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MG MGF Technical - Low coolant level alarm - Road Test

We have just returned from our 2000+ mile road tour down to the Pyrenees in southern France and Spain (and what a great tour it was), so I thought I'd write an update on how my Coolant Level Alarm system worked.

See http://www.apttony.co.uk/Servicing/CoolantAlarm.html for the original details.


During all normal road driving there was no problem, and it wasn't until we started driving in the actual Pryenees mountains, with lots of very tight hairpin climbs and descents that the alarm tripped, but only very, very briefly, and only the once. Driving speeds were relatively low, probably down to 10 / 20 mph on a lot of the corners.

With the timer set at approx. 4.5 seconds, the most the the float could have been down near the bottom of the tank was about 5 seconds.

However, as we started our return journey, heading north on the A75 from Beziers and up towards the new bridge at Millau, the experince was a lot different. This route is a relatively new motorway, climbing and dipping through a very rugid area, with long sweeping curves. At high speed (no - all I'm saying is 'high' so don't ask!) the alarm tripped regularly on the left hand bends, especially those that dipped down, swung left, and then climbed, and it stayed on much longer, to the point that I began to worry. Out of the bend, a quick flick of the wheel, and it shut up.

I've tried to estimate how long the buzzer might have been on, and I guess somewhere around 6 - 8 seconds is near the mark. That means a total of about 10.5 - 12.5 seconds.


I have just done some experiments with the tank itself. It's full to the maximum level mark, (effectively the tank seam level). Unbolting it and tipping it sideways, i.e. driver's side down, it has to reach approx 37 degrees for the alarm to trip. At this point, the outlet pipe is still submerged, but not by much.

Now I'm no hydraulics engineer, but I suspect that the behavior of the coolant in this test is not the same as when subjected to centrifugal forces on a long bend. In my test the surface of the coolant remains a flat plane, but I suspect (correct me if I'm wrong) that when driving as above, the greater bulk of coolant, if not all, is forced to one side, away from the tank outlet. The question is, does it then start pumping air?!!!

A much narrower, taller tank, or the same tank turned though 90 degrees might be subject to less problems.

Comments please.
Tony Thompson

Tony,

I maybe about to show my complete and utter ignorance here... believe me, it wouldn't surprise me :-)

But... when the coolant is hot (hang on, isn't that a contradiction in terms!) and the engine is running doesn't the level rise (read this in someone's post a couple of days ago) so there would be less chance of your suspected situation happening - unless you've managed to get the car on two wheels like those stuntmen (stuntpeople sorry... must be pc) can do?

Chris

ps. I'm ready for the "don't be so damn stupid Chris" replies so keep 'em coming :-) Just to point out that I got a C in A'Level physics so thats about the extent of my theory in this :-)
Chris Tromans

The principle of the expansion tank is that it isn't actually part of the normal circulation of the coolant system - i.e. water isn't actually pumped through the tank. The purpose is for it to act as a resevoir - where when the coolant volume expands under increasing temperature, the excess drains into the tank. Then, after the ignition is switched off, the contracting coolant draws fluid in from the expansion tank.

The two jiggle valves constrain the fluid entry to the expansion tank. To my knowledge, there is no non-return valve in the expansion tank outlet. However, the system is under pressure, and therefore, there ought not to be any flow through the expansion tank outlet unless, for what ever reason, there is a loss of pressure in the main coolant circuit.

Therefore I would say that there isn't anything to worry about Tony, even if the expansion tank outlet is exposed to air for a period of seconds under hard cornering.

An interesting observation though!

Perhaps a much longer delay period needs to be built in (especially for those MGF/TF owners who want to take their MG on track)? 10 or 12 seconds perhaps?
Rob Bell

If you run remove the expansion cap (when cold) and run the engine coolant in pumped in through the top pipe. Does your car work differently to mine Rob...?????
Tim

>>If you run remove the expansion cap (when cold) and run the engine coolant in pumped in through the top pipe.

Using my handy Babelfish I think Tim's saying that he does see a flow of coolant into the tank ;-)

Indeed, with the PRT thermostat installed there is a significant flow of water into the expansion tank - as there is no jiggle valve in the top hose.
Dave Livingstone

Tim, if you remove the expansion cap, the tank is no longer pressurised. As the pressure in the rest of the coolant system rises through operation of the water pump, you'd expect coolant to come pouring into the expansion tank.

You'd also expect coolant to run into the expansion tank when the engine is cold: the coolant expands with increasing temperature - so this is expected.

What you shouldn't see is coolant flow once the engine is up to normal working temperature and at equilibrium.

Dave - do you see flow even when the engine is at temperature? If so that says something for the normal operation of the jiggle valves in the standard cooling system!
Rob Bell

Hi Tony,

nice to see you back from hols. We did the short weekend ride to the Alps/South-Tyrol in last week.

Images on the subject :)
Taken from a nearly new TF135 a week ago

Sensor, view from behind the boot wall direction header tank
http://www.arcor.de/palb/alben/41/1056041/1280_3136363766336637.jpg

Sensor/Connector location seen from above (high lighted section)
http://www.arcor.de/palb/alben/41/1056041/1280_3831323435626265.jpg
and
http://www.arcor.de/palb/alben/41/1056041/1280_3237623663383262.jpg
You can use it on your site if you like.

Anyway, I'd not worry about what you found strange and adjust the timer to above 12 seconds.
Cause if the water level drops whyever, then it's anyway whether you stop the engine 10 seconds earlier or later. A minute or more may be bad, though.

Regards
Dieter
PS. the picture gallery from our ride is linked to the mgfcar.de ebtry page. 200++ images of people common to you from last year... except Carlo :) You know, the yellow Elise ... see yourself. :)
Dieter

Rob,

Doh! You've been reading the Service Manual again, haven't you!

Okay, maybe I should have as well. All is now clear, but it is easy to fall into that trap when you run the engine with the tank cap off and you see coolant being 'pumped' around, even though it is coming UP the outlet pipe!

As to seeing flow into the tank when up at running temperature......well, I couldn't swear to it, but maybe...???? I'll have to look closer, now that I have a new clean and clear tank!

Yes, I had already thought of the the idea of adding a switchable secondary RC circuit to give an alternative 12 second timeout. It gives the best of both worlds.
Tony Thompson

I'm glad the babel fish is working...

Rob, your theory cannot be faulted, I'm going to go off to set up a vigil of the coolant tank to convince myself.

Tim
Tim

This thread was discussed between 16/06/2005 and 17/06/2005

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