Oldsmobile Diesel V6 Engine

Oldsmobile Diesel problems

Head bolts

GM used "torque to yield", commonly known as "stretch" or "angle torque", head bolts. This allowed the bolt pattern to remain the same as the gas powered counterpart with an increase in clamping load when compared to standard head bolts. A total of 10 bolts per head were used. Four per cylinder with the center three pairs shared. This permitted the use of the same tooling and reduced setup costs. This design did not provide adequate clamping force under the severe conditions these engines were subjected to. Overheating or excessive cylinder pressure can breach the seal of the head gasket and in severe cases break the bolts.

High strength aftermarket head bolt sets are now available to make the engine more reliable in this area.

Pump timing

The Stanadyne injection pump was chain-driven. With normal use, the chain stretches and the pump delivers fuel too late. The pump timing could be adjusted to return to normal operation.

Water in fuel

Arguably a major portion of the real problem would have been quite simple to avoid. A water separator was not part of the fuel system on these engines. Water-contaminated diesel fuel was quite common.

Water will rust the steel internals of the fuel system. Rust will damage the precision parts in diesel fuel injection pumps and high pressure diesel injectors causing erratic operation. Injecting fuel too much prior to TDC on the compression stroke will cause pressure conditions similar to pre-ignition/detonation in gasoline engines. Water in varying amounts will also be injected with the fuel charge. Any rust in the steel fuel lines, fuel filter, pump etc can damage replacement parts and continue to cause injection cycles out of time.

Consumer-created fuel problems

Water in gasoline is dealt with by adding anhydrous alcohol drygas to the fuel. Drygas is incompatible with diesel fuel. Unwitting consumers used drygas in their diesels to combat the water. The alcohol attacked the original governor rings inside the early Stanadyne DB injection pumps and dislodged chunks that blocked the fuel return line flow. These pumps include an ignition advance / retard mechanism based on pump housing fuel pressure. The restriction in the return line caused abnormal ignition timing. The troublesome governor flex ring was replaced by a much improved governor cage version known as the EID assembly(elastomer insert drive) and was introduced by Stanadyne in the mid 1980's to avoid this problem.

The above mix of conditions originating with water in the fuel can combine to create extreme cylinder pressures far exceeding those foreseen by GM engineers damaging the head gasket and sometimes breaking head bolts.

A head gasket leak effectively quenches ignition in the affected cylinder. This allows unburnt fuel and coolant to leak into the crankcase thinning the lubricating oil. It also combines with combustion byproducts to make mild acids that will attack the copper / babbitt bearings and aluminum pistons. A head gasket failure can be particularly damaging in a diesel. A diesel engine has effectively zero piston to head clearance at TDC on the compression stroke. The introduction of coolant into the cylinders can cause hydrolock. Hydrolock typically results in bent / broken crankshafts, pulled threads on main bolts, and bent connecting rods, effectively destroying an engine.

Because the various failures these engines encountered were causally interrelated, and dealership technicians were unfamiliar at best with passenger car diesel engines, recurrent failures were possible because only the most obvious symptoms of trouble were addressed. The "one use only" head bolts were commonly re-used and symptoms in other interrelated systems ignored. Thus, cars could suffer multiple head gasket / head bolt failures from re-use of head bolts or a damaged injection system.

The Oldsmobile Diesel V6 engines, although sharing much of the same production history, were produced on different tooling, where it was feasible to upgrade the head bolt pattern to what is arguably a superior design capable of withstanding more consumer abuse.

LT6

The LT6 was a very rare diesel engine produced from 1982 to 1984.

Applications:

19821984 Oldsmobile Cutlass Supreme, Supreme Brougham, Calais

19821984 Buick Regal

LT7

The LT7 was a transversely-mounted diesel engine produced from 1982 to 1985.

Applications:

19821985 Oldsmobile Cutlass Ciera and Supreme

19821985 Oldsmobile 98 Regency

1985 (Listed in options 19821985) Chevrolet Celebrity

19821985 Buick Century

LS2

The LS2 was a front wheel drive version produced in 1985 and 1986.

Applications:

1985 Oldsmobile 98