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One month ago, I started car, ride and still had cold air from vents. Normally after few km it started to be warm. This lasted for a few days, a slight warmth after a long ride. The next symptom is when driving: cold, cold, overheating for a few seconds, cold. The next day, cold, cold, overheating and the message "power reduction", stopping the car did not help, I could feel the heat from the engine but the radiator was only warm. When I connected the computer during the next drive, the engine did not want to exceed 130F for a long time and then went to 220F and dropped to about 170F. I decided to replace the thermostat.
When the car was stopped with a fuel system failure, I drained the coolant and installed a new thermostat. I filled cooler tank and after turning on the engine, the temperature does not rise, the fluid remains motionless in the tank, the hoses are empty, the engine starts to heat up.
I think it's not a head gasket because the engine doesn't heat up when cold and I don't have any blow-by in the radiator tank.
The thermostat was replaced and it didn't seem to change anything.
So is it the water pump, that's why the water stays in the tank and doesn't go to the hoses?
What do you think ?
EDIT:
I add coolant and remove air from coolant system. After few minutes of drove had warm air from wents. Temp still rose and about 200F started to blow cold air from vents and temp still rose, about 240F started to drop and have warm from vents. When car is on iddle - engine temp goes high, when I rise revs - temp goes down. If temp is about 230 I can open radiator tank cup, coolant goes to maximum level and stop, I have no boiling coolant, no discharge to the outside.
I ordered new pump, if nothing change... I don't know what next.
Sounds like no flow or a blockage or a trapped air bubble. Thermostat is first thing I'd replace, which you already did. After 20 years, the water pump impeller may be corroded away, so I would replace that next. Have you tried a coolant flush? If the impeller is gone, a coolant flush won't help because there won't be any flow.
I checed exhaust gas blowing from a damaged engine head gasket.
On warm engine I put pipe with control fluid to radiator tank :
started engine wait few minutes. I rose RPM to 3000 and wait few minutes.
Tester is still blue, if fluid change colour to yellow I have exhaust gases in coolant, green means a small exhaust leak. Test showed that the problem was not in the system containing air from exhaust gases.
I removed old water pump and looks good. It's looks like from my friend Mustang engine but i have aluminium romero and maybe it is the reason.
Old pump on left, new on right:
I changed it and its better, I think that wider pump blades take more coolant. I checked system and is better. Still have warm, and hot air from blower and if I drive I have about 192 - 196 F ( thermostat schould be opened at 190 F). Problem is when I stop or drive really slow, engine temp rise to 235 F and goes down when I go higher with RPM. I checked viscous fan clutch too. Maybe here is problem. On cold engine fan can be rotete by hand, on hot engine nothing change. The same easy to rotate. Maybe it is reason why need higher RPM for cooling ??
I use Liqui Moly 2699 cooling system cleaner but inside was clean.
Yesterday I observed :
On cold engine. Start, warm up engine, warm air from blower. Nice to about 190 F ( thermostat should start to open) and... cold air from vents till engine has about 245 - 250F, after that warm air from blower and engine works normal. Temp oscilating about 200 - 205F. If I stop and start hot engine I can drive without problem. Everything works fine till I stop engine to cold down.
Maybe new thermostat not working like should and need overheating to start. I ordered another one. Originally I had Motorcraft, second was Motorad now I ordered Delphi with 180 F open.
Today I picked up another one thermostat. I thought it was Delphi but... it is Gates I chose this time 180F, not because overheating but I have impression that on warm summer days and when traveling around southern Europe, my car has less power than on colder days. Second thing, I most often use the "Performance" program from Superchips and the fuel has much higher octane than in the USA.
I removed termostat, and I solved the overheating mystery...
On left Motorcraft, on middle Gates, on right Motorad:
First I had ( I think from new ) Motorcraft : it was stuck open and the car wouldn't heat, and once it was closed, it couldn't open and the engine overheated.
Second I put Motorad : the engine warmed up, then overheated and started working normally until it cooled down completely. Then the situation started all over again.
Now is Gates : engin at first start warm up and worked without problems. After cooled down I refill coolant and works like should.
One difference is that on Motorad 190F engine works ( after first overheating ) about 205 - 220F on Gates 180F oscilating about 188 - 196F
Where was problem with overheating on Motorad thermostat ?? On photo you can see on upper side of Motorcraft and Gates little hole. Motorad is without it. Air that accumulated in cooling system during operation of the open thermostat passes through this hole and coolant can fully fill thermostat. In Motorad in Motorad, the air bubbles surrounding the thermostat did not allow it to reach temperature, which resulted in overheating.
And one more thing... on Gates thermostat visco fan clutch started to work.
Hi,
I didn't have time to write earlier, but the problem quickly returned. I drive my Expy but still the same. After start coolant temperature and heating increase, then the vents start to be cold and the temperature increases to overheating. I set to N and increase RPM several times. The air vents heat up and the engine temperature drops. I can drive 1000 miles until the next cold start.
What I replaced with new ones:
- 2x thermostat (now I have 165F)
- water pump (old one looks like new)
- viscous clutch (works better than the old one)
- new drive belt (the old one was slippery and shiny)
- the water pump pulley were sanded to make it non-slip
- heater valve (the old one sometimes got stuck if I checked it by hand)
I observe :
- heater valve not react, it is no matter cold engine, hot engine, vents temp set to low or to high.
I was almost sure that last winter after start in car was cold no matter what I set on AC panel, engine temp rise and after that my vents start blow and i have warm inside car. I'm wrong or it can be as I wrote ??
Now I started cold engine and after few minutes I have warm and warmer air from vents.
My theory was that maybe this heater valve should be close till engine warm up. Now if my valve not react over 5 gallons of coolant are heated and flow through the thermostat is not enough to stabilize temp. I need some higher revs to mixed coolant and after this everything is ok.
Maybe it is stupid theory but my wife said the same, Last winter car was cold inside after few miles start to blow warm air.
I have underpreasure in feed line for this valve ( inlet colector to feed line ) but on valve side is no preassure or under preassure. I can check only that both lines are on place, nothink loose, i can't put my hand to this module - not enough place for my hand.
Second observation:
From auxiliary heater still blow cold air but I observe knocking from rear right side something and looks like something stuck and not react on change temps ( flaps or gears ??) .
Or maybe it is connected - cold rear vents and engine overheat ??
. I will flush with new water pump.
I chose this time 180F, not because overheating but I have impression that on warm summer days and when traveling around southern Europe, my car has less power than on colder days. Second thing, I most often use the "Performance" program from Superchips and the fuel has much higher octane than in the USA. 
