Why do head gaskets fail in diesel engines?

Question

I'm considering the purchase of a 2001 VW Golf TDI "mechanic's special", so named because it apparently has a failed head gasket (white smoke). I'm not a mechanic but I'm fairly handy with tools and figure I could probably fix it.

My concern is that head gaskets are not the kinds of things that just fail for no reason, and that this one might be the result of some problem that has done other damage to the engine or other parts of the car. So I'm wondering what the likely causes of head gasket failure might be, and what other damage those might have caused.

Bonus points for indicators of these problems that can be seen without splitting the engine first. :)

Edit: 225,000km (140,000 mi), if that makes a difference.

Answer 1

By far the most common cause of failure of the gasket is overheating.

So why would it overheat? Check for coolant leaks in the following

• engine gaskets
• water pump
• radiator
• pipes

Also check for oil leaks or low oil - these can have knock on overheating effects.

Check your radiator isn't full of sludge - that could be a cause of the problem, or it could get filled with sludge as a result of the gasket failure.

Some gaskets are just badly made or fitted, so check these as well.

Answer 2

Be aware that a diesel head gasket is not necessarily a job for an amateur. It is generally not as straight forward as a gasoline engine head gasket.

You may need special tools that many amateurs lack -- a torque wrench, for sure.

Be sure to consult a repair manual for your particular engine. Some VW diesel engines will have multiple head gaskets available, and you have to use the right one for your engine's serial number. The wrong one can result in piston damage if they impact the head. Remember, clearances on a diesel are much tighter than on a gasser!

You may have to replace the head bolts, as well. Some VW diesels use "stretch bolts," that are designed to elongate when torqued properly. These cannot be re-used, and must be replaced.

Be sure to examine the head carefully when you have it off. In fact, many shops will x-ray the head before replacing a gasket. Some VW diesel heads are prone to micro cracks that could be allowing coolant to enter the cylinder, even with a good gasket. These cracks can result from hot-running, just as a blown gasket can.

I wouldn't do this on my only car. It could be laid up for weeks as you go from problem to problem! If it's your "daily driver," pay someone who knows this engine to do it -- they will make sure they have all the possible problem parts on hand.

On the other hand, if you have something else to drive while you dink around on this, it can be a challenging learning experience!

Answer 3

I own several cars of this vintage (ALH engine) as well as a large number of previous VW diesel vehicles.

First on the smoke, one cause not listed is turbo failure. That would cause over fueling, which would cause white smoke.

If the cyl head gasket were failed, causing white smoke, the smoke would likely smell sweet, if there was glycol in the coolant. The coolant level would also be lowering, and might show some contamination.

Earlier VW IDI engines tended to blow gaskets more often, however, I know of NO ONE personally who has blown a head gasket in a ALH engine. They are much more bullet proof in that regard.

To evaluate this car, you need to better determine the cause of the white smoke. While it is true that it can be caused by a blown head gasket, that failure is uncommon on this engine, compared with others assuming normal maintenance and operation.

How fast will the engine turn? If you cannot get it close to redline, perhaps 2000 to 4000 RPM, that would point towards the turbo.

Also, if the turbo bearings have failed, and it is loosing engine oil through them, there may be pooling in the intercooler area. Ingesting that oil while zooming the engine could cause a "runaway" which could cause destruction of the engine. So pop the lower hose off the intercooler to assure that there is not allot of oil there. There will normally be some, but not more than a teaspoon or so.

Also, if the MAF is suspected, you can disconnect the MAF. If it runs about the same, then it is unlikely that a bad MAF is causing overfueling.

I would look at the effluent from the exhaust. If it has fuel in it, then your smoke is due to overfueling, and is likely a turbo which is not working. The turbo can readily be replaced, and is about $560 from places like Autozone. You will pay more if you get higher end turbos. Don't go buying any parts until you have isolated the cause.