MotoMojo – Motorcycle Cooling Conundrum

May 20, 2015 View Comments by

Fan switch, jump terminalsSolving Your Overheating Problems –

Hot weather really taxes a motorcycle’s cooling system. Problems that weren’t apparent in cooler weather will quickly show up. Running hot can manifest itself in different ways. Overheating may cause engine “pinging” and detonation (also known as spark knock) when it’s under load, even if you’re using premium fuel. This is often the only warning you get with an air-cooled engine. To avoid engine damage, ease off the throttle until the pinging sound stops, and check the motorcycle for problems as soon as you can find a safe place to do so.

With air-cooled engines, ensure the oil is full and cooling fins are clean and unobstructed. Use recommended summer-weight oil (preferably synthetic). With air/oil-cooled engines, also check the oil cooler for obstructions or damage. Other, less common overheating causes (in all engines) are lean fuel mixtures and retarded spark timing. If you don’t find a problem and are not using premium fuel, try the next higher octane to eliminate pinging (if it is occurring).

With liquid-cooled motorcycles, you may be warned of overheating by a high reading on the temperature gauge or a warning light. Don’t ignore overheating, as severe engine damage can occur and coolant may boil over, which can burn the rider or get on your rear tire and cause a crash. Overheating can warp heads, score cylinders, and seize pistons; damage can occur quickly, so heed warnings. Shut off the engine, pull over, and let the engine cool while you determine what’s wrong.

Never remove a radiator cap when the engine is hot. If coolant level in the overflow tank is low, look for a leak, which should leave drips, stains, or puddles. Sometimes you’ll get lucky and discover it’s just a loose hose clamp. More often it’s a leaky hose, which can be taped in a pinch. Check the radiator for leaks, bent fins, and clogging from bugs, mud, and debris. An internal leak, such as a head gasket, may allow coolant in the oil (very bad).

Coolant (typically 50 percent water and 50 percent antifreeze) provides corrosion and freeze protection, but it also raises the boiling point of water. Normally you should use coolant approved by the motorcycle manufacturer, but if you are stuck on the side of the road, water will do temporarily. The radiator pressure cap significantly raises the boiling point of the coolant, so it should be checked and replaced if suspect. But you can leave it loose if you are limping home with a taped-up hose.

Failed water pumps usually leak. Sometimes pump impellers disintegrate or slip on the shaft, and some bikes have belt-driven pumps; check for this. To check for pumping (with a warm engine, thermostat open), place a gloved hand around the radiator hose that carries coolant from the engine to the radiator. Rev the engine briefly. If the pump is working, the hose should bulge as coolant flow increases.
Thermostats close to allow warm-up and open to maintain operating temperature. If the engine is overheating due to a stuck-closed thermostat, the coolant hose coming from the engine to the radiator should be fairly cool. In an emergency, you can temporarily remove the thermostat’s inner guts.

If the engine gets hot while going but not at idle and low speeds, the problem may be due to a partially clogged radiator (either inside or out). Bugs and dirt can clog the external fins. Mineral deposits and other crud can form insulating deposits inside the radiator. Sometimes you can check the radiator by partly draining coolant and peeking inside through a hose fitting with a hose removed.
If the engine overheats all the time without apparent external leaks, it may have a blown cylinder head gasket or cracked head. Clues include foamy “milk shake” froth in the coolant and/or oil; a blown head gasket may also make the radiator bulge from excessive pressure. Call a tow truck.

If a bike runs hot only at idle or low speeds, the electric fan may be inoperative. To check fan operation, allow the engine to idle until fully warm. As coolant gets hotter, the fan switch should activate when coolant reaches a preset temperature, completing an electrical path that activates a fan relay. The relay then provides 12 volts to the fan. If the fan doesn’t work, inspect the fan and wiring for damage or debris, and check fuses. The thermostatic fan switch is typically located near the bottom of the radiator. To bypass the fan switch, jumper the two wires going to it together. If that doesn’t work, try running a wire directly to the fan motor from a 12-volt source.
Hopefully you’ll never need any of these procedures. Emergency repairs should not be considered a substitute for proper maintenance, and factory-approved products should be used as soon as possible after field fixes are performed.

Text and Photography: Ken Freund


Want to receive free Touring Tips, reviews, deals and contests, and additional content? Sign up for your free newsletter now!

Tags: , , , , , , , , , Categories: Technical Tips