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Q: Overheating and gas mileage on 1994 GMC Jimmy

The car has a new water pump, oil& filters changed, thermostat, and coolant. Why is it still overheating?
Also, we are getting horrid gas mileage. suggestions?
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Horrible gas mileage can be caused by a poor running engine. Does your engine run rough or not a smooth as it used too? I would suspect the over heating may be due to a mildly or more serious blow head gasket due to the fact that you have replaced a lot of other cooling system components. The most accurate way to test for a blown head gasket on a gasoline engine is to test for the presence of carbon monoxide in the cooling system. Snap-On, Matco (part # CO 2000), and Napa sell a tool to check to carry out this test. Another link to a supplier of this tool is Carbon Monoxide is only present if there is a blown head gasket or cracked cylinder head. I think the napa part number 700-1006. . Signs of a blown head gasket are coolant loss (frequently have to top up the coolant level of the engine without seeing an obvious leak), experiencing over heating or rough engine running or the presence of white sweet smelling steam from the exhaust. A badly blown head gasket may allow engine coolant and oil to mix.
We also suspect a clog in the coolant system somewhere. The engine runs rough like it did when the egr valve needed cleaning, so we are having that done too. We bought the screen for it but hav seen no real change.
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