Is is safe to drive with your radiator cap loose?

Question

My radiator is leaking pretty badly and needs replaced. It usually doesn't seem to leak until it builds up pressure and then after you shut it off it keeps leaking for a few minutes then stops. The cost of the new radiator is $150 in stores, or I can order one online (from a reputable dealer) for $75. So, I ordered it and when it comes in I am going to drive 3 hours up to my father-in-laws and he is going to swap it out for us for free (he is a mechanic).

Anyway, my wifes radiator went bad about a year ago and they told her to put the radiator cap on loose to keep the pressure down. Like with mine, if you turn once it is attached but loose and if you turn it a second time it is tight.

1) If I keep the cap on loose until I can get it fixed (at least on the 3 hour trip) will that help reduce the leak?

2) Is it safe to do that?

I don't need a ride to pick up the radiator, I need to drive the car up to my father-in-laws garage (he is a mechanic) which is about 3 hours away. He is the one that told me to leave the cap loose. Just to see what it would do I left it loose to drive to work (about a 20 minute drive) and it didn't leak a drop. When I leave the cap on tight it starts leaking before I even leave the house and is still leaking until about 5 minutes after I shut the car off.

Several people said it wasn't safe but I didn't lose any coolant and thats why most people say it is dangerous. So, is it safe to leave the cap loose and stop every hour or so when I make the 3 hour trip?

Answer 1

It is totally safe. did this for a year when I was younger (like 17) to keep from putting my truck out of service when I had a water leak. The way you would lose water with the cap loose in an over heat condition or the first time the thermostat open and puts pressurized water into the radiator, otherwise, with the cap loose, you have no pressure in the system to push the water out. You can do this,just keep an eye on the temp and keep in mind that your leak is in the radiator, so even with no pressure you will lose SOME coolant even with no pressure. Keep it full during your trip and it wont run hot.

Answer 2

Hello okay, this is what's gonna happen, let says this is a 3 hour drive on the highway right, if you leave the cap loose just a bit (enough for the pressure to be released but not that much so that you start losing a lot of fluid through there you will be fine, reason being is because when let say you are doing a constant 60 mph in the freeway the fluid rotates constantly through your engine because the thermostat is open, the moment the thermostat closes is when water is gonna start to try to come out from the cap. Another thing you can do is get to Autozone or Walmart and get one of those radiator caps that release pressure with a small red latch on top, if you put that one on and leave the latch open i think is your best chance to lose the less amount of fluid possible, should be about $12 and remember to take at least 2 gallons of water in this trip and keep an eye on the temperature gauge at all times. Good luck!

Answer 3

Loose Radiator Cap

Answer 4

It's perfectly safe to just turn the cap one click or go to Carquest and buy a 4 pound pressure cap and twist it all the way on. You're a smart man! As the coolant heats up it builds pressure against the inside of the cooling system and pressure cap. Less pressure reduces coolant leaks. The one and only problem might be less pressure lowers the boiling point of the coolant. I have a rather old fashioned friend who use to replace the radiator pressure cap with one with a four pound rating every time he bought the biggest baddest Ford in town. Keep an eye on the coolant level inside the radiator and coolant recovery tank. Low coolant pressure is a double edged sword. It won't expand the radiator core. Low pressure might cause steam pockets cylinder the head water-jackets as that's by far the hottest part of any internal combustion engine. I rescued a guy on the highway one day who had a top expansion tank tear loose from the vertical radiator core. I did the one click trick and he managed to drive 20 more miles home after work one night. It was a rotten old Mopar radiator.

Answer 5

You didn't list what year make and model car you had. American car radiator necks are designed with a safety click, just prior to removal of the cap so you don't get hot coolant in your face. See if your car has the second click on the radiator neck.



Why do you ask the same question you did yesterday? #1. If you put the radiator cap on one click instead of two you will not loose any coolant regardless what Bandit says! If you feel uncomfortable with that suggestion buy a 4 pound radiator pressure cap from Carquest.



I'll repeat this one more time. The object of doing one of these things is to lower the coolant pressure which will slow down the coolant leak. If you're driving 3 hours to your father in-laws to get it fixed you'd better have at least two five gallon plastic Kerosene cans full of water with you. Keep in-mind that when the coolant level drops it uncovers the temperature sensor in the motor so your dash gage will no longer be accurate.



I'm an old man who's done this a time or two and it'll work sucessfully without engine damage. I'm talking to you, not the inexperianced people above who have no clue!

Answer 6

By design your cooling system is a "closed loop" system, meaning that the coolant should not be exposed to the atmosphere. The purpose of the radiator cap is to allow a certain amount of pressure to build up in this closed system. The pressure that builds up actually keeps your cooling system from boiling, as water under pressure has a higher boiling point than water exposed to atmospheric pressure. For instance, if you put a pot full of water on the stove top, it might start to boil in about 4 minutes with a high flame under it. That same pot with a closed lid would take 5 minutes before it began to boil.



