Engine Bay Cleaning – How To Auto Detail

Engine bays can get grimy and oily, but cleaning them requires some extra care. A clean engine bay can help if you’re trying to track down leaks since they’ll be more apparent against a clean surface, and it’s a fantastic way to win points at a car show if you’re popping the hood.

Take Caution in Engine Bays

A preface and warning from Jeyy:

The more modern the car, the safer it usually is. I say usually, because all cars are different, even from the same manufacturer. Has it been modified? Is it in good mechanical order? Is there a loose connection? It’s impossible for me, sat here at my keyboard to know, but generally speaking, car engines are absolutely fine getting wet. Some offroad cars are even designed to function when entirely submerged. The electronics inside an engine bay are typically housed inside plastic boxes which may or may not be fully weather sealed. Electronic connections on modern cars are typically fitted with o-rings to prevent water ingress.

As I said, all cars are different and you should do your own checks and research to ensure that you are not running the risk of damaging someone’s pride and joy. And for the love of god, make sure you have the owner’s permission. Some might be quite irate to find out you took a pressure washer to their engine bay – whether they’re right to be worried or not is not the point. Especially with older cars, there are vehicles out there which should not get this treatment at all.

All of this to say: I accept no responsibility for anything you do to your own vehicle, or someone else’s vehicle. Like with all aspects of detailing, you assume all responsibility for your actions. If you are unsure of anything regarding the engine bay, you may wish to consult a mechanic first.

My method isn’t necessarily the safest, but below is what I do to clean engine bays. It works fine for me, and I’ve never had any issues afterwards. I’ve also included some details on other steps you could take if you wanted to. Make sure you are happy with the entire process you’re going to take, before taking any steps at all. Sometimes it’s best, not to aim for perfection, but to simply aim for better.

The Long Version

Tools for Engine Bay Detailing

All-Purpose Cleaner (APC). Consult the instructions for suggested dilutions. For spot cleaning particularly bad areas, either use a higher concentration or decant a small amount neat in a suitable container.
Various brushes. A hand brush for large areas, a wheel brush (bottle brush type), detailing brushes (cheap boar hair brushes, don’t bother with nice ones).
Water source. A regular garden hose attachment will do fine, pressure washer is what I use. If you can adjust the pressure on your PW, turn it right down. If not, use the widest nozzle you can. If you don’t have access to running water, you could change the methodology to use product-soaked cloths and water in a sprayer etc.
A suitable dressing.
Steam cleaner. Preferably with a plastic brush attachment and a “jet” tool. This isn’t a requirement to use a steam cleaner, but it can make life easier on grimy hard to reach areas. You should take care using a steam cleaner when detailing, don’t point it at anything that can’t take superheated, pressurized steam, and do not underestimate how easy it is to burn yourself.
Rags. You could use old microfiber (MF) towels if you have ones you’d be OK with relegating to engines only, but be prepared to bin them if they’re too far gone afterwards. This is the perfect job to use those towels that you used for ceramic coating removal. I prefer to buy a “bag of rags” – these are available at all motor parts stores here, and you get a couple of kilos of cotton rags for only a few £. These are perfect as I don’t care about binning them after a single use, they’re dirt cheap and better for the environment (they’re almost always offcuts from cotton t-shirt manufacturing). They can also be used in areas where you’d like to prevent water ingress, like air intakes, dipsticks, filler caps.

Additional Measures

Grease. You will likely want to re-apply grease to any areas that require it, as you may end up leaving them without sufficient lubrication to work properly and/or prevent corrosion. Ensure you use the correct type for the application.
Polythene dust sheeting. I use this if there are any sensitive areas I’d like to cover. Really, any thin plastic will do.
Pet dryer/car blower/leaf blower. I don’t own a blower yet, but I’ll be buying a pet dryer, which will be used during my future engine bay cleans.
Finally, PPE. Chemical resistant gloves (this depends on the product you’re using and can also depend on allergies you may have, so do your own research on what you should use). Goggles (if you don’t have goggles, specs will mostly do fine). If you’re working indoors, or with poor ventilation, you may require a respirator, depending on the products you’re using; again, do your own research. Clothing: wear a long-sleeved top that you don’t mind getting degreaser-soaked dirt flicked on, alternatively use a painter’s suit. Depending on any powered equipment you’re using, ear protection may be required.

