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Hi all!

I recently bought a 88' seville. During normal driving the engine temp is at 165-170, but once the car is stopped and idling the temp slowly climbs to 200, at which time the coolant overflow tank starts boiling and smoking. I know about a month before I got the car, the previous owner replaced the radiator. when coolant is boiling and smoking, at no point is the car throwing any dash lights or error codes. The cooling fan is working properly. Where would everyone suggest I start? Thanks!
 

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but once the car is stopped and idling the temp slowly climbs to 200, at which time the coolant overflow tank starts boiling and smoking.
If you are boiling over at 200°, the radiator is not holding pressure. With a 50/50 mix and a 16 psi cap you should not boil until about 260°. I'd start by replacing the radiator cap.
 

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If you are boiling over at 200°, the radiator is not holding pressure. With a 50/50 mix and a 16 psi cap you should not boil until about 260°. I'd start by replacing the radiator cap.
Thanks Ranger! there was actually a brand new radiator cap in the glove box. I guess I'll give that a try!
 

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well I put a new radiator cap on today, and that didn't make a difference. still staying at 165-170 when driving and will rise to 200 at idle and coolant bubble over
 

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You have cooling system problems - and someone tried to fudge a fix by installing a 160 degree thermostat.

That engine should run at 185 - 210 degrees coolant temp. 165 - 170 is too low for correct efficiency and for some systems (emissions) operation.

Is the coolant level set correctly for an engine cold (ambient) system fill ? There should be a decent amount of air space in the radiator tank or coolant overflow jar. Coolant expands a LOT as it warms up and that necessary air space is what prevents overflow under normal operating conditions.

Your original thermostat was either a "180" or "195" degree unit.

As posted, here's the boil point table for engines that can hold correct system pressure - n your case, probably 16 psi. Note that the (recommended) 50/50 concentration line is out of sequence.

Coolant Boiling point.jpg
 

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Okay, I appreciate your input and I will get a new thermostat in there. should I go with a 180 or a 195?
 

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I see 2 different thermostats listed (RockAuto.com) for your car.

The correct one for your engine will be shown in the GM/Cadillac service manuals. Either find a used paper copy on eBay or subscribe the car to www.alldatadiy.com - the online GM service manual set.

Perhaps an owner/member with the same car/engine will chime in. Not sure exactly which engine options were offered for the 1988 Sevilles. Alldata says it's the 4.5L.
 

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Don't quote me, but if I remember correctly, I think those engines used a 190° or 195° stat.
 

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Okay, I changed out the thermostat. The old one looked pretty bad. I used a 195. The car is staying at 170-175, I haven’t had a overheat situation yet.
 

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So I was sitting in traffic and it started overheating. It was bubbling hard in the overflow and steaming. Temp showed 199 on the dic. It has a new radiator, radiator cap and 195 thermostat.. any other ideas? Thanks!
 

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You're losing system pressure somewhere. 50/50 coolant, pressurized to 16 psi, should not boil until 267 degrees F.

Please read and understand the several previous posts on the boil and temperature subject - back past #5 at least.

Think about it - even plain water doesn't boil until 212 degrees F. You have problems somewhere. Have you used an IR thermometer to verify the coolant temps at various places in the system?
 

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Assuming that your temperature is accurate (199°), you where not overheating. What you saw may have been "false boiling". That is the cooling system being over pressurized by a bad HG and venting via the cap. Even with no cap, 199° is well below the boiling point.
 
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