95 Deville 4.9 low coolant temp.

Devillesforlife

My 95 Deville with a 4.9 will not heat up past 170 degrees Fahrenheit while driving, but it does fully warm up past 200 if let to sit and idle. I replaced the thermostat with a new 160 rated thermostat & Still having the exact same issue. I read the 160 rated doesn't even fully open until 180 and if I am moving it won't go above 170. The heat does blow warm air but on extra cold days, it isn't hot enough. If it sits and idles long enough to hit 200+ the heat is as warm as it ever needs to be and once I start driving it will cool down from 200+ down to 170 in a matter of a few minutes So basically I know the coolant is a perfect 50/50 mix & it has a new thermostat. Any help would be greatly appreciated

Mr_Shiftright

Why are you using a 160 thermostat? I believe your car requires a 195 thermostat. You may be over-cooling your engine.

Devillesforlife

I am at best a back yard mechanic, but I've done a lot of repairs over the years. When I went to buy the replacement thermostat I saw all the different temperatures & called an older friend to hear his 2 cents. He had said the lower temp would lower pressure making for less wear & tear. so I went on his thoughts. Yes 195 is the OEM for my Deville. Would the thermostat I have in there now be why it won't Heat up? thank you for input

Devillesforlife

How could the lower temp thermostat be the cause of this: If it sits and idles long enough to hit 200+ the heat is as warm as it ever needs to be and once I start driving it will cool down from 200+ down to 170 in a matter of a few minutes

imidazol97

The thermostat restricts the flow of coolant from the engine through the radiator until the temperature rating of the thermostat is reached. Then it opens gradually over several degrees to allow increased flow through the radiator and back to the engine.

There is another circuit for the coolant that goes through the heater and that is not restricted in order to enable the heater to put out as much heat as quickly as possible.

When you have a 160 deg F thermostat, it opens at a cooler temperature. The maximum temperature the coolant should reach with the car moving down the road and air flowing through the radiator cooling the coolant flowing through it and back into the engine is 160 deg plus a few degrees more as it opens gradually.

Put the 195 degree F thermostat in and you'll be happy with the result. The lower coolant temperature is not good for the engine because it is keeping the mixture of fuel richer to compensate for the lower temperature. Also the oil in the sump is not being run hotter as it would with the higher temp thermostat. That higher temperature helps purge the condensate and combustion products that get past the rings into the oil--essentially boiling it off.

Sitting and idling, the coolant is heating above the 160 deg F. So the heater starts working much better at the higher temp of the coolant water going through it.

All the parts of the engine have been designed for the higher thermostat. You'll likely notice the oil life computer reading is going down faster because the engine is spending more time colder than the optimum temperature.

I have no idea how someone could logically justify a 160 deg thermostat in a GM sedan like the one recommended to you. The people at the counter at the parts store should have helped guide you to the correct thermostat.

Devillesforlife

Thanks I already bought it about to put it in

Devillesforlife

Fixed. After installing the new 195-degree thermostat I found my answer as to why the 160-degree thermostat would allow the system to heat up and then cool back down. The hole allowing for the passage of coolant is larger on the 160-degree thermostat allowing for a higher-capacity flow than on the 195