by Jim Kerr

Automotive engines contain hundreds of parts that operate over wide temperature ranges and speeds. They are expected to run smoothly, be economical on fuel, produce low emissions and last for hundreds of thousands of kilometres. The technology in modern engines allows all of this to be possible, but sometimes things go wrong.

Twice in the last week, I have been asked to diagnose engine noises. The two vehicles had low-pitched intermittent knocking sounds that would change when the engine speed varied. It was also louder when the engine was warm. It didn’t take long to diagnose – I have heard this sound before and it was an expensive noise. Both engines had failed connecting rod bearing and damaged crankshafts.

The first engine, a V8 in a 1995 Dodge Ram pickup has already been repaired. It needed new engine bearings, a rebuilt crankshaft and an overhaul gasket set. The replacement parts alone totalled several hundred dollars and ten hours labour added to the repair costs. This truck had always been pampered and has less than 90,000 kilometres on it. This is low mileage for a modern vehicle, so why did it fail?

The second engine, a V6 in a 2000 Montana van still needs to be repaired, but the knock in it was even louder than the Dodge, so I expect the repairs will be even more extensive and expensive. This van has only 75,000 kilometres on it. Again, the real puzzle is why did this engine fail and how could it have been prevented?

A little further diagnostic work helped find the root cause of the problem with both engines. Although the failures were similar, the causes were not similar at all. The Dodge engine failure could be traced back to an antifreeze leak while the Montana engine was done in by driving it with a misfire. We can learn from their mistakes.

Several weeks earlier, the Dodge V8 had started loosing engine coolant. Usually coolant leaks are external and you find a puddle on the ground but this engine had an internal coolant leak. A failed intake manifold gasket had allowed coolant to seep into the engine and down into the oil pan. The only indication there was any problem was the coolant level in the radiator overflow tank kept getting lower.

The manifold gasket was replaced, the manifold surface was machined and the engine oil was changed but the damage was already done. Enough coolant had seeped into the oil pan that some had been sucked up by the oil pump and pushed through the engine. Unfortunately, coolant and engine bearings don’t mix well. The bearing surfaces smear and begin to fail. So the real cause of the bearing and crankshaft failure was a coolant leak.

How could this have been prevented? Changing engine coolant at the recommended time intervals (every two or five years) found in the owner’s manual will help maintain anti-corrosion protection in the cooling system. Monitoring engine coolant level regularly could have also prevented this failure because the leak could have been corrected before major damage occurred.

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