I've noticed with this car on a 60 mile trip last week on interstate (75 mph) 30 to 35 degrees outside with heat on norm that car average 140 degrees according to DIC, which seems right with the amount of heat it was putting out.
I changed the T-stat yesterday and let the car idle till 218ish and fans kicked on and heat work good(it was 50 degrees outside then). When driving the car however, the coolant temp dropped and it seems to like to run at 150-160 degrees on a 25 degree day with heat on norm, even after letting it warm up initially. The T-stat is 195 F, and coolant is full, no service lights. Shouldn't it warm up to at least 190-195? then hover around there while driving? I'd really hate to blow up a 54k mile 4100 in winter due to it overcooling.
Just cut a piece of cardboard half the size of the radiator and stick it between the condenser and the radiator, NOT on the side the cooling fan is. There is no way to heat the engine anymore and putting a really hot t-stat in there is a pain on those days it heats up and cools right back down. The cardboard can pull out and go right back in in minutes and can be trimmed to match what ever running temp you want. you want it cooler, cut it back; if not, leave it alone.
Thanks for cardboard idea, might have to try that as I drive the car to work daily which is a short trip(3 miles stop and go), but i do let it warm up for a few minutes in morning. I did run a few errands after work, it warmed up to 195-197 then cooled to 185 then cycled again, it was about 20 degrees outside and heater worked good. I've had the new t-stat in for a couple days now, I think I'll see what it does for a week or two, now that i've got it to warm up to 195 while driving it in cold weather with heat on. It just seems like this car takes longer to warm up than the other cars I have had.
I thought the t-stat might be bad, but after today, maybe it is good. It has a Stant 195 t-stat in it now, thanks.
Automobile(s): 58 Coupe Deville, 77 Coupe deville, 90 Deville
Re: 1986 Seville HT4100-Overcooling issue
Have you measured the actual eng temp with a non contact thermometer? That seems low, my 90 4.5 with a 195 deg thermostat runs around 200/210. In fact that is one thing I like about this car is that the heater starts working quickly and puts out a lot of heat. I have used the cardboard trick that Recluse mentions and it will work if all else fails. But you may have a defective temp sensor, idk. Some people recommend that you test the thermostat in a pan of water on the stove before you put it in to make sure it is working as it should.
Have you measured the actual eng temp with a non contact thermometer? That seems low, my 90 4.5 with a 195 deg thermostat runs around 200/210.
Depends on the state of the cooling system. Years ago when I got the car it ran a little warm, but the cooling system was less than par because the radiator had to be changed. Once changed, and an aggressive flushing, mine has been doing the same thing as the OP and you can tell when the t-stat is cycling.
Since 195 is when the t-stat starts to open, a 200-210 operating temp is the correct normal range.
I've got a new rad (cracked plastic side tank), water pump (leak at shaft seal), heater core (cracked plastic side tank), and thermostat in my '90. All less than one year old. Strangely, mine runs at 195 all day with no fluctuations, city or highway, hot or cold ambient temps. Tried two thermostats with no change. This is different than my other 4.x's that ran 203-205 all day. I'm not going to worry about an 8-10 degree difference. Everything functions normally.
Automobile(s): 1988 Allante' (sold), 1984 Eldorado, 84 Sedan DeVille
Re: 1986 Seville HT4100-Overcooling issue
I think you have trapped air in the system and that would account for the up/down temp readings. Remove the radi. cap with the engine off and cold. Coolant should virtually overflow from the radiator. If not start the car turn on the heater to 90 degrees and with a friend holding the idle above 1500 RPM, top off the radiator once the level drops as the thermostat opens. ( could take 10 min) While idling high, quickly reinstall cap. I assume the cap is proper and new? As the prior poster said, the thermostat doesnt open below 195 so that should be the minimum temp displayed. The radiator fan comes on at approx 220 degrees.
Update-after a couple of weeks of driving the coolant temp and heater are functioning flawlessly. maybe there was some air in the system after i changed the t-stat? At any rate, the car warms up to 195-200, then t-stat opens up and cools it down to 175-180ish, then cycles. The heat is soooo much better, no cardboard either. it was bitterly cold in Nebraska yesterday evening and heater worked great.