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99 Deville Overheating when driven over 60MPH


PAUL T

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Have you replaced the cap? Maybe the cap is releasing low?

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

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After running at about 3000 rpm for about 5 minutes or so coolant started spewing out of the surge overflow, temp. was back up to 224 and the fans were on.

Paul T,

That seems like an awful low temp to have a BOILOVER.

Is this a pressurized surge tank?

Is it possible that it is nothing more than a bad pressure cap that is not holding the pressure that it is supposed to?

Yes it is a pressurized tank. He is not running a 50/50 mix due to a previous boil over, more water than antifreeze. I will ask about a cap replacement.

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Yes it is a pressurized tank. He is not running a 50/50 mix due to a previous boil over, more water than antifreeze. I will ask about a cap replacement.

I don't know for sure, but I THINK I remember reading about some of the guys having a lot higher temps than 224 without it boiling over.

Hope it is nothing more than a bad pressure cap.

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Coolant of proper 50/50 concentration should not boil over at 224F even if the cap does not hold pressure. Another possibility is that a limited volume of coolant gets too hot and boils because of lack of circulation, that would incease the pressure in the system and result in boiling over. I would start with bringing the coolant concentration to 50/50 and go from there.

The saddest thing in life is wasted talent

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NO question about that, I have had 235 to 240 with no issues. The boiling is caused by less than a 50/50 or a lack of system pressure (cap, leak, etc). I would look to cap first as it may not be holding 16 psi.

I have not heard Paul say anything about steam from exhaust, rough idle or coolant loss, so this is not leading itself toward head gasket. The high temp DTC codes detailed above could have resulted from coolant boiling due to a lack of system pressure compounded by WATER being added to the system.

Listen, for this car to be at the dealer and for them to say its NOT head gaskets, knowing the history of head gasket problems is telling. Now they could have made a mistake, but lets give this the benefit of the doubt first.

I still think testing the coolant for combustion by-products would help to take that off the table, after it is determined that the cap is good or bad.

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Wait a minute guys. I don't think a less than 50/50 will cause an overheat situation, IF IT IS UNDER PRESSURE. In fact I recall the Guru once mention that plain water is a better coolant than anitfreeze and that is what NASCAR engines run. The only reason for "coolant" is freeze protection and lubrication.

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Cars operate in a wide variety of temperatures, from well below freezing to well over 100 F (38 C). So whatever fluid is used to cool the engine has to have a very low freezing point, a high boiling point, and it has to have the capacity to hold a lot of heat.

Water is one of the most effective fluids for holding heat, but water freezes at too high a temperature to be used in car engines. The fluid that most cars use is a mixture of water and ethylene glycol (C2H6O2), also known as antifreeze. By adding ethylene glycol to water, the boiling and freezing points are improved significantly.

Pure Water

Boiling Point 100 C / 212 F

50/50 mix

Boiling Point 106 C / 223 F

C2H6O2/Water 70/30 mix

Boiling Point 113 C / 235 F

The temperature of the coolant can sometimes reach 250 to 275 F (121 to 135 C). Even with ethylene glycol added, these temperatures would boil the coolant, so something additional must be done to raise its boiling point.

The cooling system uses pressure to further raise the boiling point of the coolant. Just as the boiling temperature of water is higher in a pressure cooker, the boiling temperature of coolant is higher if you pressurize the system. Most cars have a pressure limit of 14 to 15 pounds per square inch (psi), which raises the boiling point another 45 F (25 C) so the coolant can withstand the high temperatures.

Antifreeze also contains additives to resist corrosion

The boiling point of ethylene glycol is about 198 C. -- much higher than the

boiling point of water -- so mixtures of the two can be used as a heat

exchange fluid at temperatures significantly greater than 100 C. at significantly

lower pressures than water alone (in a closed system). In principle, water alone

could be used as a heat exchange fluid, but its vapor pressure doubles from 1 to

2 atmospheres at a temperature just over 120 C. So the problem becomes one of

engineering hoses, fittings, etc. that could withstand high pressures if water

alone were used as a heat exchange fluid.

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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I had to come home so I am not where I can check the car anymore. He said the cap was changed, so I suggested that he have the system back flushed to see if there are any blockages and to check the bottom of the surge tank for excessive stop leak. Then fill it with the right concentration of coolant and see what it does. He also has access to another 99 Northstar motor if all else fails. He bought this car off of someone who had it overheat and did not know what to do with it. I am sure that it was probably filled with stop leak and either some passages, pipes or surge tank is partially clogged. Being that the radiator and cap are new I can rule them out. It is a good running car with only 75,000 miles on it. I will keep you informed on the findings when I get the word.

I had the same thing happen to my 94 Concours. The only reason it is a parts car now is that there was alot of steam coming out of the exhaust.

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The dealer said it was not a headgasket but they didn't diagnose the problem further? The cap has been replaced but that is not a guarantee that it is holding the correct pressure. The large auto parts stores will loan a pressure tester - it might be wise to check the cap. If the cap passes the test, then have the coolant tested for exhaust gasses.

It seems very strange that it is boiling over at 224 degrees.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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The coolant does not boil at a lower temp like 224F when it is properly under pressure and of a correct mixture. The approximately 15 psi cooling system pressure acts to raise the boiling point to 270F or so.

Bruce

2023 Cadillac CT4-V Blackwing

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The coolant does not boil at a lower temp like 224F when it is properly under pressure and of a correct mixture. The approximately 15 psi cooling system pressure acts to raise the boiling point to 270F or so.

I should have been more clear, the coolant was probably 70% water and 30% coolant. I agree it probably would not have boiled over at 224F if it had been at the correct mix. I will let you all know what the radiator shop tell my wife's Uncle when he gets it there. I will bring this thread back up then and report the findings.

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Based on the info, I'm inclined to also believe this is a head gasket failure.

The cooling system can still hold pressure overnight (mine did) and it would "boil" over after driving for a while. The head leak does not have to be significant - just a little bit will overpressurize the cooling system and force out the water.

Sometimes you can smell the exhaust odor in the reservoir or see gray deposits on the cap. Get a tester from NAPA and check for hydrocarbons.

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He bought this car off of someone who had it overheat and did not know what to do with it. I am sure that it was probably filled with stop leak and either some passages, pipes or surge tank is partially clogged. Being that the radiator and cap are new I can rule them out.

Please don't be too concerned about the use of stop leak in this car. It was actually REQUIRED on the '99 Northstars by GM to avoid nusance leaks and headbolt corrosion. The mention of that material being in the engine does not mean that the cooling system was abused by the previous owner, nor was it the "old school" leak fix that most of us old timers are familiar with.

In fact, a system drain and refill (never flush a Northstar unless you can't avoid it) requires that a FULL TUBE of Bar's Leak Golden Seal Powder be added to the cooling system. It will cause no harm.

It also WON"T fix a head gasket leak. Which at this point I'm not certain that you have. I must admit though that I'm leaning in that direction.

Never underestimate the amount of a persons greed.

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