what happens when car runs without radiator cap on

The damage could be anywhere from minimal to head warpage. I'd recommend getting your car inspected for damage by a licensed mechanic. Best wishes! -Jackie

An automobile engine needs a cooling system to remove heat. For the system to work correctly, it must be self contained--sealed because it uses pressure to function properly. The pressure keeps the coolant from reaching a boiling point as quickly. As the temperature increases the coolant becomes steam. Without a radiator cap, the liquid overflows. Engine overheating can have devastating consequences. The engine begins to "ping" and lose power. This can cause piston, ring and rod bearing damage. Pre-ignition can occur, which creates weak places or "hot spots" which could burn holes in the pistons. Blown head gaskets can occur, cams can freeze up and break. Hoses and radiators can burst.

I agree with all the above. Cap must be good and proper pressure to work properly. Does it over heat while normal driving or does it mainly happen when you come to a stop and idling? If it happens after you stop check to see that your fan is running. It should come on at idle if temp is getting to high.

First thing is to now change your thermostat... I had a Chev Wagon 1989 and it needed that... If your car is older then the rad may be now pooched. if you look at the rad and run your fingers across the bottom rad weaves they may be no longer connected. Replace with a 2 core Rad. Also consider a transmission oil cooler... Makes a better running car and your tranny and alternator will last longer. If you have an older than 86 forget the car, Head gaskets blow when overheated.

"what happens when car runs without radiator cap on" - it depends on how long it was off and what happened. When the cap is off, the coolant boils at a lower temperature. --- Full the reservoir, put the cap on and see what happens.

You Wrote: ......." car is over heating after ten minuets of driving "....... I'm assuming, that after refilling the cooling system, and replacing the radiator cap, the car overheats after 10 minutes. Question: After refilling the cooling system, Does the coolant Stay in the system, or is it leaking out of the block or radiator, somewhere other than the radiator or coolant recovery tank???.........if the Coolant is staying in the system, and the car seems to run fine, but overheats after 10 Minutes.....Replace Your Thermostat....... it's very possible it's Stuck Closed........ it's True as previously stated..... all sorts of Evil can happen to your engine, after it has a serious Over-Heatage..... my sweetheart once drove down the highway for quite a while in my old '91 Pontiac Sunbird with a 3.1/V6, before noticing the red lights on the dash..... surprisingly, everything was fine..... I changed the Thermostat anyway.... it's Good Policay to do so, after an Over- Heatage...... after you change the thermostat, add a good quality cooling system sealer (in case you have a minor leak somewhere, like at a freeze plugh), and refill the system. Run the engine. Check for Leaks. On top of your Thermostat Housing, there may be a small fitting, it's an air bleed valve. If it's there, open the bleed valve, til no more air comes spitting out of it... keep an eye on the coolant level. In fact, if everything works out OK, keep an eye on the coolant level for days, since there might be more air in the system, even after bleeding. Also, keep an ear out, and inspect regularly for leaks, your water pump, especially underneath. They too often go bad when they are run dry after an overheatage. Good Luck! .......I hope everything is Cool! ......sorry, just had to add that....... jvb.......PS. if everything works out OK, change your oil, it too may have gotten cooked as well.

I had This problem Today I pore the coolent and forgot to fix the Radiator cap as a result My Radiator Runs Hot on every Signal Light and smoke Is coming out from the Radiator So I stopped My vehicle and Purchased Coolant and Radiator cap. Now it is working fine

I must be stupid. Besides the radiator cap there is a drain hose, open to the air. It is almost the same to work without cap.

I let my sister borrow my car vy commodore 03 model, i forgot to put the radiator cap on b4 she left, it over heated halfway to Forster from taree and now it won't start what's the worse outcome any comments available?

If it ran fine before, but now it won't start after overheating, you've most likely warped the cylinder head due to overheating and compression for startup can't be achieved. Head gasket repair and resurfacing the head is most likely the next step. Sorry.