520i temp gauge/overheating?

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Hi - hope someone can help with this. I'm new to this group so apologies if this has come up before, but I couldn't find the same problem described.
I have a 520i (P reg UK) and last night as I was driving home, I noticed the temp gauge go into the red zone (where the red light came on, which is what made me notice). I thought the engine was overheating and was about to stop, but when I took my foot off the accelerator it went back down to 'normal' temp. I wouldn't have expected this if the engine was genuinely overheating (e.g. if the fan belt had broken or a hose had burst) - in that case I would expect it to stay hot until I switched off. I continued to drive home slowly (only a couple of miles in all), and this behaviour repeated itself a couple of times.
Does anyone know what this could indicate? Could there be a thermostat problem? I was hoping someone would recognise the symptoms and be able to advise.
Thanks
Flummoxed
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When I blew a headgasket on my 735, exhaust got into the cooling system. what would happen, tho not as rapidly as you are experiencing; would be the engine would be hot (indeed, overheating) becuase the air was at thermostat, preventing the coolant to circulate and the temp to raise to the "danger" mark.
If I QUICKLY raised teh rpm or dropped it it seemed to inject enough <something> so make the coolant circulate again and coolant from teh radiator would hit the temp sensor/thermostat & then the temp would drop (only to raise again later)
How's your coolant level?

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Thanks for the reply. Of course I need to check the coolant level anyway, and I will do tonight, but I didn't have time last night (it was late and dark, and I didn't need the car today).
But I would have thought that if it was a simple case of the coolant level being low, the temp gauge would have stayed 'high' until I stopped the engine altogether. The strange thing here was how it went back down to normal (fairly quickly too) when I reduced revs and then went up again a few minutes later, then back down, etc. That's why I suspected the thermostat or something similar rather than lack of coolant. Anyway I'll let you know how the coolant level was once I've checked it tonight.
Thanks
Flummoxed
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I checked the level and it was low - I put about 0.8 litres in to bring it up to the "Cold" level. Does anyone know if that would be low enough to cause the effect I described?
(Of course the question remains as to why it was low, but it could be a long time since I checked the level. I couldn't see any obvious signs of a leak, although I've yet to drive it since I topped up the cooling system).
Thanks
Flummoxed
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Did you bleed the system? or just top it off?
If there's air trapped in the system, topping it off won't fix it.....

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Ooerr, good thinking. No I didn't bleed it, I don't know how to at the moment. I'll have to look it up.
Cheers
Flummoxed
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Could well be. The temp gauge on these cars isn't a straightforward thermocouple device that gives a true read out of the temp. It appears to work like one, but in fact the ECU will keep it in the middle over a wide range of actual temperatures that are within the normal running ones. Exceed that and it goes up very quickly.
Under normal conditions these engines don't need topping up between services.
First thing to check is that the fan clutch is working - it progressively 'locks' up when it senses the airflow through the rad getting too high and runs up to a higher maximum speed. Only usually at town speeds and you can hear the increased noise from it.
As regards a leak it's often the water pump and difficult to detect without a dye/pressure test. But get the engine up to temperature and then stop it. Squeeze the top hose after a couple of minutes. It should feel pressurized. If not - ie it squeezes easily you've got a leak.

If it turns out to have a leak and it is the pump get the thermostat and housing changed at the same time - and also the serpentine belts unless you know when they were last done. They're not as expensive as the labour to have them done later.
If you say where you are in the UK you should get some recommendations for specialists. You might also consider joining the group in my sig for E39 expert advice.
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Thanks ChrisJ and Dave for the further advice/help, and the pointer to the other group. (I didn't even know my car was an E39, just to show my ignorance).
I've done nothing further yet to the car as I've been too busy, but I will want to sort it out soon.
I'll report back once I've had a chance (hopefully by the weekend) to look into it.
Cheers
Flummoxed
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are you 100% positive about that? I had heard that before, that the guage really is more of an "idiot light" made to look like a guage.
Yet; when I put a cooler thermostat on my car, the guage absolutely settles a touch lower than it used to.

