Why would a car have a burning smell, but only when going uphill?

Question

My girlfriend has a 2008 Mazda 3 which has developed a strange burning smell. It only does this when going up steep hills (8% grade or so) for more than a mile or two. The smell seems to be the strongest in the rear of the engine block near the oil cap.

She has taken it to a mechanic, who said that it smells a bit like burning antifreeze. However, they did a pressure test on the cooling system, and there was no loss of pressure. They also didn't find anything else abnormal during their inspection, so they failed to diagnose the problem. Her temperature gauge is always perfect. Her transmission is behaving nominally; shifting happens at expected times, and the RPMs appear healthy.

This car has had transmission problems in the past: It was repaired in June 2020, and replaced entirely in September 2021. The transmission is still under warranty until September, so if that is the issue, she'd like to diagnose it quickly.

Thanks in advance for any ideas!

Answer 1

Since the mechanic performed a pressure test on the coolant system that should rule out the cooling system leaking.

The transmission is under warranty for a limited period of time so you may want to start there to rule it out as a possible source of the burnt smell. It sounds like your transmission is working well, but it could still be leaking transmission oil onto hot parts of the vehicle causing the burnt smell. Did the mechanic check for leaks around the transmission?

Oil leaking onto the exhaust manifold can cause a burnt smell, although not a burnt coolant smell. The leak often occurs around the valve cover gasket. It is common when going up hill because you have to rev the engine more.

I suspect you have a leaking valve cover gasket. They are cheap and easy to replace on a Mazda 3.