Topping up the Oil – Can You Open the Oil Cap When the Car Is Hot?

Almost everyone remembers you need to top up the engine oil while they are driving. As soon as they turn the engine off, they want to get on and do it before going back indoors. That leads to whether you can open the oil cap when the engine is hot.

When the engine is hot, the oil temperature can rise above 250 degrees Fahrenheit (121 degrees Celsius), resulting in serious harm. I’ve learned this from personal experience, having often burnt my hands being impatient. Therefore, it’s important to wait before opening the oil cap. Additionally, checking the oil level and adding oil is more accurate when the oil is cool. I have explained these reasons in greater detail below.

Can You Open the Oil Cap When the Car Is Running?

There are no restrictions from the car that will stop 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 with some force; with the oil cap off, it is normal for the oil to spit out quite ferociously. Hot oil burns: it makes a mess and stains clothing. If you must open the oil cap while the engine is still hot, it is best to switch the engine off before opening the oil cap. At a 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 15 – 20 minutes for engine oil to cool sufficiently. The oil needs to drip down and 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; it will still be surprisingly warm. 

How to Open the Engine Oil Cap and Top-up the Oil

All 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 remember 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 sideways with the bottle, as shown in the picture below. The oil does not spit out of the bottle and pours steadily, so you do not risk chucking 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 if you spill any, but be aware that burning oil smokes a lot. 

When topping up the oil, remember that the minimum marker to the top marker on the dipstick is around 0.9 Quarts (1 liter of oil); it is best to top up by small amounts, 0.2 QT (200ml) at a time, wait a few minutes for the oil to get to the sump and then re-check the level, repeat where necessary.

How to correctly and safely 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 re-insert it into the engine.
  3. Remove the dipstick from the motor 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 measurements, consider topping up a small amount, about 0.2 quarts (200ml), 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 want to avoid accidentally overfilling the oil. 
  7. If you have topped up the oil, wait a minute and then re-check the oil level. 
  8. Make sure to re-insert the dipstick and refit the oil cap.

What to Do if the Oil Cap Is Stuck

Oil caps can get stuck, especially on cars parked for a long time or not 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 fails, you must replace the camshaft cover, the gasket, and a new oil cap. This is a relatively easy 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 cover comes off; you do need to remember to clean away all of the old gasket 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 must take the car or have it recovered to a mechanics workshop, who can open the cap or replace the camshaft cover for you.

Frequently Asked Questions

Can You Put Oil In A Hot Engine?

Yes, you can put cold oil into a hot engine. It will not cause any damage to engine internals. However, it is safer to wait for the engine to cool.

Do I Need To Warm The Engine Before Checking The Oil Level?

No there is no need to warm the engine before checking the oil level. The oil level is most accurate when the engine is cold.

How Tight Should The Oil Cap Be?

The oil cap should only be hand tight. All modern vehicles have a safety stop restricting you from over tightening the oil cap.

Bottom Line 

You should wait 15 – 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 hurts 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. 

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.

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