Hi all,
I changed the upper and lower radiator hoses in my 2005 XK8. Decided to change the water pump while I was at it. Job went easy enough, but now my car is running very hot and will hit the red zone if I let it. I notice the fans are coming on very early, also. There doesn't seem to be a strong circulation when observing inside the overflow tank. And this is without having a thermostat in the car as I am waiting on a replacement. The "new" (refurbed) pump has the metal impeller whereas my original one has plastic. I guess I will put the old one back in and see what happens. Such a simple component but can a pump be bad other than a broken impeller? Thanks!

 

First thing to check is the wire to the sensor that is in the same general area. Maybe it came off. Second, and confusingly, our cars are known to overheat without a thermostat. Maybe it shunts the flow and nothing goes through the radiator...

 

It's counter intuitive, but removing a thermostat results in less cooling. To cool , the flow rate into the radiator must be regulated to allow heat to be extracted as the water moves through the radiator core.
With no thermostat , the flow rate can be so high that only limited heat extraction can occur. Get that thermostat back in.

 

I had a problem with an AC Delco pump that the (plastic) impeller cracked around the shaft such that the shaft was not turning the impeller and no water was pumped. It was only in service for a few months when it happened but thanks to the Real Gauge no permanent damage was done.

I had never heard of such a problem before but only OEM water pumps for me from now on.

 

Interesting. I've never encountered this before. I still wonder why the fans come on way before operating temp is reached and why I don't see agitation in the overflow tank, especially with no thermostat in.

 

I actually got to hear the Real Gauge alarm go off for a few seconds. Fortunately, no harm done.

 

SOLVED!

You guys nailed it! I put the thermostat back in and all returned to normal. I must admit I was skeptical. It's always been commonly held that a car without a thermostat in it probably won't even reach operating temperature, much less overheat. Ha! Not our cars. When I was younger, the first thing you did in summer was often to remove the thermostat to keep the engine cooler in the hot weather. I guess today's cooling systems are a bit more sophisticated that that.

Thanks so much! Cured my issue and I learned something new and valuable.