Boiling over from the expansion tank cap.
First post for me so be gentle!
Problem I have with my '96 155 2.0TS 16v is that when crawling in traffic the coolant boils over through the filler cap on the top of the expansion tank.
Recent history of the car:
Cam belt (& associated bits) needed replacing and boiling over problem evident, skint and was out of work so car used infrequently for a few months. When I got the dosh together to have cam belt changed (too big a job for me) my mechanic mate said that the filler cap was knackered as water poured through it when he had the header tank out. Replaced (with what looks like non-alfa original) filler cap and combustion gas tested (bottle of liquid on filler hole) whilst in for the belt. The test showed no bubbling and no discolouration of the liquid.
Problem continued, another mechanic mate gas tested again, still looks good, so assume head gasket is OK. Rad looked a bit iffy, so put a new one in a couple of weeks ago, old one had lots of fins missing. Temp gauge whilst motorway driving stays a little lower that before rad change. No overheating seen since, but then I'd not got stuck in traffic for a while. Today, encountered a traffic jam. Noticed after a while that temp gauge was rising quite rapidly, fan cutting in (as it always has) and spotted a trail of liquid in rear-view mirrors. Put on the heater which helped to drop it a bit and luckily got moving shortly after. At home refilled header (about a litre) and did some testing. Temp gauge sits happily on first white mark (about 70 c?) as it did when I first had it, fan kicks in for a few seconds about 4 or 5 times, everthing looking stable and then temp gauge starts to creep and coolant starts to boil out of the cap. I suspected that the cap "may" not be operating at the correct temp (as not an Alfa one) so topped up tank again (another litre roughly), fitted girlfriend's 146 (1600) filler cap and re-ran test. Same outcome.
My thoughts now are that:
1) Filler cap is not correct pressure (don't know whether the 146 one is the same as the 155).
2) Thermostat is not functioning properly (from reading some other posts)
3) Head gasket after all (as first mechanic said). Arrghh!!
Any guidance would be very welcome.
Sorry for the length of this, thought it best to give as much detail as possible.