The LML's factory water pump impeller erodes away until it's barely moving coolant — a quiet failure that ends in overheating. We replace the pump and overhaul the cooling system at your home, before it costs you head gaskets.
The 2011–2016 Duramax LML shipped with a Delphi water pump whose impeller has a documented habit of eroding — the vanes that push coolant through the engine wear away over time. It's an insidious failure because the pump doesn't seize or scream; it just moves less and less coolant while looking perfectly normal from the outside. The engine's cooling capacity quietly shrinks beneath a truck that's still towing heavy loads.
The early symptoms are easy to rationalize away: the temperature gauge sits a little higher on grades, the cab heater isn't as hot as it used to be (weak coolant flow through the heater core), and eventually coolant starts seeping from the pump's weep hole as the shaft seal joins the decline. Each sign alone seems minor. Together they're a water pump in its final act.
The end of that act is an overheat event — and on a hard-working diesel, overheating is head-gasket territory, where the repair bill multiplies by an order of magnitude. That's why this job is an overhaul rather than a part swap: new pump, new thermostats, and a system flush, so the LML's entire cooling capacity is restored at once. On a truck that tows through Ontario summers, it's some of the best engine insurance money buys.
If your GM is doing any of these, this is the likely cause:
This failure's whole danger is its quietness — the truck keeps working while its cooling margin disappears, and the day that margin hits zero is a heavy tow on a hot day on the 401. A diesel overheat event doesn't forgive: head gaskets at minimum, warped or cracked heads if it's bad. The arithmetic is stark — a cooling overhaul now, or that plus a top-end rebuild later. Weak cab heat in an Ontario winter is also its own daily argument for not waiting.
Yes — water pump, thermostats and flush are all front-of-engine work done in most of a day at your driveway. The truck never leaves home, and we handle the coolant, the bleed and the disposal.
Access. The pump hides behind the LML's accessory drive, so the book hours are real even before dealer rates multiply them, and parts at list price finish the math. We quote one flat price for the full overhaul — pump, both thermostats, flush and coolant — before starting. One number, agreed upfront.
Because they're the same age, they're a fraction of the cost while the cooling system is already open and drained, and a sticking thermostat after a pump job causes the exact overheating you just paid to prevent. The pair costs little now and a second labour bill later — easy call.
We verify before we replace — weep-hole evidence, heater output, flow behaviour and a pressure test together point at the pump or away from it. Running warm has other causes on these trucks, and the diagnosis is part of the job, not an extra. You pay to fix the actual problem.
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