520i temp gauge/overheating?

One point I'd like to reiterate (as no-one has picked up on it) is that the coolant level in the reservoir went UP rather than down after I drove the car. Can anyone say what circumstances would cause that? Could it just be that when the coolant gets warmed up it expands into the reservoir? If I had a leak or a blown head gasket I'd expect the level to go down not up.

Does anyone haev any ideas?

Thanks again for all the help.

Flummoxed

"Flummoxed" wrote

1) The level in the radiator/system *usually* goes up when warm. Extra is trapped in the overflow system and gets sucked back when the engine/radiator cools. 2) If *both* go up, that shows air infiltration (i.e., bad head gasket) (after entire system is cool.) 3) A bad head gasket doesn't necessarily mean loss of coolant - remember that cylinder pressure is higher than the coolant's vapor pressure. 4) Another source of air might be a leak in the water pump inlet pipe (from the bottom of the radiator). This isn't high probability, since the system pressure usually overcomes the pump negative pressure.

FloydR

Right - I'd say that rules out the fan coupling as it will be all airflow that cools the rad at that speed.

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I'd taken my instruments out and taken them apart a little.

I put them back without doing up the screw bits that put the two halves of the instrument panel together. This caused the clocks to indicate that the engine was overheating.

Hi again and thanks for the continued help with this.

Maybe I should have pointed out earlier: in Aug 2004 I had the Thermostat & O rings, and fan coupling replaced, then in Feb 2005 the waterpump was replaced. Therefore I think it's unlikely to be any of those components, as the car has been driven since then without any problems, until now.

I suspect it might be that the cooling system needs bleeding, so if anyone can tell me how to do that I'd be grateful. As I said, I've found two cross-head plastic plugs that might be the things to use to do this - can anyone confirm this? What's the correct procedure?

Thanks again for the help.

Flummoxed

Bad head gasket. That's the only thing I know of that will cause the level in the overflow tank to increase. The leak injects some exhaust gas from the combustion chamber into the water jacket. Once you have air (exhaust) bubbles in the coolant the pump will start to act up causing all manner of strange temperature displays.

A cooling system that needs bleeding will not result in a higher level in the overflow tank (assuming you are checking when cold)

Thanks for the replies.

This morning I started the car up and waited for it to warm up, then I slackened off the two plastic plugs (one at a time) that I thought might be the bleed screws. I noticed a very small amount of liquid coming out of them, but not very much at all, then it stopped. I'd have expected liquid to keep coming out once it started (like bleeding a rad in a central heating system) but it didn't. Anyway, this time I did notice that I seem to have a leak, although I couldn't work out where exactly, somewhere underneath the coolant reservoir. It's probably only a slow leak and maybe only happens under pressure (when the engine's warmed up?). Guess I'll have to get it fixed anyway. This time the coolant level in the reservoir had gone down (once it had cooled again) and it took about a half litre to bring it back up to the Cool/Kalt level. The temp gauge never went above normal the whole time, which is good I suppose!

One thing I wondered: should the two large hoses from the radiator to the engine (one top right as you look from the front, and one bottom left) be full of coolant? I can squeeze them very easily as if they're empty, whether the engine's running or not. I guess they must have liquid in though, but I'd expect them to be hard to squeeze if the system's under pressure.

Anyway looks like I do have a slow leak, and that must have caused the coolant to gradually leak out and then air to get into the system which probably caused the weird effects on the temp gauge. I'm still not convinced I've removed the air from the system though, as I didn't really know what I was doing. I just hoped that slackening off those plugs would allow some air to escape and fill the space with liquid. It didn't seem to do that though :(

Anyway thanks for the help. Guess I'll be calling the garage in the morning :(

Flummoxed

Correct - they should be hard to squeeze when hot and for some time after stopping the engine.

You've got a leak. I had a leak on my E39 which you couldn't see. A dye test showed it to be coming from the waterpump. Exhaust all possibilities before deciding it's the head gasket.

Dave Plowman wrote: "You've got a leak. I had a leak on my E39 which you couldn't see. A dye test showed it to be coming from the waterpump. Exhaust all possibilities before deciding it's the head gasket."

No, it isn't the head gasket, the leak's somewhere below the coolant reservoir, but I couldn't see exactly where from. It was just a trickle of liquid coming from somewhere round there. There's a narrow hose with a jubilee clip coming out of the bottom of the reservoir, and I wondered if it might be that but I the jubilee clip didn't seem to need tightening (at least I was unable to tighten it any more than it was already). I couldn't see any other obvious thing to try.

Anyway I've booked it into the garage now - hope it doesn't cost too much :(

Thanks for everyone's help.

TP