Professional Engine Bay Detailing Guide:

Never clean a hot engine. In Australian summer, an engine that has been running can retain enough heat to flash-dry chemicals instantly, causing permanent staining on plastics and metals. The engine should be cool to the touch or only slightly warm (less than 35°C). If the car was recently driven, pop the bonnet and let it sit in the shade for at least 2 hours before commencing.

Use a vacuum or a leaf blower to remove loose organic matter, such as gum leaves, twigs, and insect remains, from the scuttle panel (base of the windscreen) and the corners of the engine bay. These areas often trap moisture and salt, leading to rust in the firewall. Pay particular attention to the battery tray where acidic buildup often occurs.

Identify and protect sensitive components. Use aluminium foil to wrap the alternator, exposed ECU plugs, aftermarket air filters (POD filters), and the alarm siren. Foil is superior to plastic wrap as it holds its shape against water pressure. Ensure the oil dipstick and filler cap are fully seated to prevent water ingress into the lubrication system.

Prepare your solutions based on the soil level. For a standard Aussie engine bay with red dust and light oil, dilute your APC at 1:10 in a spray bottle. For heavy grease around the lower block or older 4WD engines, prepare a 1:5 degreaser solution. Keep a bucket of clean water nearby to rinse your brushes frequently during the agitation phase.

Start with a gentle rinse using a wide fan spray to dampen the entire bay. This lowers the surface temperature further and loosens the top layer of dust. Avoid direct high-pressure spray on electrical connectors or sensitive sensors. Aim to move water from the back (firewall) toward the front (radiator) to ensure dirt flows out of the bay.

Apply your primary degreaser to the lower half of the engine bay first, focusing on the engine block, subframe, and hoses. These areas typically hold the heaviest grease. Allow the product to dwell for 3-5 minutes, but do not let it dry. If you notice the chemical drying due to high ambient heat, mist it lightly with water to keep it active.

Using a stiff-bristled synthetic brush, scrub the heavily soiled metallic areas. Work in small sections, roughly 30cm x 30cm. The mechanical action breaks the bond between the oil and the metal. You will see the degreaser turn brown or black as it emulsifies the grime. Rinse the brush frequently in your water bucket to avoid cross-contamination.

Spray your pH-neutral APC onto the upper plastics, including the engine cover, coolant reservoir, airbox, and battery cover. These materials are more sensitive to harsh chemicals and require a gentler touch. The APC is effective at breaking down the 'traffic film' and dust that settles on the top of the engine.

Use your soft-bristled boar's hair brush for intricate areas like bolt heads, ribbed hoses, and electrical loom coverings. Use a circular motion to create a lather. This step is what separates a professional detail from a quick wash; focus on the undersides of hoses and the gaps between components where red dust tends to settle.

Don't forget the underside of the bonnet. If there is a fabric heat liner, clean it gently with a damp microfibre and a very dilute APC—do not soak it, as it may sag. If the underside is painted metal, treat it like the exterior paintwork, using a soft brush and APC followed by a thorough rinse.

Rinse the entire bay thoroughly using a 'flood' technique (low pressure, high volume). Ensure all chemical residue is flushed from the crevices. Pay close attention to the areas around the headlights and the radiator support, where soap often hides. Continue rinsing until the water runs completely clear of suds and dirt.

This is the most critical step. Use a blower or compressed air to force water out of spark plug wells, coil packs, and electrical connectors. Water trapped in these areas can cause misfires once the engine warms up. After blowing out the crevices, use utility microfibre towels to hand-dry the remaining flat surfaces and hoses.

Once dry, inspect the bay for missed spots. Red dust often leaves a faint residue in corners that only becomes visible when dry. Use a microfibre dampened with APC to wipe away any remaining film. Check that no foil or plastic wrap remains on the components you protected earlier.

Apply a water-based dressing to all plastic and rubber components. For best results, spray the dressing onto a microfibre applicator and wipe it onto the surfaces. This prevents 'overspray' on belts or the windscreen. A quality dressing like 303 Aerospace provides a satin finish and essential UV protection to prevent the Australian sun from cracking your vacuum lines and plastic covers.

Allow the dressing to sit for 10-15 minutes, then use a clean, dry microfibre towel to lightly buff the surfaces. This 'levels' the product, removing any high spots or greasy residue and leaving a factory-perfect, non-tacky finish that won't attract new dust.

Start the engine and let it idle for 5-10 minutes with the bonnet closed. The heat generated will help evaporate any microscopic moisture remaining in the bay. Monitor the dashboard for any warning lights. If a light appears, it usually indicates moisture in a sensor; shut down and use the blower again.