Truck radiator leaking coolant fluid

Truck radiator leaking coolant fluid

It’s not very surprising to find a leaking radiator in old ’70 / ‘80’s trucks. Older radiators have soldered joints making them vulnerable to small coolant leaks over time. As it’s the case for our 1719AK truck from ’74. Here’s the story to get a leaking radiator fixed.

Leaking truck radiator

About 2500 Km in our journey, the first signs of a leaking radiator showed. A sweaty corner on the bottom of the truck radiator core. The metal had a different darker corner as if it was splashed with water – with dry weather for weeks.

Mercedes Benz Truck leaking radiator OM401 with 30 liter of coolant fluid
Mercedes Benz Truck tropical radiator with 30 liters of coolant fluid - OM401 engine

The good part about the old truck radiators is - they can be fixed by resoldering joints or fixing / replacing the radiator core. This in contrary to a newer aluminum / alloy radiators with a plastic head and bottom – usually requiring full replacement.

Mercedes Benz Truck leaking radiator corner
Mercedes Benz Truck radiator - leaking core - OM401 engine

Weeks later the first drops of coolant fluid appeared in the morning with a cold engine. A temporary radiator fix sealed the leak – more about that further down. Small leaks in the radiator show when the engine has cooled down. Bigger leaks show also during driving, sometimes spraying the engine with a fine mist on the rear side of the radiator.

In our case the coolant leak had now grown to about 2 liters a week, time to fix the radiator before it gets worse. During driving the leak was hardly visible or noticeable in a higher engine temperature.

Leaking radiator fix

For small coolant fluid leaks in a radiator upto approx 0.8 mm in size, there are coolant fluid additives. In the old days pepper grains were added to the coolant fluid to plug small leaks.

Radiator sealant on US AMAZON with product reviews

In our case, Pattex Nural 70 (only marketed in Spain) sealed the radiator hole after 1 day of driving. And it stayed like this for about 4000 Km. Even a leak of 2 liters / week is not an immediate showstopper and a repair can be postponed for quite some time. 


Pattex Nural 70 is a mix of small pellets which should be diluted in water before adding it to the cooling fluid. The mix results in a thin white solution which leaks through the radiator hole causing a build up, closing the tiny channel of the leak. The pallets should not be thrown in undiluted, risking a blockage of narrow engine cooling channels.

Popular brands are K-Seal, Bars Leak and Sonax - a must have for those on long journeys in regions without support infrastructure.

Repairing a leaking radiator

The older radiators with soldered joints can be repaired by resoldering the leaking areas or closing tubes in the radiator core .

Mercedes Benz Truck radiator resoldering the holding frame
Mercedes Benz Truck radiator - removal of the frame

The mounting frame is also soldered – and be easily removed by heating the corners.

Mercedes Benz Truck radiator leak
Mercedes Benz Truck radiator - with small leak in the corner

The leak showed in the sweaty corner, a tiny leak easily closed by the sealing additive.

Mercedes Benz Truck radiator fixing a leak by rsoldering a joint
Mercedes Benz Truck radiator - re-soldering a joint

It’s a dirty job, the lead fumes are toxic requiring suction ventilation for those who repair radiators on a daily basis.

A day in the workshop is usually enough to get the radiator out and in again including resoldering. 

Truck radiator replacement

Old style radiators are usually refurbished by replacing the heater core. The top and bottom half of the radiator is re-used.

New cooling channels are soldered into the top and bottom half resulting in a brand new radiator. A job like this is done in 4-6 hours of work including cleaning.

For popular brands refurbished radiators can be on stock also, trading the leaking radiator in.

Have a look at the 1974 Mercedes Benz Expedition truck