740 Heater blocked

Best (and possibly only) way to clear it if it's indeed blocked is to replace it. It also could have jammed up the heater control valve. Stop-leak additives usually cause more harm than good, they can mess up the whole cooling system, better to fix the leak correctly.

As James Sweet says, the best way is to remove and replace the heater matrix. Sadly, that is a huge job.

You may have luck with disconecting the heater hoses (both of them) and duct taping a garden hose to the outlet pipe (the upper one, I think) of the heater matrix. Open the tap a bit and try to flush the heater backward that way. If dirty water starts coming out you can increase the flow - ultimately you should be able to get water through it like you'd get out of a kitchen tap. If it resists, try it on the other pipe - and keep pushing back and forth until you give up or you get a good flow of clear water.

If the heater still doesn't work, don't forget there is a heater control valve near the matrix in the lower hose. Those valves get grungy and jam.

Mike

You were lucky! I had to replace the entire unit. WHAT a job! The steering wheel and pedals were left. But I managed to fix dodgy wiring, get all dash lights to work, and a few squeaks as well. But never again I hope! Paul

Reverse flush using a hose or low setting on a pressure washer with pulse jet nozzle. You may need to remove the matrix to make sure you don't need to swim in your seat.

Cheers, Peter.