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Failing Head Gasket?


Rich Philpot

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Dear all,

Went to drain and refill the coolant in the 948 saloon this weekend, to find black sludge in the header tank.  The water was also pretty black too.  Is this a sign that there is a head gasket leak somewhere?  Otherwise, everything seems normal.  The engine doesn't appear to be getting too hot, there is no loss of coolant and no smoke from the exhaust.
Any ideas?

Cheers,

Rich

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Black sludge is unusual. As you know, the car was laid-up for over 20 years, everything flushed out before going back into service, all apparently nice and clean. It's possible that the (limited) use over the past few years has freed-up muck which was already sitting in the cooling system.

Cheers,
Bill.

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I'm inclined to agree with Bill.  Having done a certain amount of digging around in various 6 cylinder water jackets recently I can certainly vouch for the large amounts of muck that lurk in there. You might find that the black muck is slightly magnetic (Fe3O4 is magnetite) - which would prove it's just ancient deposits disturbed in the water jacket.  I did find some magnetic black powder in one of the blocks I've been working on - which was also from a car that had been stood a long time.  Probably worth flushing the cooling system again before doing anything drastic.

Nick

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  • 1 month later...

Well, flushed out the 948 rad again, more black stuff, this time a black oily film left on my hands etc with a 'rainbow' on the pool of water on the drive.  Refilled it (it had lost a couple of pints anyway) and revved it up without the cap on.  A large amount of greasy black froth appeared, so I'm guessing that the gasket has been bridged between an oilway and the water jacket.  thankfully, the oil looks OK (it was changed 200 miles ago)
Going to bite the bullet and take the head off tomorrow.  I managed to pick up a NOS Payen head gasket set a couple of years ago, but I have two concerns ...
- the head gasket is steel and there are one or two tiny rust spots on it - should I use it?
- the workshop manual advises to coat the new head gasket in 'Wellseal' - what is this, how thickly should it be applied, and is there a modern equivalent product?

Many thanks!

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You can get Wellseal from most motor factors.

I wouldn't mind about the rust spots once you clean them off assuming you are going to use the Wellseal.

Having said that — if you are in any doubt just wait until you have a new head gasket and use that. You don't want to go through the misery of changing the gasket again.

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You're very unlikely to find a 948 gasket without rust spots, I have about 15-20 of them and they all have spots to some degree, spidering under the shellac finish.
Cleaning this off is no problem, Wellseal is also a shellac product, so it will replace what you take off,

Cheers,
Bill.

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Leave only the manifolds on and take everything else off, that way you can use them as a handle if the head is a tad stuck. As soon as it's off the car I'd take them off so it's easier and lighter to put back on.

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