MGC Forum

Aloha from hurricane drenched Hawaii. I received my copy of C Notes yesterday from the AMGC register and read it with my usual enthusiasm until I got to the 'Tech' article about cooling by Tim Hodgekinson. In it, he gives several warnings about overfilling the expansion tank. He claims that overfilling will cause a 'hydraulic' situation that will cause ALL the coolant to run out unto the road!! I do not buy that scenario at all, how could that happen? Did anyone else read the article? What do you think? Thank you, David Hume.

I agree that the warning about the engine going "hydraulic" seems far fetched. If the coolant is overfilled, the cap with the pressure relief on the overflow tank would relieve enough fluid to reduce the pressure then close. When the fluid cools down, air will be pulled in to replace the lost volume. That should be the end of the problem.

Thank you Bill, my thoughts exactly. Strange that there is always someone wagging their finger (perhaps with the best intentions) telling you that unless you do exactly what they say, the sky will fall on your head. Aloha David.

Tim is on here so I'm sure he will elaborate.

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Regards

Kevin

David

How about overfilling your exp tank drive around the island and report back
Whats the worse that can happen

Regards
Ray

No problem Ray H, I do it often, about 230 miles all the way around and way nicer than the M25! Come try it sometime, hardly any traffic, just empty roads and 85 degrees, mostly sunshine. How is Surrey this time of the year? I do not think that a few extra cups of water in the expansion tank would make any difference at all. Aloha!

David,

In respect to Tim's Hodgkinson detailed and in depth knowledge of the MGC, I didn't staff the article through Mark Miller, our Technical Director before I published. My bad.

I have no problem with the discussion here or on the MG Experience site and I very pleased that you started the discussion, thank you. Let's see if our collective experience stands or if a technical engineering explanation might support Tim's conclusion that over filling the expansion tank could result in a catastrophic loss of fluid.

The first lesson I learned is that our cooling systems work under pressure. Pressure is essential for proper operation and to maintain that pressure you must have a completely closed system, no leaks, no drips, no worn or perished hoses, gaskets or hose clamps.

The other lesson I took from the article is resisting the temptation to "relieve" the pressure in the expansion tank before allowing the system to return to ambient temperature. The pressure cap I have on my expansion tank has a relief lever on the cap that enables the relief of the pressure before removing the cap when the system is hot. When you relieve that pressure, before the system is cooled, you loose the advantage of the pressure stored in the expansion tank that aids the fluid returning to the system. Worse yet, if the pressure is relieved, and the cap resealed on the expansion tank, then the fluid will not flow back into the radiator as the air trapped in the expansion tank then creates a vacuum as the system cools and that works against the design that restore fluid to the radiator. Once that pressure is relieved, keep the expansion cap off until the system returns to ambient temperature.

I am glad you started the discussion.

V/R

Mark Hanna
AMGCRA Editing Director

David,

I think you are on the right track with this one.
If the 10 lb pressure is removed with the engine at operating temperature then coolant will not return to the radiator when cool and the thought about coolant loss could of started this discussion.

If the cap is removed when hot several times then all the expansion fluid and air would never return to the radiator eventually dropping the level in the radiator well below it's correct level which is to the top of the main filler cap when cold.
I have had my overflow tank well overfilled when I cannot see the indicator clearly (not full up) on occasion and it makes no difference. I usually withdraw coolant once the system is cold. I now use a torch to actually see the brass level indicator properly.

The caps clearly say to remove when cold.

Bruce.

in defense of Tim , remember that this system works because of the overflow tank , allowing for fluid to expand and contract. When cold there should be room for expansion , then your system can operate as designed if all is working as it should. But I agree , if it can build enough pressure it will purge the extra.

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Understeer is when you hit the wall with the front of the car and oversteer is when you hit the wall with the rear of the car.
Horsepower is how fast you hit the wall and torque is how far you take the wall with you

The tank is part of the closed system and needs room for building the pressure, like being the top tank of a radiator. It isn't a recovery tank like most modern cars have which isn't under pressure. The pressure is wanted to raise the boiling point around 2.5 degrees per pound so a 7 pressure cap will gain around 17 degrees. As Mark says if you open the system you will loose a lot of fluid and most likely get scalded.
If overfilled it should relieve itself until the level is correct thru the overflow tube while driving. This is why attention needs to be used when hooking up the hoses properly to the tank.

I think there are some key elements missing from Tim's article, and the use of the word "hydraulic" misleading. Siphon may be more accurate.
Firstly, pressure loss is pressure loss, and while air is compressible and liquid not, a cooling system doesn't know what is getting past the cap. The cap only knows to re seat when system pressure drops sufficiently, and that will happen, under normal circumstances, when enough liquid has escaped.

The issue with the MGC cooling system, is that the expansion tank is at the lowest point in the system, and the fill point at the highest. If the pressure cap is unseated by expanding liquid, and there is air in the system and perhaps a localized overheating has occurred, steam or expanding air pressure will continue to drive coolant out of the system until the air or steam pocket reaches the "swan neck".
Usually when most of the coolant is already gone, and the engine well overheated.

When re filling the cooling system, or topping it up, often only one short run cycle will be insufficient to expel all the air from the system. Several cycles may be required to do the job properly.

I will often top mine off at the swan neck, re fit the cap, remove the pressure cap from the expansion tank and run the engine up to normal operating temperature while watching for coolant to enter the expansion tank. If there is any air in the system this stream will "burp" until it is all expelled. The tank can then be adjusted for proper level and allowed to cool before doing a final top off at the swan neck filler, and fitting the pressure cap.

BTW, I could not get the link at the end of his article to work directly. The link takes you to a redirect page where you have to choose where to go. Using the link on the left of this page then searching for "archives" and selecting "cooling" will get you to the article I think Tim intended to reference with his link.

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Andy

NAMGBR# 20-7738

OK guys, thanks for all the input but please let me restate my beef on Tim's article.
In it he warns.... many times.... of the peril of overfilling beyond the 'MAX FILL TAB'. (I am sure Max Philltab was at my school).
The 'peril' being the total evacuation of ALL the coolant from the engine. (If I put in a little too much coolant, that causes ALL the coolant to get out??)
He says " the system has become fully hydraulic. The coolant is now being forced out of the block under pressure and ALL the coolant is lost" How Tim? You said it was a hot day Crikey! there would be not a lot MGs left if that was the case.
The pressure cap burps a little coolant, the pressure drops and the cap closes

That was strange, I got posted before I was finished.
Tim, I don't mean to slag you off, but what you say does not make any sense. In my reality, the cap vents and resets, a little coolant comes out. That's it. There is no hydraulics. The only way all the coolant is coming out is if you have already blown the head gasket!
And for what it's worth, I modified my first MGC in 1985 to run with the pressure cap in the 'swan neck'..( I was young, didn't know any better and I worked in a radiator shop) and it ran like that for years!!

I haven't seen this article so my comments aren't directed to it. I'm just saying the system on the C with the tank is not an expansion type tank system, it is part of the closed system and the overflow is directed to the ground.
Everyone has had experiences with their cars and maybe this is one that is unique to the author of the article.

Good to hear from you Dave! I was beginning to think you were target by a bevy of beautiful Virgins and sacrified to the Vulcan Gods in Hawaii!

I did not read the article but the suggestion sounds baseless. A lot of good commentary here, but having an overfilled relief tank suck away all the coolant isn't logical by any measure. On the other hand, not having the correct lid, NA spec is higher than UK and Bruce's vehicle, or putting a blanking cap on the tank and pressure cap on the thermostat housing could create issues of over heating for sure.

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Regards,
Robert Kirk

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