Stage 1: Inspection and preparation

Engine bay cleaning is best done before you clean the car. It’s almost inevitable that you’ll fling dirt and chemicals onto the paintwork/windscreen.
It’s a good idea to check that there aren’t any pre-existing warning lights on the dashboard before you open the bonnet.
The engine must be cool. If you work on a hot engine, you risk major damage when cold water hits a hot, cast, metal part such as an exhaust manifold.
Now’s the time to take any before pictures.
Remove any loose dirt, such as leaves, twigs, soil, moss, mouse nests (PPE essential for rodents!)…
Inspect the engine bay for any potential risks, such as damaged connections, cables, hoses etc. Also familiarise yourself with the layout and keep in mind where potentially sensitive parts, such as the ECU, fuse box, sensors, alternator etc. are located. Bear in mind that petrol engines will have ignition coils and they’re often recessed down into the top of the engine, creating a spot where water can pool. Try and be extra careful around these areas.
If you’re not comfortable proceeding, don’t.
Cover areas as you see fit using plastic, rags etc. I normally only cover the alternator and any exposed intakes, and often the oil filler cap, if it’s possible for water to pool.

Stage 2: Cleaning the bonnet (hood)

Spray the underside of the bonnet down with your diluted APC. If the bonnet has fabric sound deadening material, give this a good spray too, as you want to get enough on there, so it properly wets the material.
Pay attention to the bonnet release mechanism. If the car’s older, this is likely to be caked in ancient grease that’s turned nasty and a bit of an eyesore. Spray a liberal amount of solution on this area if needed.
Allow product to dwell for at least 1 minute, up to a maximum of 5 minutes. These are general dwell times, consult the instructions for your specific product.
During the dwell period, use the hand brush to go over the entire area, making sure the fabric is well agitated, and if required use the smaller brushes for hard to reach and intricate areas like the bonnet release mechanism.
If there are areas that require a 2nd hit of chemical, use a stronger solution, or (and this is what I prefer to do), dip a brush in some higher concentrated solution and work it into the area.
Rinse the bonnet down and ensure that all the product has been sufficiently removed, especially from the fabric.
It’s a good idea to rinse the front end of the car down too, as you will almost certainly have gotten some product on the paintwork, and it’s best to not let it dwell or worse, dry.
If any areas require extra attention, see to them now. Work until clean, rinse, rinse front end.
If you’re happy with the results, move onto the next section

Stage 3: Cleaning the engine bay

Note: I’m going to leave this generic and vague, use your own judgement.

If: you’re new to this, the engine is particularly dirty, the engine is particularly large/intricate, or you just work slowly – you may want to work in sections, to avoid leaving the product dwelling for too long, or drying.
Spray the area down with diluted product.
When it’s had sufficient time to get to work loosening the dirt and grime, using your largest brushes, cover as much as you possibly can, before swapping to a smaller brush for the areas they can’t reach. If you go in with a detail brush to begin with, it will take you far longer. Just cover as much as possible as efficiently as possible, then go in with something smaller if needed.
The hand brush is great for the top surfaces, and exposed, flat areas. The wheel brush is great for getting vertical surfaces that are hard to reach, including in and around hoses and wiring. Detail brushes for anything that those two can’t reach.
As before, apply a stronger or neat solution to areas that require it.
Once everything’s been cleaned, move onto rinsing.