Unless he's got a headgasket leak where exhaust gases are getting into the cooling system -- then it would indeed still feel pressurized.
Of course, those are based on my experiences and not specific technical expertise
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Josh Assing wrote:

Yes, BMWs are actually sensors and meters (not switches like idiot lights) but the display is non-linear such that middle part of the display represents a much wider variation of temps than the sides. The intent was to keep owners from complaining about small (normal) variations in temperature.
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hmm Interseting.. so I think what I read about it being a "an idiot light looking like a gauge" was only partially right.
so it's an averaging sort of thing -- becuase otherwise, it'd be dead in the center...
As an interestin gside bit -- when I had my overheatin problem, we used a non-contact thermometer, and teh guage consistently rose & lowered to the engine temp -- same now; as the engine heats up; the guage slowly rises, until it settles just below the center mark. I put the normal thermostat in; and it returned dead center..
Another bmw mystery I guess
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Not if there's a leak in the cooling system. With the engine stopped it will depressurize quickly. A good system will keep pressure for several minutes till it cools down.
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Mine didn't.
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Not sure why you'd want to change the spec of the stat but that sounds like it's running well outside the normal range. When warming up it does act like a true reading unit.
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wrote:

Yes; it rises slowly; if I turn the car off before fully warmed up and then turn it back on -- it returns to the mid point & then once the car is back on and wamring up again; continues up until it stops right below the center line. so yes; just like a real guage.
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Had a similar problem witht the bizarre temperature guage problem, mine lasted like a day and then disappeared. It was followed a week later by the engine cutting out problem which was diagnosed as a faulty cam position sensor. During the replacement of the sensor, the mechanic discovered that the thermostat housing was broken. The thermostat/housing might be what's causing your problem..
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Right, here's the latest: Having topped up the coolant reservoir to the "Kalt/Cool" level (it's a kind of see-through plastic reservoir next to the radiator), I took the car out for a short drive (about 5 miles) to warm the engine up.
A few minutes after it reached normal temperature, it started to creep upwards towards the hot zone. But before it reached the red zone I used the electronic push-button heating controls inside the car to increase the temp to 32 deg (the max in this car), and turned the heater fan up high too. The temp gauge almost immediately returned to normal, and it stayed like that until I got home (another couple of miles). So somehow turning the heating up inside the car had the effect of returning the engine temp to normal (or so it seemed).
When I got back I left it for an hour or so to cool down. I now notice that the coolant level in the reservoir has gone *up*, i.e. the level is now higher than the 'Kalt/Cool' mark.
Would the above be the symptoms of the system needing to be bled? Or maybe the symptoms of some other known problem?
I can't see any signs of a leak under the car or in the engine compartment, it all looks dry as a bone. When I slackened the filler cap on the reservoir I could hear a slight hissing sound indicating a pressure difference, but this might be because the engine /cooling system was still a little warm (it's actually a warmish day here for a change!).
Does the above give anyone any clues as to what the problem might be? I haven't tried bleeding the system as I don't yet know how. There's a cross-headed plastic screw near the reservoir filler cap, and a similar one near the 'engine' end of the top hose from the radiator. Are these what you use when bleeding?
I'd be grateful for any advice on this.
Thanks for your help so far.
Flummoxed
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How fast were you going when the temp shot up?
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Dave Plowman (News): "How fast were you going when the temp shot up?"
I wasn't going all that fast, maybe 50 mph, 60 at the most. I was being careful and keeping my eye on the temp gauge. I wouldn't say it shot up, it just crept up from normal towards the high zone. I slowed down to see if it made any difference (like it did when I first discovered the problem), but this time it didn't seem to make any difference - the temp gauge kept creeping up. But as soon as I turned the heating controls and interior fan up, it went back to the normal position.
Thanks again for the help, I appreciate it.
Flummoxed
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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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