I have a 2016 GX 460 (125K miles) that is using coolant fluid. I can not find the leak so I'm wondering if it's a valley plate leak. If it is and I have to get it fixed what else should I have done to the engine while it's in the shop. What to get another 75K miles out if it.

 

Common GX coolant leaks:

Radiator
Valley plate
Water pump

Less common is a head gasket failure.

If it is the valley plate leaking, check for the common GX oil leak at the front cover. If it is bad, there is incentive to do the valley plate and front cover at the same time.

 

While (as mentioned) less common - we have been seeing a few more head gasket issues lately (me being one ). Test your coolant for combustion gases to confirm that. Mine was odd that I had an over-pressurized coolant overflow (combustion getting into coolant), but it would hold positive pressure once it cooled down - so it was a "1-way leak" from Combustion chamber into coolant - but coolant would not leak back into combustion until a few thousand miles later. CEL came on for P0304 misfire, could clear for a few hundred miles and it gradually got worse, and a new head gasket was needed (also getting a new water pump for good measure). Replacing coolant well-before the 10-year/100k Mi interval is apparently recommended for head gasket longevity.

Also had the radiator done earlier last year - that one is pretty obvious and generally leaks at the upper hose connection area. Also had valley plate done last year under warranty - symptoms of loosing coolant w/o any visible signs of a leak. I did have a light coolant smell for that and light staining in the garage (rear of engine), and luckily had that done while my transmission was being replaced under warranty.

Good luck whatever it turns out to be!

 

If you can't find traces of coolant in common leak areas like water pump and radiator (cracked radiator or hose), then a slow leak is likely to be a valley plate seal. Do a search on this forum for how to find evidence of this leak, but you will likely need a borascope to confirm. If you are getting the valley plate re-sealed, then also have them replace the PCV valve and even the spark plugs if you haven't done those recently as they are easier to access with the intake cover removed.

 

Put some UV dye into the coolant in the radiator then try to find traces of coolant via a small UV light.