Stage 4: Rinsing the engine bay

If you’re using a pressure washer, turn the pressure down (if able to), use the widest nozzle you have, and don’t get too close. You’ll have to use your own judgement here, but you’re only rinsing the product and loosened dirt away, there’s no need to get in close. If there’s dirt that isn’t rinsing right off, then you need to clean it again, properly; do NOT use the pressure washer to blast dirt off.
If you’re unable to reduce pressure, or don’t have changeable nozzles, then use the PW with a good few feet from tip to engine, this will give the jet time to slow down so it’s not as aggressive.
Be careful around the sensitive areas you identified earlier.
If you’re happy with the results, move on to the next section.
If not, repeat previous steps as necessary

Optional: Dressing the engine bay

Now’s the perfect time to apply a dressing to the engine bay. There are various ones available, some of which should be applied wet (self-levelling dressings), some which should be applied dry. I’d highly recommend using a self-levelling product that can be applied wet, as it makes the process much easier, in my opinion. Avoid applying dressing to areas with belts and pulleys.

Using a dressing that can be applied wet

Following the instructions, apply product over the surfaces whilst they’re still wet.
You might as well clean the rest of the car as you want this to sit for a while.
Once dried, come back and apply dressing to any areas that require it.
Wipe over all surfaces to ensure even coverage and remove any excess water/product.

Using no dressing or a dressing that can be applied dry

If you have a blower of some sort, they’re perfect for this. Go to town getting all the water blasted out of the various nooks and crannies. Bear in mind that you may dislodge some dirt that wasn’t cleaned/rinsed off properly earlier.
If you don’t own a blower, get started with your old MFs or rags, soaking up any excess water.
Allow the engine to air dry a bit, so it’s not wet to the touch.
Apply your dressing, following the manufacturer’s instructions. Skip this step if you’re not applying a dressing.

Final Stage: Cleanup

Now that the engine bay and bonnet are clean, dry and dressed, we’re almost done.
Remove any protective plastic, cloths or rags that you used.
- If any of the areas you covered require cleaning and dressing, do this now, using some product applied to a microfiber, then wipe dry and apply dressing, if applicable.
Using a suitable automotive product, re-grease any areas that require it, such as the bonnet release mechanism, hinges etc. Make sure you apply the right amount. Too little may lead to corrosion, failure to operate properly, or even damage. Too much will be messy, attract dirt and may require cleaning prematurely.
Start the car and ensure everything is working as expected.
Take some after photos and admire your work! As a bonus, share them on the HTAD Discord.

The Short Version

Tools Required

All-purpose cleaner (APC)
Brushes
Cloths
Dressing (water based)
Plastic bags
PPE: Gloves and goggles

Precautions

DO

Let the engine cool
Check for anything that could cause engine issues (connections, frays, leaks)
Learn the locations of sensitive components (ECU, fuse box, alternator)

DO NOT

Use a pressure washer at short range, or high pressure. Use a wide spray and keep your distance – or just use a hose. If your machine has the ability, turn the pressure level to its lowest setting.
Spray unprotected, sensitive components.
Get dressing on belts.
Underestimate how painful (and potentially injuring) getting a piece of APC-drenched grit on your (non-goggle-protected) eyeball is.

Steps

Cover any sensitive areas.
Open bonnet and spray the underside with APC. Allow to dwell for a couple of minutes.,
Work the APC into the fabric (if present) with a handled brush. Clean the painted areas with brushes/cloths.
Rinse the underside of the bonnet. Rinse the windscreen and quarter panels of any overspray/dirt.
Spray APC over the engine bay as required. Particularly bad areas may require more (or more concentrated) solutions. Only cover as much area as you can work without it drying before you’re ready to rinse.
Allow to dwell.
Working methodically, section by section, use brushes to get into all the crevices and agitate the APC, loosening the dirt. Wheel brushes are great for deep, hard to reach areas.
Rinse all the APC and dirt off with either a hose, or a pressure washer (see precautions).
Whilst it’s still wet, spray it down with dressing. I usually leave the dressing to sit and clean the rest of the car and return later.
Once the dressing has had time to dry, give it a wipe down and soak up any excess water or dressing. This will also help give an even finish, if necessary.
Start the engine and check all is well.

Contributors

Jeyy, jobobcul