A discussion of diesel head gaskets is more about the past than the present. Diesel engines from this century do not have the endemic problems that plagued the first two decades of diesel engines, but occasionally an overheat problem will cause failure. The diagnostics remain the same and the repair procedure is still quite similar.
The first 1500 and 1600 diesel engines with the 11 mm head bolts had 5 thicknesses from which to choose. All engines with 12 mm head bolts since mid 1980 have 3 different thicknesses. Obviously this means some engines end up with more compression than others. Last century, a client buying a new diesel Volkswagen was well advised to drive a number of them and choose the fastest one. The difference is still there, but far less perceptible since all modern diesels have turbochargers. Slight boost variances have more effect than the varying initial compression, and the dynamic compression under boost is the issue.
The diesel head gaskets from the pre-Tdi engines were made of cardboard with steel rings to seal in the compression / combustion pressure. Seepage is so common on those engines that we include the gas analysis test with every Pre Purchase Inspection on those cars. The head gasket can seep for years until that last big hill in hot weather that leaves one stranded. With the inception of the Tdi, the head gaskets changed to a multi-laminate metal construction that has eliminated the problem. We only see these head gaskets blown as a result of overheating.
All the VW / Audi diesel motors have always required choosing from a selection of head gasket thicknesses. In a gasoline motor, there is a dimension called “Deck Height”, which is the distance down from the top of the cylinder bore to the top of the piston with the engine at Top Dead Center or TDC. Diesel engines however have “Piston Projection”, which is the distance above the cylinder bore to the top of the piston. Simply put, the pistons stick up out of the block. That measurement varies because of differences in the pistons, rods, blocks and crankshafts. To achieve the proper compression ratio, the distance from the top of the piston to the cylinder head must be between .024″ and .028″, which is thinner than the paper of a matchbook cover. Most times it is best to use the same thickness gasket that was removed. They are marked with notches or little holes in the protruding front tab of the gasket.
Diesel head gaskets can seep compression slowly without any perceptible symptoms. A tiny trail of bubbles coming through the overflow hose to the reservoir might be seen. The cooling system eventually operating at the maximum pressure that the coolant cap allows might be noticed. One giveaway is that the cooling system might still have some pressure in it after a hard run then cooling down completely. The definitive way to check for seepage is to capture a sample of the gases from the cooling system, then test the sample with an exhaust gas analyzer. Any hydrocarbons present indicate a compression loss to the cooling system, with one exception. Some of the aftermarket additives sold as cooling system sealants emit large amounts of hydrocarbons. For a further discussion of those cooling system additives, see our webpage on Coolant System Flush.