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Q: Car running hot. on 1997 BMW 318is

Gauge on dash panel keeps showing hot. Changed out thermostat and water pump. Still shows hot on gauge after about a mile.
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Could be a blown head gasket. 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.
First, make suer it's bled correctly. This is how to do it: Bleeding cooling system and checking for water leaks

Top up cooling system, bleed cooling system, refer to Coolant, Service and Repair.

Check cooling system for water tightness, refer to Testing and Inspection, Procedures.

Set heater control to maximum temperature and set fan to low speed. This opens heating valves and installed auxiliary water pump runs if necessary. Run engine and flush out engine cooling circuit with three or four short bursts of throttle (approx. 4500 to 5000 rpm) (sealing cap open). While doing so, do not run engine for longer than approx. 30 sec. to prevent coolant from heating and and expanding. If coolant level in expansion tank drops during this, top up expansion tank to the brim with coolant. Attach screw cover and run engine to heat it up until thermostat opens.

If it won't help, follow Ron and Patrick suggestions.

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If you have a way to pressure test the cap then you can tell if it might be the problem.
If you cannot test the cap then swap it with a known good one. If stops overheating then replace the cap with one from a dealer.
Been burned on this concern once. good luck
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