Hi everyone and sorry in advance for this leeengthy post

I own a 1995 Fiat Uno 1.1 i.e.S (1.1 L FIRE engine). I've long had problems with coolant loss, the expansion tank was empty after about 200km of drive or a week after topping-up. There was an obvious leak in my old radiator (you could see it dripping when the vehicle was not moving) so after a year of so, I decided to buy a new one.

So, I recently bought a brand-new Marelli radiator. Unfortunately, things went worse. Even though there was no "static" coolant loss any more but the expansion tank emptied after 50km drive or so. As I found traces of sprayed coolant around the expansion tank cap and hearing a hissing noise after driving, I decided to buy a new cap. Still, with the new cap, even though no more sprayed coolant is found around the cap, the expansion tank goes from full to empty every 150km of drive or so. I've also observed that the loss is much more faster when it's hot outside, i.e. when driving at mid-day. In addition to that, it seems that driving at 120kph (70mph) in the highway for the same amount of time than in the traffic jam, also accelerates the loss. Finally, a veery quiet hissing noise can also be heard around the (new) cap after driving, even though it's much more silent than before, and no coolant spills around.

I've checked the radiator fan cutoff thermostat and it seems OK, as the hoses to/from the radiator. The engine always works at the "nominal" temperature range, except when the expansion tank is empty (waaay down from the "min" sign), so it slightly overheats (the temperatures go past the mid, but have never reached the red zone).

Any ideas please? I honestly do not know what to do. One friend suggested adding another o-ring at the radiator cap, so that there's better sealing. Is that OK? Should I tighten the cap as far as I can? Also, I'm thinking of "smoothing" the expansion tank rim, if this is the problem.

Any help greatly apreciated, I do not know what to do any more!! It's a shame all this coolant story, because the car is in a very good condition otherwise.

It could be a blown head gasket. Check for white smoke from the exhaust.

To test you could try and fill the radiator right to the top (with the engine cold) then put the cap on loosely and run the engine for a minute. If you get the hissing noise or water comes out then the head gasket would be my sugestion.

If it waits until the engine is warm before giving problems it could simply be an air bubble in the system. bleed the system to fix. If this appears to be the problem and bleeding doesnt fix it, then it could be your heater giving problems.

Yoinks!

The radiator cap is designed to release pressure when it gets to 1.0 bar.. adding a new O-ring to it won't affect this ability and in any case, you shouldn't want to.

The beast has a blown head gasket. You're getting combustion gas blown into the coolant and that's causing an air-lock - that increased pressure gives the coolant nowhere else to go (it's getting "pumped up") and so it escapes out of the coolant cap.

A pressure test on the cooling system (or a test for CO - carbon monoxide) will prove the gasket has gone for sure, before you start dismantling it.

Ralf S.