overheating problem

Yesterday was the warmest day (low 60s F) since I started driving my 91 Caprice again and it overheated. It overheated when I last drove it , a couple of years ago. I replaced the water pump and hoses then stopped driving it, thinking the problem was fixed. Evidently not. It has a new thermostat. the heater works fine. It does not lose coolant until it gets hot, then it overfills the coolant jug to overflowing. I will have to work on it this afternoon because I need to drive it to work in the morning. I have searched and read the overheating threads on here. Any suggestions? Thanks.

First thing to try is a coolant flush.

When you say "overheated" do you mean all the way to the red? My Hilda has the electric fans and if her AC isnt running, the fans dont kick on until like 195 and she's on the last white hash mark. She wont boil over, even in stop and go in the Florida summer, but she really smells like coolant at that point.

If your AC doesnt work, you cant just run it anyway and hope that tells the ECU to kick on the fans. There has to be pressure in the AC system.

Your mileage may very, tho, being a 91.

91-93 TBI cars have mechanical fans. So that should be an issue. But the coolant overflowing the jug is a telltale sign of a possible blown headgasket. Usually that means the cooling system is being over-pressurized.

Ok, I flushed the coolant and replaced the water pump.My temp guage does not work. When not overheated the car runs good. The timing is a little off ( by a couple of marks on the indicator) but I didn't change it because I could not find my distributor wrench.The car is a 91 so it has no electric fan. I drove it about 20 miles after the flush and installing the new pump . The temp here was 60s. It seems ok now. Related or not, my "check engine" light comes on from time to time. I will drive the car to work this morning , I hope.

I made it to work ok. The outside temp was in the upper 30s. I hope the problem is solved. I plan to buy a gauge set to go under the dash this week so I can keep an eye on my temp, oil pressure and volts.

you can pull the codes with a paper clip.

https://www.obd-codes.com/faq/read-gm-2-digit-obd-codes-free.php

Overflowing to the coolant bottle can be a stuck thermostat..... Obviously you cant run without one (like I do) where the weather is that cold......Thermostats are closed at rest and failure is defined as failure to open. Some parts guys will try to sell you a "Fail-Safe" thermostat that (supposedly) fails in the open position..... It's marketing (a fraud)
You may have already replaced the thermostat, so it must be good, right? Did you heat water to the advertised temperature and test to see if it opens? I've been told that one in three are defective right out of the box, but I've found personally that more of them are defective (over 50%)
It almost never freezes where I live and is usually quite hot, so I run without a thermostat. I also like solid fan hubs in place of a fan clutch....

Thanks. Actually, somewhere in my seldom used tool storage, I have a tool that you plug into the connector that does the same thing as a paper clip jumper. It was given out by one of the ignition companies when ECMs first came on American cars. The Echlin Co , I think. I worked at NAPA auto parts at the time. I will check the codes probably tomorrow. I know the EGR valve is disconnected and the vac hose plugged. The cat converter has been removed.

That could give you codes, a disconnected EGR

I checked and got code 14 - coolant sensor. After the water pump replacement and flush over the weekend I thought maybe the problem was fixed. Then , after work yesterday I drove 18 miles on the interstate , to get a temp sending unit for the car. After the interstate drive the car was hot enough that the water was bubbling in the jug. After I got home, another route, not on the interstate, I flushed the system again, installed coolant and water, and installed the new sender. I then drove a few miles and kept a watch on the now working gauge. It barely went above the half-way mark, nowhere near the red zone. I'm wondering why would it overheat on the interstate, a pretty much straight and level drive, and not in stop and go driving. Bad fan clutch, maybe? PS-When I checked my timing I forgot to unplug the tan/black wire , so the timing is probably correct. I have not rechecked with the wire unplugged.

8 miles to the interstate and 8 miles on the interstate to work this morning , in the rain, the gauge didn't get over the half-way mark.

On the drive home last evening the needle went almost to the red zone. My service engine soon light comes on about 15 minutes into every drive. I've noticed that when the SES light comes on the temp gauge starts to climb. I suppose it's because of the default mode/ loop that kicks in when the light is on? If I stop and turn the car off and restart it I'm good for another 10-15 minute drive.

That's a difficult one

Joy, Joy, Joy, 16 +- miles to work, 2-3 at lunch, and 16 +- home today and not a miss, no service engine soon light, and the temp gauge never went above the half-way mark. This heap might be a daily driver yet!
Yesterday it died a couple of times and ran rough. I found the vacuum line from the throttle body to the MAP sensor had become disconnected at the throttle body. Good reason for rough running , huh? The connector at the t.b. seems a little loose fitting so I will replace the hard line with a piece of rubber vac hose. I don't trust the hard plastic lines Gm used much, anyway.

Come to think of it, a bad ground could do crazy things to