XJS & XJ from KWE Cars
Classic Spirit Reborn

Coolant Sealants

You have a slow coolant leak – a few drips on the drive, occasional overheating, heater running cold. Solution – add a sealant such as Radweld, K-Seal or Barr’s leaks. WRONG! These materials will more or less permanently damage the engine, radiator and heater matrix.

We have had several cases where well known sealants from Halfords and others have been added by the car’s owner and subsequently the heater runs cold. One particular car needed flushing 3 times, but still in the end suffered localised overheating and blew a head gasket. A stripped engine and nearly £10,000 later we got the last of the stuff out. On one of our personal cars we made this mistake some years ago and to this day the material (in this case K-Seal) is still popping up and causing problems.


V12 cylinders (head removed). Note gungy stuff around studs and liners. This area should be pristine

What happens is this: These sealants are in liquid form within the coolant and harmless. However, as soon as it encounters air – usually at a leak – it solidifies and seals the leak. What it’s supposed to do. The problem is that in old, unsealed engine cooling systems there is often air trapped inside and so the sealant solidifies in the engine. Usually the first thing to get clogged is the heater matrix which has quite narrow channels, then the cooling radiator and then the engine cooling galleries. This latter is very dangerous because the solidified clumps of sealant prevent various nooks and crannies inside the engine from being cooled by the coolant, so they overheat and distort.

Once solidified the material cannot be washed away or, as far as we know, dissolved in anything. It becomes a clay-like pudding which can only be scooped out by hand having removed the cylinder head(s). Physical force with a water hose can dislodge the heater matrix clogging, but if there’s any of the material left in the system it will simply clog up again.

To make it more difficult, especially in the V12, simply flushing the engine’s cooling system via the radiator hoses will have very little effect. This is because the bottom of the engine block collects all the rubbish, but the top end flush just skims over this area without really disturbing it. In one case we left it flushing continuously for over an hour, and having connected all the hoses again the system clogged almost immediately.


We have now worked out a way to reverse-flush out of the block, but it means removing both thermostats on the V12 and blocking certain galleries – not a simple job.

Here is the first evidence you may find of sealants having been used: SAM_4652A heavy coating of brown gunge around the coolant filler caps. If you’re about to buy such a car – don’t!


Clear filter inserted in heater feed hose on a V12


Filter full of clogging stuff


Large particles of solidified sealant – after the system had been carefully flushed

We carried out some experiments on afflicted car. We flushed the engine in the conventional manners and then fitted a see-through coolant filter. Within a few moments of running the flushed engine the filter was clogged with old sealant.

So, what do you do if you have a cooling leak? Replace the leaking part – usually a hose. In an emergency one can bind up a hose dribble, and can ignore a minor leak from the radiator provided you keep topping the coolant up when the low coolant light comes on or there are signs of overheating.

With a serious leak, no amount of sealant is going to fix it, so we’re only talking about minor, persistent leaks – and there is only one right way of dealing with it. Replace the offending part.





Ironically, in early Jaguar manuals the owner is advised to fit Barr’s Leaks additive when changing the coolant. This is wrong advice!

2:57 pm KWE Cars