Engine overheating after thermostat change

[eseethal]

I need some help here please: I diagnosed a stuck open thermostat (it was really stuck open). No problems, the car only was heating up very slowly, temperatures were always in the normal range. I changed it to a new one from SpecialAuto. I had raised the rear end of the DeLorean up and bled/burped the system as recommended. So far, so good. Thermostat is closed, engine warms up quickly, thermostat opens, the lines to the cooler get warm, the cooler gets warm bottom to top. But when driving the temperature gauge quickly rises to the first orange mark and a little above that. I tried burping/bleeding several times, but I still see a quick rise in temperature under load. The fans kick in later, telling me that way that the temperature sensor gets a higher reading than the temperature switch for the fans. So is there still some air in the system trapped? Or the thermostat is not opening up completely. I put the thermostat in like the old one was (looks right when I check it in the parts manual).

Any ideas what I could try? The next step would be to try a new thermostat.

Many thanks..........Eugen

[DMC-81]

Hi Eugen,

You may still have some air trapped, especially under the thermostat. Does your thermostat have a check valve?

If not you can drill a 1/8" hole in the frame to aid bleeding. Like so:

image.jpg

Then, raise the car again and repeat the bleeding process. Sometimes it takes more than one to remove the air.

[eseethal]

Dana, thanks, the old thermostat hat the little valve, but the new one did not. I will open up and drill this little hole.
Maybe this will fix my issues here. thanks........Eugen

[DMC5180]

Dana, thanks, the old thermostat hat the little valve, but the new one did not. I will open up and drill this little hole.
Maybe this will fix my issues here. thanks........Eugen

Also be sure to release air from the top bleed hose on the right side of the radiator.

[eseethal]

Thanks, yes, I bleed the right side of the radiator too (twice, really messy!). I even changed back the old thermostat but the engine still runs much hotter than before. It's like the waterpump is not working full. With an infrared thermometer I can measure temperatures: the lines to the radiator and the radiator itself are much cooler than the DeLorean temperature gauge says. I am an experienced mechanic and have bleed alot of rear/mideengine cars, but that really I cannot explain. Is it possible the water pump has gone bad?

Eugen

[DMCMW Dave]

Thanks, yes, I bleed the right side of the radiator too (twice, really messy!). I even changed back the old thermostat but the engine still runs much hotter than before. It's like the waterpump is not working full. With an infrared thermometer I can measure temperatures: the lines to the radiator and the radiator itself are much cooler than the DeLorean temperature gauge says. I am an experienced mechanic and have bleed alot of rear/mideengine cars, but that really I cannot explain. Is it possible the water pump has gone bad?

Eugen

Not likely. More likely you have air trapped in the radiator (you bled that) or the engine. That's really all it can be if it came on suddenly.

What is your thermometer reading on the 4 hoses right at the water pump?

Is the engine really as hot as you think it is? The gauge sender is at the back of the engine. Once in a while it will trap air (steam) and read much hotter than it really is. If that's the case, you run it enough at higher speed to clear it. But keep your thermometer on the hose going into the water pump to make sure it's really not overheating.

[eseethal]

Thanks, Dave! The thermometer reading is something in the range from 160-170 degrees. It's just the surface temperature, depending on the material these readings are not very precice. But I can feel these hoses are not really hot. That's why the fans are kicking in much later - I have to idle the engine for a while. I replaced the engine coolant sensor a couple of months before. The readings on the gauge were perfect then - 220 maximum. The radiator gets warm from bottom to top, so no air there, I am pretty sure. I'll try bleeding a couple of times again. It can't be much air in the system. When I pressure up the expansion tank manually with a pump, the level of the coolant does not change (as it would when air is in the system). And when I open the cap on the expansion tank, when the engine is really overheated, boiling cooling water would be the result. But there is not much pressure inside, just a little escape of air.

Eugen

[eseethal]

I have now been driving about 200 mls with this issue. I even once more change the thermostat to a new one, but still I get the high readings of my gauge.
- independend of outside temperatures, when I put a little load on the engine (accelerating, highway driving), the needle quickly sits a the first red mark
- I rule out trapped air, I have the auto bleeder system installed, the radiator gets warm evenly.
- when I come to a stop, the fans do not kick in - only when I let the car idle for a couple of minutes (telling me, that the temperature of the radiator and the hoses and the otterstat are not as high as the gauge wants me to believe)
- when I open the coolant bootle, not boiling water sprays into my face (which would be the case when the engine is really way above 220F)
- the needle of the gauge rests at the 100F mark (ignition on, engine cold, ext. temperature 70F). This is probably not correct, right?

So IMHO it is either the gauge or the newly installed temperature sensor. I will check the ground at the gauge (the ground pulls the needle down, the sensor voltage pulls it up, so a bad ground could lead to a higher reading, when I understand the principle right.)

Or it is a bad new sensor, that I had recently installed. The thermostat was stuck open so the engine probably ran too cool and the gauge readings where in the right range - now with the functioning thermostat the engine gets hotter and the readings are to high. Resistance values for the sensor would be great (ohms/fahrenheit) - then I could take ohm readings and compare it to the displayed temperaturr.
Any opinions on this highly appreciated.....

[David T]

If you got rid of all of the air in the system and it still gets hot and you still have air you may have blown your head gaskets. That would cause overheating and would push air into the cooling system. Confirm with a combustion gas test of the coolant. As for temps you can check with a non-contact infra red temperature gauge or stick a thermometer into the header bottle.

[eseethal]

Thanks, but I politely have to disagree: The cooling system does not pressure up, no signs of a blown head gasket, everything is fine so far, no air in the system. I also checked with an infrared temperature meter - the hoses are much cooler than the gauge wants me to believe. I am pretty sure it is either the sensor or the gauge. Even if the waterpump was not working or the thermostat was stuck (that would give me these symptoms of a hot engine and an not so hot radiator/hoses and fans not running), when I open the coolant bottle the coolant must be boiling. Coolant above 220F has to be boling if not under pressure.

Eugen