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Excessive Steam or Smoke
After Overheating
Alpha

Engine Overheating

Most cases of overheating are caused by failed sea water intake pumps. On an Alpha drive, this pump is in the drive. Inboards, Bravo, TRS and all the Speedmaster drives have a separate intake pump on the engine.
Of course, you should never overlook the possibility that the water intake is plugged. If you have a through drive pickup like an Alpha or Bravo,  even thought it is not likely it is possible to pick up a plastic bag that will cause a mystery overheat. When you stop, the bag drifts away causing severe hair loss as you pull it out trying to figure what happened.
Especially with transom or bottom mounted intakes sea weed or sand can make it all the way to your sea strainer (assuming you have one) and plug it up solid. One of our customers with a Bravo drive just told us that  the opening for the water intake hose was corroded almost shut!! So pull the drive and take a look.
In any case if the intake pump has been starved for water for more than a few seconds it is likely that the rubber impeller inside has been damaged. These impellors also fail simply because of age.
A bad impeller will occasionally shed some blades. It is important to trace the water flow and find all the pieces.
In most cases the thermostat is not the cause of the problem, but it can be damaged by high heat. A bad thermostat will overheat the engine, but there will still be plenty of water coming out of the exhausts.
Clogged exhaust manifolds or risers (elbows) can restrict the flow of water enough to reduce the flow through the engine. This is most common in salt water. Rust accumulates and eventually clogs the water passages. In fact, it is best to replace risers and manifolds every few years to eliminate this problem. 
It is possible to monitor the condition of your risers by simply touching them with the engine running. MAKE SURE THE ENGINE AND RISERS ARE COOL ENOUGH TO TOUCH! Assuming your sea water pump is good, a clogged riser will be noticeably warm to the touch. 
Even though it is unusual, we have seen circulating pumps where the cast impeller is corroded so badly that the pump no longer pumps. If this is your problem there will still be plenty of water flow out the exhausts.
Last but not least, excessive steam behind the boat is usually the result of a water leak in to the exhaust. This could be as simple as bad gaskets between the riser and manifold or could be something more serious. It is common for a manifold to crack after it has been overheated.
After you have fixed your over heating problem, here are some other things to check
 

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