Should be OK - but make sure that any electronics are well protected and don't direct too much high pressure water onto the alternator. Make sure the plugs and leads are dry before you run the engine. Be careful of any sound deadening material - high pressure water can tear the outer cover off, and generally rip it to pieces.

Same applies to the 5 - even more important to make sure plug holes/leads are dry - and because they are so deep it's difficult to do. A spray with WD40 helps to get rid of moisture, and it can provide a small amount of protection to help prevent nuts and bolts rusting after a thorough clean. (Spraying with WD40 also reduces that white rash on the alloy you get from salt in the winter)

Don't overdo engine cleaning - it isn't necessary and you can do harm if you get solvents and water in the wrong places. But it does look nice - better little and often with a rag than an occasional blitz with loads of water and solvent. (1/01)

I was thinking I ought to clean out my baby's engine, I popped into Halfords, and was greeted with an enormous range of cleaning fluids - maybe 20 different bottles and sprays, most made by the same company (Redex), so not even competing against each other!

Other than ensuring it is for an injected petrol engine, which one/type should I go for?

Most seemed to be petrol additives, but one was a spray that is sprayed into the plenum chamber.

Any thoughts? What will this sort of treatment do the cat? (1/01)

I have to say that I rather think a lot of these are snake oil, but three might be useful:
STP/Wynns/etc Engine Flush - you add it to oil before an oil change, run the engine for 15 mins or so then drain the oil - gets rid of gum in the engine. Use at every oil change - Mazda dealers do (or should offer it at least) (They use a Castrol Workshop brand - at least Stratstones did)

Redex etc Petrol Injector cleaner - add to fuel - it cleans the injectors - theoretically. I use it occasionally, but can't say that I've noticed any effect.

Carburettor cleaner - various brands - can be dribbled (gently sprayed) into the inlet manifold with the engine idling (eg via the cruise control vacuum take off - the blocked up connector near the throttle body on top) This cleans the internals of the inlet tract - again, not noticed any real benefit.

None of these should harm the cat as long as you don't overdo it.

One of the last resorts for curing uneven idle and idle droop is to clean out the Idle speed Control valve (under the throttle body), but you need to dismantle it to be effective. Carburettor cleaner will get it nice and clean.

If you remove the air intake pipe from the throttle body, don't clean off the sealing ring of gunge around the throttle butterfly - it's intentionally there. Don't spray that area with carb cleaner.

Another technique that can be beneficial is to bleed water into the inlet manifold (through that same take off) - it will clean carbon out of the combustion chambers (ever seen a combustion chamber when a head gasket has allowed coolant in? At least it's nice and clean!). But you're probably better off just making sure you drive it hard regularly. Anyway, again, don't overdo it.

All MHO, and at your own risk! (1/01)

Try using paint brush cleaner - about a £1 a bottle at the discount decorating stores. Scrub it into the oily muck with an old paint brush and wash off with warm water. Absolutely superb at cleaning off the dirt and doesn't smell like some of the other propriety cleaners. Don't use paint brush restorer though this will soften plastics. (1/01)