I removed the factory thermostat from my '99 E450 V-10 6.8 Motorhome...reinstalled a 180 stat from a local parts house... now my fuel mileage seems to be down by about 20-25% ...is this common? What temp stat should I use in the vehicle? I live in the southwest desert where ambient temps run quite high in the summer. Thanks for any input,buford8

 

I don't know of any recommendation of using a 180 thermostat. The one you removed should have been a 195, was it?
Your engine system is designed to cancel out alternating injectors to prevent an overheat condition which could destroy the engine. You might want to check this out.

 

Thanks for the response Hanky, I don't understand about canceling out alternating injectors? When does this occur? The stat I removed from my V-10 is a 185 degree stat.

 

In order to protect the engine from overheating the ECM will block /turn off individual or pairs of injectors so no combustion takes place in alternating cylinders and they act as air pumps to remove the heat from the engine so the vehicle can be driven for repairs. It's a good feature and you might want to get a shop manual to learn more about it.
I would suggest you check with a Ford parts dept and they should be able to tell you the correct stat that you should use. Also they should be able to answer the questions you might have regarding the overheat protection system.