Topping up the oil – can you open the oil cap when the car is hot?

Almost everyone remembers to topup the engine oil while there driving, so as soon as they turn they engine off most people want to get on and do it before going back indoors. Which brings people to ask can you open the oil cap when the car is hot, or at least should you?

When the engine is hot, the oil can reach temperatures over 250 degrees Fahrenheit, so the risk of harm is relatively high. This is the main reason for waiting to open the oil cap. The other reason is you’re probably checking the oil level and topping up the oil, which is done more accurately if the oil is cool.

Can you open the oil cap when the car is running?

There are no restrictions from the car stopping you from opening the oil cap when the vehicle is running. The thing to be careful of is the hot oil itself. When the engine runs, the oil is pumped around the machine with some force; with the oil cap off, it is normal for the oil to spit out quite viciously. Hot oil burns: it makes a mess and stains clothing. So, it is best to switch the engine off before opening the oil cap. At the minimum, the oil won’t be spitting at you.

How long should you wait before opening the oil cap?

As we’ve mentioned, you can open the oil cap immediately; however, if you have just checked the oil with the engine hot, the level will be incorrect. It takes around 20 minutes for engine oil to cool sufficiently and for all of the oil to find its way back to the oil sump. It is best to wait at least 20 minutes before opening the oil cap and checking the oil level.

How to check the oil level

To check the oil level, you will need a clean cloth you don’t mind getting oil on and some gloves if you don’t want oil/dirt on your hands. With the engine sufficiently cooled:

  1. Remove the dipstick carefully.
  2. Clean the dipstick with the cloth and reinsert it into the engine.
  3. Remove the dipstick from the engine again.
  4. Do not bring the dipstick up by more than a 90-degree angle but bring it up to the light to check the oil level.
  5. It should be between the minimum and maximum marker on the dipstick.
  6. If it is below halfway on the dipstick, you should consider topping up a small amount so that you don’t run the risk of running the engine low on oil. If the engine oil is halfway on the dipstick, it isn’t worth topping up just yet, as you don’t want to overfill the oil accidentally.
  7. Make sure to re-insert the dipstick and refit the oil cap if it was removed.

How to open the engine oil cap and top-up the oil

Oil caps undo anti-clockwise (righty tighty, lefty loosey); most caps will click into place when screwing the cap back in, so there is no need to force the cap further than its stops. When removing the oil cap, place it somewhere in the engine bay, and do not forget to refit it; the scuttle tray or upside down on the top of the engine are good places to store the cap while you top-up the oil.

The best tip anyone can give you when topping up the oil is to do it with the bottle sideways, as shown in the picture below. The oil does not spit out of the bottle at a steady slow, so you do not risk pouring it everywhere. It is always wise to have an old rag to hand to clean up any spillages immediately. Tiny drops of oil will burn away when the engine is running but be aware burning oil smokes a lot.

When topping up the oil, remember that the minimum marker to the top marker on the dipstick is around 1 liter of oil; it is best to top up a small amount, wait a few minutes for the oil to get to the sump and then re-check the level, repeat where necessary.

What to do if oil cap is stuck

Oil caps can get stuck, especially on cars that are parked for a long time or don’t get regularly serviced! You can sometimes use some big pump pliers to get a good grip on the cap, and brute force can unscrew the oil cap. Be very careful, though; if the cap oil is weak, it can snap, which is a real pain to remove from the engine and requires using a chisel and hammer to beat the cap around the thread, which takes ages!

If all else, you will need to replace the camshaft cover, the gasket, and a new oil cap. This is not a difficult job that can be done at home and doesn’t require you to drain the oil. You only need to undo and remove the bolts securing the cover, usually 8 – 12 bolts, and the body comes off; you do need to remember to clean away all of the oil gaskets before replacing it with a new gasket and the cover. Failure to clean the old gasket away will result in an oil leak later. If this is above your mechanical skills, you will need to take the car or have it recovered to a mechanics workshop, who can open the cap or replace the camshaft cover for you.

Bottom line on can you open the oil cap when the car is hot?

You should wait 20 minutes after turning the engine off before opening the oil cap when checking the oil level to get an accurate reading and to stop any injuries. When an engine is hot, the oil temperature can reach 250 degrees Fahrenheit (120 degrees Celsius), which, you can guess, will hurt a lot. Occasionally, oil caps can get stuck and require a bit of work to undo; if you find yours stuck, your local mechanic may assist you in getting it undone without damaging the cap. It may end up that you need other items replaced to remove the oil cap.

My name is Tom although my friends call me Tommy. Messing around with cars and bikes has always been a hobby of mine even from a young age. So I made it my day job 17 years ago. I am a fully qualified mechanic as you would expect. I've worked in all different areas of the motor trade, valeting, panel beating, engine repairs, I'm sure you get the idea. I enjoy sharing my wealth of knowledge and experience with others, which is the reason I spend a lot of time here writing for this website.

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.