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Overheating Minivan

My 1999 mercury villager, 3.0 liter automatic, overheats every time it gets hot outisde (over 95 F) and the AC is on. This problem first started when I changed the radiator fluid 3 years ago. Think I have blead out all the air, but it is impossible to really know (unless somebody has a trick to do this). I have since rodded out the radiator (estimated 10% blockage at the time), installed a new thermostat (with tickler hole up), new water pump, and new radiator cap. No significant changes. The new radiator cap did seem to prolong the car from overheaing, but only slightly. The main radiator fan comes on when the engine is hot and when the max AC is turned on. Why does this engine contiue to overheat? I’m guessing there is some kind of air bubble still in the system, but the vaious bleed holes seem to say otherwise.

Have you done a pressure test?

No I have not done a pressure test on the cooling system. What does that do to diagnose the problem? Judging from the pumped up hoses, and the small amount of steam that came out from the old radiator cap, there is plenty of pressure in the system. There is no steam coming from the new radiator cap.

A pressure test is to check for leaks. You pressurize the cooling system and see if it holds. If the pressure leaks down there is a leak, possibly a head gasket.

A fan should run whenever the AC compressor is engaged, not just when you set the controls to MAX AC.

The AC condenser dumps extra heat right in front of the radiator. Are you sure the air passages through the radiator are clean?

Also, have you measured the ACTUAL temperature of the coolant? If you’re going by the dashboard to determine overheating you could be misleading yourself.

Does it boil over or just run high on the gauge?

All good questions. Here is what I know.

There does not seem to be a leak based on the overflow tank maintaining the same level. I check this peridialcally over the last couple of months in the morning when the engine is cool. I put a mark on the tank just to be sure I was looking at the same level each time. No change.

I have not let the car boil over. I turn off the AC when the meter in the car pegs max hot. That is when I start to reach max hot. The car and I take turns going to max hot after that until we get home.

When the car gets really hot, it looses power and starts to ping, slightly. I use cheap gas, so pinging is probably normal for this case. I think the tming is OK since it runs fine at cooler temps and has plenty of power the rest of the time. I have never adjusted it or actually inspected it either. I guess I could do this jsut to check it off the list of possibilities. Measuring the actual coolant temp would be slightly more difficult, but I could problby do that too.

I did hose off the AC condensor coils and the radiator coils about 2 months ago when I replaced the water pump. I actulally took out the radiator and inspected it after I hosed it off. Both the condensor and radiator look clean with no blockage.

I will re-read my workshop manual about the fan coming on whent the AC is turned on. I don’t think this is true for this car. I had a 93 villager that did not not come on when the AC was engaged and my friend’s Nissan Quest is the same way too. Right now it comes on when the engine is hot or when the max AC is engaged. Anyway, I don’t think the problem is with the fan. If it were, then why does it overheat when the Max AC is on and the fan is on?

Somehow I think there is an air bubble in the system that I cannot seem to get rid of. Do you have any techniques to ensure there is no air in the coolant system?

Here is something you might try: the next time the temperature gauge heads for the danger zone, turn off the air conditioning and turn on the heater. If the engine temperature drops, this seems to indicate that the radiator may not have enough cooling capacity.

The coolant system depends on pressure to disperse heat. You can have a leak that does not loose fluid that would account for your symptoms. If you are experiencing coolant loss you need to find the source.

I’m pretty sure the temp will drop if I turn on the heater. It drops when I just turn off the AC, so adding in the cooling from the heater will only help more. I have to beleive the radiator is big enough to keep the engine at the right temp all the time even under severe conditions. My 93 villager never had these problems and it had the exact same radiator.

This all started when I changed the coolant the first time.