Modern Vespa

I have had on order for 2 weeks a cap for my expansion tank and still dont have one and cant ride because it is leaking around the existing cap. Has anyone ever tried putting an o ring inside the cap to sop it from leaking. I can fill the tank with water and let the scoot run and when the engine reaches about half way on the temp gauge the fan kicks on and water starts leaking around the cap. This is on a mp3 400. I am missing a lot of riding time this summer and cant wait to get it fixed. I ordered the cap from the vespa dealer in Atlanta my closest source.

Dang that sucks - call Scooterwest and see if they can order it. Also - sounds like it's cracked - have you looked to see if perhaps you can do a temp repair with epoxy and then wrap around with fiber tape

Thanks for the advice so I went out and checked the old cap for cracks but cant seem to find any. The water is being forced out the bottom of the cap but I know the cap is tight. Could something else be causing this. Has anyone ever had a thermostat not open and cause excessive pressure when the engine heats up.

Sounds like a bad gasket - get a piece of rubber and make a gasket.

slight chance it is doing what it should be doing, and the impeller water pump is the issue? With no cracks or issues for leak showing, could be something else, a bad thermostat?

If it is running at normal operating temperatures when it leaks, thats a bad cap/gasket. If it is getting hotter than normal, that's other problems and the cap is, in fact, doing what it's supposed to do.

ok so i take my scooter in to the dealer to tell him about the scooter pushing water out the cap on the expansion tank. they just called and said it was ready but they could not get it to duplicate the problem and said it was low on coolant. I filled it up with water and cranked it at home and soon as it got to normal operating temp the fan would come on and it would start leaking out the cap. Could this problem have started by me running it low on coolant until it overheated and after I added water to the tank it had some type of air in the tank to cause it to push out the water. I just dont want to drive 100 miles to pick up the scoot and start to ride it back home for it to get hot again. Anyone have any thoughts on this.

How bad did you over heat it? You could have taken out the head gasket and it's the exhaust gases pushing the coolant out.
Not sure why the shop couldn't duplicate. They most likely let it idle in the parking lot, instead of taking it for a long ride.

yes have the dealer take it for a ride for 10 miles or more prior to you coming for it just to make sure. a short spin around the block is not good enough.

Okay I got the scooter back from the dealer yesterday and boy did I feel like an idiot. The scooter got to low on coolant and overheated then pushed water out the expansion tank cap. This is what should happen when it gets to low on coolant. All they had to do is drain the remaining water and coolant I had left in. They replaced the coolant and got the air out of the tank and everything is fine. Also did 6000 mile service but said my belt was fine. I thought the belt was always changed at the 6000 mile mark.

Take it back and tell them to change the belt
Don't understand these shops saying the belt "LOOKS GOOD"
Will it make it another 6,000 miles, which is when you need your next service? No, and they are doing you a disservice not changing it.
And if they had the damn thing apart to look at the belt, why the hell would they not change it?