Radiators get dirty. Internally it gets blocked with a mixture of sediment and salts from the coollant, and the detritus of corroded aluminium. Externally road dirt and engine bay grime clog up the holes.
Even the best kept Imp will have a dirty matrix. The reason being that the Imp fan is really a giant Hoover ! As you drive along, it sucks in all sorts of debris. The situation is made worse if you have bad oil leaks, so make an attempt to cure the worst of these. Otherwise, you will have an oily gunge being spread all over the radiator matrix.
Flush the entire system with descaler. [scale: chalky deposit from the lime in hard water] This might also show up a leak or two. Holes in a radiator aren't easy to weld... Best start saving for a new radiator.
Purchase a tin of degreaser, a large tin of antifreeze or a tin of coolant Inhibitor or better still: both.
Jack the Imp up and place it securely on axle stands or ramps. Drain the radiator and keep the coolant. Undo the radiator top hose at the thermostat housing end and the bottom hose at the water pump. Undo the three bolts holding the complete radiator assembly in place and carefully remove it.
Once the radiator is away from the car, remove the front metal cowling. Get your degreaser and work it into the radiator with a stiff brush. Note the circular pattern of dirt on the matrix, with a curious collection in each corner !
Now get the garden hose and vigorously blast the matrix until you can once more see daylight through it. An oil slick should now be travelling down your previously clean drive. While you have the hose available, stick it in the radiator bottom stub and reverse flush it. You'll be surprised at what comes out. It will pay you to check the condition of the tubes inside the radiator. If they appear to look like the inside of a furred up kettle, then replacement is the only effective answer.
Now bolt the radiator back in, tighten all the hoses, ensure that the rubber gaiter is secure and not trapped, and then top up the coolant. Now you have to bleed the system. Connect about 18" of transparent tubing between the tree way junction, and the header tank. Run the engine at a fast idle and give an occasional blip to clear any stubborn air bubbles. You may have to let the engine cool down and repeat this operation. Sqeezing and releasing the hose from he 3-way junction to the head may help push airlocks out. Check all connections (particularly the thermostat housing) for leaks - This is very important !
Now you can put the car back onto its wheels and go for a blast. You should be able to marvel at the reduced running temperature.
There is a moral concerning the Imp Cooling System, though: if it's working wel, leave it alone !
by Martyn Jones. - Impressions June 1986
I've noticed that 'Biotex Blue' will clean even the dirtiest bits effortlessly. Much better than any other soap / degreaser I've tried.
What you don't want to do is remove the paint from the radiator, as black works better than a nice copper shine.
Be careful of how you clean the fan. Don't roughen the surface or it will be less efficient.