I have a 2011 M37X, and started picking up coolant smells when the AC is on fresh air.

Popped the hood, and saw evidence of dried coolant spray, but have never had a puddle under the car. It seems like the leak only occurs during engine load, as I can't see any leak when idling.

I saw EdBwoy's post / video about the coolant plug gasket leak - looks like the same area, but curious if any M37 owners have seen leaks in other places before I dig into it.

Top down view above driver front (zoomed out):


Top down view above driver front (closer in):


Side-on view (camera inside passenger front engine bay, pointing slightly up and towards driver side):


(Sadly six years of Canadian / Northern U.S. road salt were not kind to the engine bay)

Any tips/experiences welcome before I dig into it. Thanks!

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I haven't noticed any coolent smell coming out of my AC, but I did take the car into the shop a few weeks ago due to high temps and risk of overheating. It turned out to be a simple case of a low coolent level, but they did notice evidence of a small leak that might have went towards causing the problem. It's reassuring to see someone else dealing with this issue since we're still not quite sure where the leak is originating. Please keep us informed though of what you find. It would really help me and I'm sure others narrow down where the leak might be coming from.

You can rent a pressure tester at most auto parts stores (or buy one off ebay or Amazon if you're a DIY guy).

Pressurize the cooling system and look for leaks (you can also get a dye kit with a UV light).

Don't mess around with that, it's clearly not holding pressure... probably a tiny leak, but can create big problems.

AZ is spot on. Your car is designed to run at a particular temp range and coolant level. If you drive your car with less coolant than required, or at higher temps than normal, you risk a domino effect including water pump, engine gaskets, heat damage to internal parts, etc. Let's hope its a simple thing like a hose or clamp that allows leaks under pressure. Best of luck.

Thanks for the great feedback. Have located a parts store nearby that rents pressure testers and will update the thread on what I find.

The hose to the Throttle Body on my M56 V8 car had a slight leak causing a coolant smell from the engine after being hot. Was not nearly as apparent as yours is in your pics, but I couldn't see mine without pulling the engine cover.

A follow-up - took off the air guide/radiator cover and driver side airbox, and at least one problem is immediately clear, there's a leak exactly where EdBwoy [1] had his problems:




Going to order the parts to fix this and then pressure test to confirm that there are no more gremlins.

[1] https://www.youtube.com/watch?v=emcmgTSsa5E

This now appears to be fixed - the upper plug was the only apparent problem. Replaced the rubber washer on the upper radiator plug and tightened it quite tight. The old washer did not appear to be in too bad of a shape. Part of me wonders if the plug is simply loosening over time, but for less than 5 bucks I'll go the safe route.

Pressure tested the system and it held at 22-23 psi for 15+ minutes (I was surprised to see the relatively high value of 23 psi as the recommended testing pressure in the service manual). The radiator cap also tested good.

Interestingly, when I pulled the driver's side airbox to access the plug, I noticed that the entire underneath of the airbox was covered in blue dried coolant where it had sprayed out from the upper plug against the front of the airbox and dripped down. Undoubtedly this contributed to the recurring smell. Gave everything a good bath so that the next time I smell coolant, I'll know it's new.

Thanks all.