Overflow tank boils over after truck is parked, temperature reads normal - Ranger-Forums - The Ultimate Ford Ranger Resource


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Old 06-10-2011
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Overflow tank boils over after truck is parked, temperature reads normal

Okay, I have a 1993 4.0L 4x4 Ranger. I have been having issues with the cooling system. I flushed the radiator six months ago and I have replaced the following:

thermostat (old one was good though)
radiator cap (pretty sure old one was good)
water pump (failed)
bypassed heater core (leaked)

I have not replaced the temperature sensor, but it does seem to function... i.e. it goes from cold to normal as it should.

Anyway, the truck runs fine. I live in Houston and we're dealing with temperatures over 100. Sometimes, whenever I park and turn off the truck, I can take the cap off the overflow tank, and the water is at the proper level and is not boiling. About a minute later, it starts boiling. Sometimes it boils so much it overflows out of the tank. The temperature gauge doesn't read past normal, though I have seen it get a little higher in the past before the new water pump.

I really don't know what to do. It's worth mentioning that I've been losing so much coolant troubleshooting the radiator, that I started using water instead. I'm going to flush the radiator again today and fill it up with 50/50. As far as I can tell there are no air bubbles.

My main fear is head gasket issues. Anyone have any input? Thanks a lot.

Have a good weekend, guys
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Old 06-10-2011
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how high is the over flow tank when its dead cold
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Old 06-10-2011
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water will boil at 212 degrees...coolant will not
drain it and mix 50/50 coolant, let it run at 1500 rpm for about 20 minutes with the cap off until it burps when the t-stat opens hold it at 1500, top it off THEN let off the throttle. you can use a pocket screwdriver between the throttle stop and the screw to maintain RPM
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Old 06-24-2011
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If boiling starts from about 10 seconds to a minute after shutting down the engine, it may not be abnormal. When an engine is running, the circulating water keeps spot head temperatures pretty constant. But, some parts of the head have more thermal mass because of increased metal thickness. Sooo, after the engine is shut off, water isn't circulating. The parts of the head with the greater thermal mass still have heat to shuck so there may be hot spots where the coolant will come to a boil. To verify, try letting the engine idle for a few minutes before shutting the ignition off. Five should be enough. This should give the hot spots time to cool.

You'll probably have even more boiling if the engine is shut off immediately after its been heavily loaded.

Boiling due to the hot spots may cause enough vapor to be generated that excess water will be forced into the coolant overflow bottle. About the only thing I know of to limit the amount of water forced out because of boiling is a higher pressure radiator cap. Not something I usually recommend. Or, check your current one for the proper pressure.
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Old 06-24-2011
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Thanks for the info, I will try that tomorrow.

For now I am running 60/40 coolant. Since the overfill tank boils over and dumps all the coolant I just stopped filling it up. I check the radiator from time to time and it is always at it's proper level - just none in the overflow tank, unless I just parked the truck, in which case enough coolant will enter the tank to fill it about to cold line.

I also noticed using my air conditioner is causing an increase in heat. I was on the 'L' of Normal with my A/C on... turned it off and dropped to between 'N' and 'O'.
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Old 06-24-2011
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Temperature should be constant whether the AC is on or not. If it goes up more than a wee bit, it may be an indication of inadequate cooling, ie too small a radiator, auxiliary fan not turning on, bad fan clutch, etc.

Use Toreador4x4's method to be certain the radiator is full, and burped. Put the radiator cap back on, and fill the coolant bottle to the "Cold Fill" line. As the engine cools and the coolant takes less space, some will be sucked from the bottle into the radiator. There should be some coolant in the bottle when its cold, but if the radiator is always full I wouldn't worry much.

One more thing to do is check the coolant bottle and the overflow line for leaks.
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Old 06-25-2011
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If it gets hotter with the a/c on, remove the top bolts on the radiator at the core support and spray the fins of the condenser and the radiator out
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