Article reproduced by kind permission of Mike Fishwick

I am always amazed by the number of BMW cars on which the body work is regularly treated to expensive polishes, yet the engine bay and underside is totally neglected. Cleaning this area is not difficult, and even on a neglected car it can be recovered with regular attention. A morning’s work every week for a month will make a dramatic change. Do not worry about getting water on the engine and its ancilliaries – just use an open hose pipe and common sense.

Engines do not have to be dirty


Do not be afraid to remove items such as spark plug covers, spare wheel, and brake pads/calipers if this will make the task easier, and in extreme cases of neglect even the alternator belt and radiator. Look for any electrical connectors where the cable entry grommet has become damaged and open to the elements – wrap these with PVC tape, and temporarily cover with aluminium foil.

Make sure that there are no oil leaks, which will spoil the cosmetic effect, even though a decent oil leak will preserve the original metal!

Grease Removal:

The first thing is to remove accumulated grease and dirt, the easiest (and cheapest) solvent being white spirit. This can be scrubbed into all the usual nooks and crannies with a small paintbrush, starting from the top and working down.

Once the grease is dissolved, work the engine over with a good emulsifying cleaner such as Jizer etc. which can then be washed away. The advantage of starting off with white spirit is one of cost – these cleaners cost as much as lubricating oil!

I find that Comma Hyperclean is the best such cleaner, and suggest you avoid the use of Gunk, unless you really want your engine to smell like a public loo!

Don’t forget the details!

Corrosion Removal:

Now that the engine can be seen without its usual coating of grease, you will see that many areas of the aluminium cylinder head etc. are coated with white alkaline corrosion, usually caused by that curse of the UK, road salt. This can be removed by use of an acidic cleaner, such as Deb Altrans, manufactured by the makers of Swarfega etc, and available from most motor factors and industrial suppliers.

I usually dilute Altrans with water in a 50:50 mix, and brush it over the aluminium, leaving it for about thirty minutes before re-treating. Do this in the shade, and do not let it dry off before rinsing off with lots of cold water. As aluminium is by nature porous, some of the acidic cleaner will have been caught in the surface of the metal, and should be neutralised with an alkaline solution. For this purpose I use BMW wheel cleaner, which is an excellent for cleaning any complex shape – it’s good on wheels, too!

On engines with iron blocks, neat Altrans makes a good rust inhibitor, turning the rust into a stable layer of black iron phosphate.

Metal Cleaning:

Warm the engine up to dry it off, and after cooling down begin to rub all aluminium areas such as the cylinder head and cam cover with coarse industrial steel wool, which will soon provide a lustrous finish which, by blunting the ‘peaks’ of the casting, is rather more corrosion-resistant than the original casting.

Some areas, such as the engine mounts and sump, are always difficult to reach, my preference being to raise the car with a pair of trolley jacks before attacking them from beneath.

Painted Parts:

There are various pressed steel brackets around the engine bay which may have begun to rust. The only real answer is to remove them, de-rust (immerse in Altrans, or better still, a stronger phosphoric acid made for the purpose) prime with Finnegans Red Oxide Primer, and paint with Finnegans Satin Black Smootherite, ideally spraying from an aerosol, which gives an excellent finish. First heat the aerosol in warm water, insulate with a few layers of kitchen towelling, and wipe the nozzle after every pass.

Plated Parts:

You will doubtless find a lot of cadmium plated nuts and bolts which are badly corroded by salt water. The bonnet catches are a particular problem area, being in the path of any salty slush being thrown at the front of the car.

Depending on their location these can either be painted silver, using a small artist’s brush and silver Smootherite, or replaced, ideally with stainless steel items. I am currently trying to source stainless steel sleeve nuts for the cam covers.

Plastic Parts:

There are plenty black plastic parts under the bonnet, and – assuming they are clean – they will respond to being treated with either WD-40 (which has a rather short life in under-bonnet temperatures) or a vinyl cleaner. I use ‘Mir’ Exterior Vinyl Cleaner, finding that it lasts very well.


Exposed cables and (on the four-cylinder engines) plug leads respond well to being periodically wiped with WD-40.

Throttle Bodies:

On cars where the throttle body can be easily reached, steel wool will produce a really brilliant finish. Do not forget to periodically oil the exposed end of the throttle spindle. If you are seeking the original finish, then Altrans will come very close, but do not let it creep into the throttle spindle bearings.


Once you have achieved a good finish on your engine you will want to preserve it, the easiest way being to regularly wash the engine as part of a weekly routine, using a sponge and car shampoo, followed by a quick rinse. When the engine has cooled down after the next run, lightly spray the exposed metal areas with WD-40, and wipe over with a clean rag.

Certain areas, such as the front of the sump, receive the worst of winter weather, and rather than clean it weekly, you may prefer to coat it with a better protective. Again, Finnegans will come to your aid with their Waxoyl preservative, which is easily removed with white spirit and Jizer when the spring arrives.

Don’t forget the other hidden parts!


Don’t pass up any opportunity to clean your engine and the underside should it be necessary to remove anything which has been in the way of cleaning, such as when replacing a thermostat, or alternator belt.

While cleaning, think about what you are looking at, and periodically check the tightness of all hose clips and electrical connectors.

Look for oil leaks from the engine, transmission and dampers, and keep an eye on the condition of the alternator and air conditioning compressor drive belts and all hoses – particularly those of the brakes.

Cleaning also provides you with an ideal opportunity to check the levels of brake fluid, coolant, and engine oil, and power steering fluid.

Also check the engine-driven fan for free play in its viscous drive coupling, as even the smallest amount of play (in the front-to-back plane) heralds the end of its life, as does any sign of the black silicone fluid leaking from its centre.

Neglect of such basics in the under-bonnet and underside areas is the surest way to suffer its consequences, while a little regular observation will save you a lot of trouble.