Removing or no sealing the radiator cap is like taking the lid off of that pot. Without a cap to keep the closed system under pressure, the fluid will begin to boil when it reaches in the neighborhood of 230 degrees Fahrenheit. Most cars are designed to run at around 220 degrees Fahrenheit, so you don't have much wiggle room. Sitting at a stoplight for 3 minutes is usually enough to raise the coolant / engine temperature 20 degrees as you have no air moving through the radiator to cool the liquid. Usually when your engine is on the verge of reaching about 250 degrees you will have an electric fan that will kick in to prevent overheating.



So if you leave your cap loose or the lever up (open) your cooling system is prevented from building pressure. Stopping at a stoplight or drive through window can allow the fluid to heat rapidly, faster than the car's electric fan can react. If allowed to get hot enough, the fluid will reach the boiling point, and this is where it could get bad. Our pot in the example above was heated from below, so the water at the bottom was the hottest and the boiling was relatively controlled. Imagine if the flames were all around the pot, hitting it from all directions. The result would be almost like a small explosion. The fluid in your cooling system would pretty much instantaneously boil, pushing boiling water under extreme pressure throughout the cooling system... some of which is not designed for boiling fluid under high pressure. Also, boiling water contains air bubbles, and these air bubbles can form in your cooling system and basically prevent ANY fluid from being pumped through the system. In a short amount of time you could find your engine's temperature well beyond the designed operating limit. Doing this for even a couple of minutes can seriously damage the engine... warping the cylinder heads, blowing out the radiator seams, damaging hoses and blowing out water seals.



I think if you keep the vehicle moving and air flowing through the radiator you will be all right. If you stop in idle for more than a few minutes, it would be best to pull over, stop the engine and let it cool for about half an hour before resuming.

Answer 7

In an effort to cut to the quick here, what we are talking about is "safe" for the engine. So my recommendation based upon a similar situation is that you can do what you are suggesting without "causing more damage" to anything. Yes, you may lose some coolant out the radiator cap. So what? You said you are going to stop every hour or so and presumably will add water / coolant if needed. Also, keep your eye on the gauge - don't know if you have a real gauge or a red light - but that's your indication of overheating to the point of damaging something. Go for it.

Answer 8

I've never heard that leaving the radiator cap loose will prevent a leak, in theory it would leak more. When your anti-freeze heats up it expands, the rad cap expels this extra fluid into your reservoir. When it cools back down it sucks the anti-freeze back into the system. I'd try replacing the rad cap unless your positive its the radiator. Even then, the cap is only ~$3.

As for the drive, if its leaking as you say be prepared to stop ALOT. You will need to continually fill up your radiator. Some good news: fill it with water. If you're driving it wont freeze. Tell your mechanic you did this though, so he can fill it with the proper mixture when he's done. Keep in mind that bit about the radiator cap though, I could picture a fountain of coolant spewing out of it. Keep an eye on your temp gauge. And if you want to get really creative rig up a system to constantly refresh your water/coolant.

Answer 9

Can your farther not drive to you?

it really is not recommended to do that sort of trip with a leaky Radiator and defiantly not recommended to leave the rad cap loose!

how ever if you really need to drive to your Fathers, then take plenty of water and stop regally to top up...oh Keep an eye on the temp Gauge too.

Good luck my friend.

Answer 10

It is probably not a good idea to drive the car with a bad radiator cap. As the fluid heats up it expands, which is why there is pressure on it. Basically the fluid is still going to heat up with out the cap on the radiator, and most likely spill all over the ground, leading to overheating. It could leave you stranded or worse, destroy the motor. Have you tried just replacing the radiator cap? That may solve your problem with out replacing the entire radiator. Good luck.

Answer 11

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HI Its ok to do that BUT keep a can of water in the car.. as every time you shut it off it well puke out fluid as the cap is not tight.. and as said keep an eye on the temp gage. Make your trip as a day trip take your time and watch the temp. good luck tim

Answer 12

You could buy some radweld and stick it in and then close the radiator cap fully. I did that and it was fine for a journey similar to yours. Failing radweld, you could get KSeal. it does the same thing. it sticks to overheating areas and forms a very crude "weld" that maintains pressure and saves you from the dreaded HG failure.

Answer 13

yes, no problem, take plenty of water, try a fixer like rad weld or putty if it is pouring out from a visible hole, stop often to check level

Answer 14

Ask your father-in-law to come to you. If he wont a reputable mechanic can fix it. It wont be free but i wouldn't risk your engine .

Answer 15

i would say no u will loose more coolant than if it was done up if u wont to keep the pressure down on the trip remove the thermostat just remember to put it back when the job is done its a simple job undo 3 bolts take out the stat put some sealer round put the housing back on bolt down that's it

Answer 16

It's not safe, you will run the risk of more damage if you do this.

Answer 17

you will be stopping a lot because you will lose all the anti-freeze.