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Outback overheating issue comes to a head

A new customer recently brought his 1999 Subaru Outback to the shop. The man said the car was overheating and making noise whenever he drove it.

Because most of the time I write about complex diagnostic scenarios, I’m sure that many of you think this is going to be a column about a really tough cooling-system problem.

But that’s not the case; the cooling-system problem was the typical Subaru 2.5-liter head-gasket failure. I am bringing up this problem to discuss the anatomy of the failure.


I first must explain the different types of motor configurations. There are four configurations of internal combustion engines: inline, V, boxster (flat) and rotary.

Inline engines are those with cylinders that are literally “in line” – lined up back-to-back. The most common inline motors are 4, 5 and 6 cylinders.

The “V” used to describe a V6, V8, V10, V12 and V16 does not actually stand for a word, but instead the shape of the piston arrangement.

The boxster motor has horizontally opposed pistons. Boxster motors come in 4 and 6 cylinders.

Rotary engines use a triangular-shaped rotor that spins, unlike the typical piston engines. There are 2- and 3-rotor engines.

All four designs have their benefits and drawbacks.


Because the Subaru has the boxster motor, I will focus on it.

All Subaru motors are boxster style. The pistons on a boxster motor lower engine vibration to almost nothing because of the counterpunch dynamic. The lower engine height also lowers the overall center of gravity of the vehicle, which improves handling. In addition, the lower configuration enables the engine to fit into a smaller area.

Along with Subaru, cars made by Porsche and early air-cooled Volkswagens feature the boxster design.


So let’s get back to the Subaru’s head-gasket problem. The most common problem with a boxster motor is engine oil leaks. Because the pistons are horizontally opposed, the cylinder heads are on the outside of the engine. When you turn off a boxster motor, the hot engine oil takes longer to return to the oil pan.

Additionally, the hot oil may come to rest on top of the head gaskets and valve-cover gaskets. Over time, the hot oil literally cooks the gaskets.

With the inline, V and rotor designs, the pistons or rotors sit directly above the oil pan. When a non-boxster motor is turned off, the hot oil returns to the oil pan much quicker and does not have an opportunity to stay behind and cause problems with the gaskets.


It’s extremely difficult to know when a Subaru head gasket is failing. That’s because it leaks or burn coolants so slowly that the customer does not know that there is a problem until his or her radiator is low on coolant.

The head gasket has a thick steel core with two coated outer thin metal gaskets. These head gaskets have a tight tolerance, so they leak slowly. The Subaru head gasket can seep oil for extended periods of time before it becomes a problem. But once the gasket coating starts to disintegrate coolant, oil will begin to leak. And once the coolant begins to leak, the head gasket must be repaired.

The best way for Subaru owners to avoid this is to follow the car’s maintenance schedule closely. It is also important to use Subaru or factory-quality engine oil and coolant. And don’t forget to check the engine oil and coolant level at least once a month.

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