i had coolant loss so changed my cap on the expansion tank and changed the waterpump as was leaking out of hole in bottom of water pump all fine for couple of days now it boils the coolant. when i stop steam comes out of the expansion tank. leave for couple of hours lost about 1/2 litre of coolant, the drive is 11 miles and the temperature gauge is half way as normal operating temperture, but no light for coolant loss. its pressurerising the system and its bleed properly. but checked hoses and cant see any leaks, drove home keeping under 3000 revs no loss of coolant travelled to work today came on motorway stuck to 65mph got to work steaming out? could it be a crack at hot temp or am i looking at head gasket?

 

Where exactly at the tank is the steam escaping? From the cap itself? Between the cap and tank neck? The coolant tanks are notorious for cracking. That is your most likely culprit.

I'd suggest hooking up a pressure tester. Once you pressurize the system, you should be able to isolate the source of the leak. Test the cap, too, but keep in mind a pressure tester doesn't test the seal between the tank and cap, so give that area a good visual inspection.

Find and fix that. I highly doubt you've got any sort of head gasket issue, so no need to panic.

 

the steam come from the expansion tank cap. i think it could be either ther the cap is doing it job as its new and reliving the pressure but that worries me that the cylinder head is leaking creating too much pressure for system? i cant find any leaks?

 

When you did the coolant change, did You thoroughly bleed any air from the system? One thing I can think is any air pockets will cause pressure build up and coolant boils in the presence of the leftover air.
Try bleeding the cooling system as well as pressure testing. Also possibly the cap seal is missing from the new part, look at that too.

 

I am thinking a stuck/sticking thermostat. The symptons are familiar.

Air in the system is also on my list.

 

I reread your original post. If I understand correctly, you found and fixed a leak (water pump) and also changed the cap as a precaution. Now you have steam venting from the cap, which is an entirely new symptom.

Call me Mister Vegas, but I'm willing to gamble the new cap is bad or the gasket is missing. Since you never had this symptom before, you've got to ask yourself what has changed.

If you still have the old cap, reinstall that. It wasn't leaking before.

If you don't have the old cap and it isn't expensive, spring for a new one. Go to a different store and get a different brand. If store A sold you a bad cap, it's very likely the rest of their stock is from the same defective batch. I ran into this one time with a distributor cap on a different vehicle. It was a real headscratcher how new parts could be defective.

Your other option is to beg, borrow, or steal a pressure tester. Test the system. Test the cap. Don't scoff at this suggestion to use a pressure tester. Running the engine and visually checking for leaks isn't nearly as good. You can find some leaks by running the engine, but not all. A pressure tester will find smaller leaks that aren't readily apparent.

Also, if concerned about a bad head gasket (not very likely), you can run the engine with the tester installed. The gauge will show if the cooling system is being over-pressurized. Just be careful because with the tester installed in place of the cap, you won't have any overpressure protection. Watch the gauge and if the pressure goes above limits, shut down the engine right away.