coolant leaking from under radiator cap, why?

[kr4ut]

Whenever I drive my car, coolant begins to leak from under the radiator cap. However, when the car is only idling, nothing comes out. I've checked the radiator cap and it doesn't show any obvious signs of fault or corrosion, and I've ensured that the cap was on as tightly as I could turn it.

So my question is, what might the problem be? Is it maybe just the cap or could it be the thermostat or is there something else I should look for?

Thanks a bunch

[White94RX]

if it's coming from under the cap, easiest and cheapest thing to do is replace the cap. if it still happens, could be the expansion tank.

[kr4ut]

Alright, I'll do that. Thanks again

[sbblowz]

chances are it is expansion tank. but its cheap to get a new cap so might as well get a new one just to make sure.

[Critter7r]

Unless there's some physical problem with the tank, the cap is the problem. The caps are designed to hold X amount of pressure before bleeding off - in theory to protect the rest of the coolant system form excessive pressure from an overheat. As they age, the caps tend to lose their pressure holding ability and begin to bleed off pressure at normal cooling system pressures.

[kr4ut]

I've replaced the cap and it still leaks out. There's not some sort of trick to making the cap completely secure and tight, is there? :x

[aspin]

Well, if you replaced the cap with a new one, and it's still leaking, it probably isn't the cap itself. Replacing the expansion tank is probably a good thing to do anyway, as they can crack over time. I'll be doing this myself soon.

[kr4ut]

Alright, I'll do that and hope it works out. Thanks

[Imperialduckm3]

If you replace the expansion tank and still have overflow issue. It could mean that you have a blown HG which pump compress air into your cooling system cause the pressure to spike, thus your overflow issue.

[Hard Dog]

I thought I had a leaking rad cap, but it turned out to be a loose clamp on the small return hose below the retaining cap that encircles the vent screw. Maybe have a look there.

Dave

[Twright]

My 2000 323i will leak from the radiator cap if the radiator is overfilled. Is yours overfilled?

[kr4ut]

To me, the hose clamp looked fine, but I decided to take my car to a dealer and he said that the clamp hadn't been properly installed by the previous owner when he/she replaced the expansion tank.

I don't think the radiator was overfilled because the coolant level in the expansion tank would continue to drop until it was empty.

So yeah, I took the car in to a mechanic, and he believes there might be a crack in the expansion tank. Thanks for all the help though, you guys really helped me out with pinpointing the problem.

[StickSeven]

why did nobody recomend simply doing a pressure test?

[tonythetiger]

why did nobody recomend simply doing a pressure test?

I have a similar problem except I have already replaced the expansion tank and a new cap but it's still leaking from underneath the cap. Could it be a head gasket? Can I check the head gasket but letting the car run with the cap off?
Thanks

[MIKYZZ4]

why did nobody recomend simply doing a pressure test?

Tonythetiger, I suggest a pressure test for your problem.

[bmwdirtracer]

+1 to the pressure test. No, running the engine with the cap off is not a head gasket test.

Who made the expansion tank? Behr or Modine is fine; Dorman is Chinese crap. Also, are you sure it's not the bleed screw leaking? Is the top hose highly pressurized? Is the car overheating, or billowing white, sweet-smelling vapor out of the exhaust? Did you correctly bleed all the air out of the system after replacing the tank and cap?

[tonythetiger]

Standard motor products inc from rock auto is where I got it from . I actually have no idea where it's leaking from now. The top of the expansion tank is wet and after replacing the cap i need to refill coolant because the light comes on. To get the air out of the system I run the car with the bleeder screw open till coolant starts coming out then close it right? Also if it was the bleeder screw it would come out while idleing?