is my engine overheating

[johns]

i have a volvo mdib which runs great but every 15-20 minutes the exaust sounds diffrent and has more steam when i feel the water it feels very warm a couple of minutes later it cools down again is this normal i have had the thermostat replaced but dont have a tempature gage it is always the same amount of water it it over heating .
i have had the engine runing for abour 3 hours non stop

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[Birdseye]

"feeling the temperature" is very subjective, but assuming your impressions are correct - no, the temp shouldnt vary like that. exhaust noise also indicates something wrong, but from a distance it is impossible to diagnose reliably.

the symptoms suggest a shortage of water flow and therefore overheating. start at the seacock end and systematically check that everything is working through to the exhaust outlet. check that the impellor is ok and that the rubber vanes have not become detached from the metal centre. since you cannot always trust tradesmen, check that the new thermostat is opening properly (and is the correct one!) by removing it and immersing in a pan of water on the stove together with a thermometer. when you do this, check how furred up the waterways are. in an old engine, this could easily be the problem.



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[Mirelle]

Just to add a footnote to the above, the new thermostat which Volvo dealers now supply in place of the original one is a different design and it is very prone to this behaviour - suddenly opening and closing. I have been told, here, (search for volvo overheating thread) that a cure is to drill a small, say 1/8", hole through the brass disc on which the thermostat assembly is mounted to allow a continuous flow of water to the bimetallic element.

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