.

Ted

And what is the outside temperature? And what is your coolant protected to, Temperature Wise? If your coolant is too diluted and your on the highway at subzero climate then yes your going to be running cooler. And around town it will show warm.

None of the above is correct.

Shoot, there is more information I already knew that I should have posted, which would have saved this... The fan actually only comes on when the temperature gets about 3/4 the way up the temp. guage. For a while I thought it was broken or a signal was getting to it somehow, but it did eventually come on. About the car not burning as much fuel at highway speed... that does make sense! I never really thought of that. The only thing is, could the car REALLY be designed that way? I mean, from what I understand, the thermostat should NOT open until the engine is very warm, and until it does open, the coolant should just keep circulating inside the engine (and through the heater core) until its gets quite warm, when the thermostat should open. So to test this operation, I tried driving with the heater set to cold (just to make sure) and the fan to OFF. It still will not allow the temp. gauge to get above the cold level. Is there really enough cold air at 110km/h (65mph?) to cool the engine completely, offsetting all the explosions happening at 2000-3000 RPM? I am no scientist, but that seems odd to think that could be possible... its not like it is -40 outside afterall... Actually, the temperature only have to drop to about -5 to

-10 celcius (25-15 fahrenheit) for this problem to occur. If the temperature is above zero, the temp. guage will read warm and I can get more heat..

Is there ANY possible was that some lines could be hooked up wrong that would allow the engine to bypass the thermostat and let some coolant get through to the radiator? I tried tracing all lines and it really seems impossible, but I am at a loss here! There has to be something wrong... surely Chrysler couldn't have designed such a poor cooling/heating system...