Blown seals - Mick's Tips 431

By: Mick McCrudden

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Has the head gasket had it, or is it something much cheaper?

Is it a blown head gasket, or not a blown head gasket? What usually happens is we see what we call mayonnaise in the sump – water in the oil – and people are immediately on the panic button. Don’t be.

There can be all sorts of areas where coolant or water can meet up with oil, with some designs running coolant through inlet manifolds and all sorts of other areas. They all create extra weak links.

One blown head gasket I dealt with didn’t put water in the oil, but it did create a seven-cylinder car.

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Your first job is to nut out which cylinder is playing up. Then work out whether it might be fuel, spark or compression. Now a simple compression test will reveal the latter and that may well be a blown gasket. You’ll also be getting a miss when the engine is running.

There are a few reasons why a gasket might blow out. In my recent case the cylinder head hadn’t been retensioned after a rebuild and that created a weakness. Nine times out of ten however a head gasket burns through because an engine has over-heated. So as sure as eggs, if the car has got that hot, it will eventually burn out a head gasket.

Now cylinder heads generally have both water and oil running through them, often in neighbouring galleries. While the gasket would rarely burn out between those galleries, a warped head (through over-heating and/or poor tensioning) will allow liquid to cross between them. There’s your mayonnaise!

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With V8s – or anything with a couple of heads – it’s common for there to be water galleries between the heads. Now if those gaskets leak, you’ll end up with coolant in the valley and quite possibly a sump full of water as well. However your head gaskets will be fine.

The message here is that if you have a sump full of water, don’t immediately assume it’s the head gasket. There are lots of areas in an engine that can cause this. Some are behind the water pump, in the block.

Here’s a simple test: pull the spark plugs out of your engine and remove the radiator cap and top the radiator up. Now I have a tool I made – it’s a hollowed-out spark plug at one end, attached to a hose with a compressor hose fitting at the other. You can also buy a commercial version, but they’re pricey.

What you do is remove all the plugs, wind the tool into a spark plug socket, hook it up to a compressor, and crank over the engine on the starter. If you see bubbles coming up through the radiator, you have a blown gasket. No bubbles, no problem. My grand-dad had a simpler test, which was to run the engine up to full temp, then very quickly pull out the spark plug on each cylinder and look for steam. That’s fine if access is easy. If it’s not the gasket, it could be one of several other areas.


If you have a blown gasket, you of course replace it. But we still have a sump full of goo that moves like caramel sauce. Here’s how you clear it out. First let it drain overnight and then put in a fesh oil filter. Then put a 50/50 mix of oil and diesel in to the usual level and run the engine at idle. Do not rev it.

A few minutes should do it. Then drain everything and replace the filter (and oil)again. Maybe you’ll have to do it a couple if times – but once usually does the trick.

Now just to reinforce what I’m saying, we recently had a fairly modern car come in, with the owner a little upset as they’d been told the gasket was gone and it would cost a fortune to fix. They wanted a second opinion and, when we checked, it was simply an ancillary cooler which cost just a couple of hundred dollars to replace.

So here’s the message: Mayonnaise in the sump may not be a gasket but something far less serious. If in doubt check it out methodically, as it could save you a whole heap of money!

| Search more of Mick's mechanical tips here


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Note: Mick runs Glenlyon Motors in Brunswick, Vic. Tel (03) 9380 5082.

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