Scum Floating on Coolant

[Hudsonian]

Yesterday, when helping orient a friend to the the Volvo Penta 2003 diesel on his new-to-him boat, I noticed a layer (approximately 1/8") of tan scum atop the coolant in the expansion tank. There was no milkiness in the crankcase oil. Is the scum a cause for serious concern? What is it?

[kd3pc]

likely oil in the coolant somewhere along the way...

[hellosailor]

Could be rust from not changing the coolant, could be oil contamination from a bad head gasket or other source. If it is oily, it will settle out and leave an oil stain if you scoop some out and let it set. That would be a source for concern since a blown head gasket can really ruin your day. If you catch it early, it only hurts your wallet.

[Hudsonian]

Thanks for your replies. I, too, had speculated that it was a blown headgasket, cracked head, or warped head. Do any of you have any suggestions short of yanking the head to inspect the head gasket and sending the head out to by magnafluxed and checked for warpage?

Can this be nursed through until October or does it demand immediate response?

[hellosailor]

You can nurse anything, depending on your luck and how you feel about gambling. Personally I feel that if there is any question for bad head gasket, you want to shut it down immediately and run diagnostics, so you don't damage the engine. You can ruin it, or get by for a long time, but you don't know without looking into it.

Any engine manual, any engine shop, can tell you which diagnostics and how to run them. There is dye to put in the coolant (to see if exhaust gas is getting into it) and the froth can be tested to see if it is oil or rust, and sometimes a radiator shop can lend you a pressure guage that replaces the radiator cap. if the pressure pulses as the engine runs--that's cylinder pressure being pushed into the cooling system.

The symptoms can vary because the gasket can leak in so many different ways. it can let cylinder pressure in/out of the other systems, or oil into the water, or vice versa, and sometimes just one problem happens, sometimes things all go kerflooie and they all get messed up.

All those diagnostics happen WITHOUT PULLING THE HEAD and any competent mechanic should be able to tell you what's up in less than an hour of his time.(Try to find a competent one, yeah.) So looking at the manuals (all engines are quite similar in this way) and DIY can be a good way to start.

If you do need to pull the head--that's not as bad as it might seem. A shop manual, a torque wrench, and some regular socket tools and you've got all you need. Do it slowly and carefully, per the manual, and a head gasket job is NOT a difficult or expensive job, in the grand scheme of things. If the gasket failed because the block or the head warped, it needs to be machined at a shop. If it failed because the block cracked, often the block can be welded rather than scrapped. You can't tell for sure until you get into it, but if it was just a gasket failure--that's a damned inconvenience, but not a budget buster.