Tuesday, April 27, 2010

Aluminum Engine and Temperature Gauges

There’s a monkey under the hood
March 24, 2010

MW sent along this hot story.

My wife fried the V6 in my beloved Mazda and was lectured at length for destroying the engine by continuing to drive with higher than normal temp showing on the gauge. Warped heads, partially melted block and pistons, the whole nine yards.

A week after the motor was replaced. I was stopped in a traffic jam on a hot Florida day with the motor and AC running. Bored, I was staring at the temperature gauge and wondering how my otherwise intelligent wife could fail to glance occasionally at it while driving. Then, as I watched, the temperature started rising and quickly pegged. Engine off, call AAA, back to Mazda. Diagnosis was low coolant level due to a leaking overflow tank. Easy enough to fix, but why did the temperature rise so quickly that I could literally see the needle move? If I hadn’t been staring at the gauge I’m sure the second engine would have fried inside of a minute and joined the first one in the aluminum smelter.

After some time it finally occurred to me that this was my first aluminum engine and I was accustomed to the low coolant level behavior of cast iron blocks, not aluminum blocks. Evidently, the aluminum block gets rid of heat so efficiently that as the coolant level fell the block temperature rise required to compensate for the drop in radiator level and decrease in radiator heat transfer area was much less than for a cast iron engine. The temperature had risen, but nowhere near the rise in a cast iron block, and not enough to be alarming, or even noticeable.

The temperature skyrocketed when the water pump impeller was finally uncovered and coolant flow stopped altogether. Without pressurized coolant being forced into the passages around the cylinders it flashed into steam and prevented liquid coolant from getting to the cylinder passages. The skyrocketing temperature resulted from steam hitting the sender.

The monkey design part is that the Mazda ECU designers didn’t put in a couple of lines of code to set off the dashboard audible alarm or shut down the engine on high temp. The marine outboard motor designers figured this out a long time ago. Nice car, but it’s my last Mazda. And apologies to the wife.

From our friends at Design News: