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MG MGB Technical - Radiator Pressure Quandry

Stock 18 GA engine with Standard 180* Thermostat and a 7 lb radiator cap.

Engine runs at a normal temp, but after stopping, temp goes up and there is always coolant that's pushed out down the tube, leaving a puddle of 3-5 ounces.

Wondering if thermostat is bad;
Radiator cap was recently replaced -- would going to a 10 lb radiator cap do any harm?

Just want to fix the over-flow problem. Not sure if the present cap is properly working at 7 lbs.

Any thoughts, suggestions greatly appreciated
G L Gimbut

GL - See the article, Expansion Tanks, Coolant Recovery Systems, and How the Cooling System Works in the Other Tech Articles on my web site at: http://homepages.donobi.net/sufuelpumps/ for an explanation of what is happening. In a nut shell, your cooling system is operating exactly the way it was designed (and how all cooling systems are designed, except later systems have extra parts that are explained in the article referenced above. As the car is driven and the cooling system heats up, the coolant expands. When the engine is shut down, the coolant is heated even more from the engine's heat, but there is no air flow to cool the coolant, so it expands even further, causing the pressure of the system to exceed the pressure of the spring in the radiator cap. This causes the primary seal of the pressure cap to lift and allow the excess coolant (expanded due to the rise in temperature) to escape out the overflow tube onto the ground. Being a dutiful MG owner, you check the coolant level the next time you are about to drive and find the coolant level below the top of the filler tube as you had left it the last time you drove, so you add more coolant and go on your drive, only to have the whole process repeated. This is normal (laws of physics), not a conspiracy by Preston to sell more antifreeze. If you refrain from refilling the radiator at each cycle, the coolant will reach its own level and stop pissing coolant on the ground. The above article also tells how to make and install a coolant recovery system that will catch the expelled coolant, and return it to the radiator when everything cools. Cheers - Dave

David DuBois

To add to Dave's wonderful description...I added a coolant recovery system...haven't had a puddle of coolant under the car since...even when i was in Boise ID with temps in the triple digits & 5000' altitude...
Bob Dougherty

David- Thank you so much for directing me to your informative site. It is comrehensive and well written. Since it's so late in the driving season, I'll probably just install an over flow tank, then, over the winter waltz about to make a recovery system.
Bob - Your comment provides the incentive to make the alteration.

Would still like to hear of other experiences and tales from others with the same over-flow situation. I'm always amazed by the ingenuity and resourcefulness of people in the LBC hobby.
G L Gimbut

The relative density of water at 4degC is 1, at 100degC it is 0.97. So if there's 12 (imperial) pints in the system when it was filled at 4degC then there's going to be about 12.3 pints at 100degC - ie a teacup more.

But filling right to the bottom of the neck in the radiator will result in much more than that being pushed out - presumably due to localised boiling which generates steam, and steam is much, much less dense than water so takes up much more space for the same weight.

David mentions the laws of physics - by which I he means the law of conservation of mass which says that mass of water in the cooling system must be the same unless you physically lose some out of the system. So the mass of water and steam in the closed system must stay the same but the volume has risen, water is effectively incompressible so the pressure rises, opens the pressure cap, and hey presto something comes out of the overflow pipe. That something is predominantly water.

I've got a two litre catch tank (a milk carton) and my tuned GT engine was filling that after a 10 lap run at Castle Combe on Monday. First a big slug of water came out then, presumably as there was less water at the top of the radiator, water and bubbles came through, then just bubbles. Then most of it was sucked back in as the engine cooled. The rest got slurped up as the engine was restarted.

HTH,

Neil
Neil

On my 73 B, to make the overflow tank, does one need a different radiator cap that would more readily allow the excess coolant to be sucked back into the system? Or does the standard one work?
Erick Vesterback

Maybe you are just putting too much coolant in. It will always heat up and expand when you turn off a fully hot engine, and any excess will come out of the overflow. A weak cap (regardless of what markings it has on it) will allow this to happen at lower temperatures and pressures than a good cap. There are also a number of engine and cooling system problems that can cause it with a *good* cap. You shouldn't need a coolant recovery system, if it is taking up coolant when you switch off then even though it may be going back into the radiator as it cools, it still means you are overfilling the radiator or there is a problem.
Paul Hunt

You will need a different cap. The standard cap only maintains the pressure in the radiator at 7lbs. When installing a recovery system a cap with a second seal at the top of the radiator cap is required. RAY '67B
RAY

If the coolant level in the radiator when cold is higher then the top of the tubes in the core by at least 1/2", you have enough coolant.
John H

No different cap. Just dangle the overflow tube in the milk carton. There's a device already on the cap to let the air back in, so water comes back in. I'll take a photo tomorrow.
N
Neil

Neil, There are two slightly different caps in use today. The standard B cap, on nonoverflow equipped cars, has one seal at its base that is spring loaded to 7lbs. The closed system caps employ a second seal at the top of the cap where it rests on the radiator lip. This allows for coolant to be drawn back in from the overflow container and hose instead of drawing in air from the atmosphere. RAY
RAY

Ray - in the UK the only cap for the MGB seems to be the dual-valve radiator cap that allows coolant recovery from a non-pressurised catch-bottle. Certainly that on my 73 roadster which was old when I got the car 20 years ago is like that, and for a while I did have coolant recovery on my V8 with the remote reservoir. All MGBs used a fully pressurised system, including those from 77 onwards and V8s with the remote reservoir, and so they all use the same cap, with the overflow tube coming off the remote tank instead of the radiator filler neck. Later practice, like on my 89 Celica, used an unpressurised reservoir, but it *still* uses the same type of radiator cap. It seems to be America where there are two types of cap, one which allows coolant recovery from an unpressurised catch-bottle and one that doesn't. I don't know whether you in the USA can get a radiator cap for the MGB that allows coolant recovery, but it seems to be the only one available in the UK.
Paul Hunt

Paul, Indeed you can get both variations of the pressure caps for earlier ( pre '68) cars. Until '67 thr radiator was vented to atmosphere via the overflow tube. When the engine cooled the cap would allow the radiator to draw air back in through the unsealed space between the cap and radiator neck. To this point the standard cap was 7lbs. Later MG increased the pressure over the years to increase the boiling point and added the coolant recovery system. I've owned my '67 Roadster since '72 and the cooling system, while reliable under most conditions, does get a bit overwhelmed on near 100 degree days. However, I am running a Moss supercharger and this does not help the situation. RAY
RAY

Hi
Just to add another variable--There are several different lengths of radiator caps available-- Have you got the correct one--If you have one that is too short it will not hold pressure properly and will allow the coolant to escape especially after heat soak when the coolant expands. Willy
WilliamRevit

Would replacing my present 7 lb cap with a 10 lb cap cut dow, or even eliminate the over flow? I know the factory/works went up to a 10 lb cap after the 18GA engine.

This may be a silly question, but is there any harm that the 10 lb cap could do the the cooling system?
G L Gimbut

The only problem that you are apt to encounter is that when you raise the cooling system pressure the weaklest link in the system will show itself. After a lifetime of running at 7lbs. a heater core, radiator, water pump or hose might start to fail under the increased pressure. RAY
RAY

Ray - like I said I can only speak for what is available in the UK. The 7lb caps here are to the same design as the later 10, 13 and 15lb caps i.e. they include the upper seal to the outer filler neck, and the low-pressure valve in the main seal. I'd be interested in getting a photo of a radiator cap for an MGB that doesn't have these, for my web site. When the radiator changed in 67 the filler neck moved position, but the design remained the same. The upper seal reduces the chance of a sudden boiling or venting scalding anyone working nearby. The low-pressure seal allows the cooling system to return to at or above atmospheric pressure when cool. If there had been coolant or pressure loss when hot then when it fully cooled there would be negative pressure in the cooling system, which would make boiling more likely next time it ran. It is serendipity that allows the upper seal and low-pressure valve to be used for a lost-coolant recovery system, MG never implemented one.

The problem with the Mk1 radiator is that you can't see the tops of the tubes, so you have to keep coolant visible in the filler neck to be sure you have enough, and it is easy to overfill which means the excess is simply pushed out the next time it runs, and you are back where you started. A catch bottle would avoid this, but only if you have the UK type cap. Checking the coolant level while the engine is *warm* rather than cold may make a catch-bottle uneccessary - as long as you can see coolant then it doesn't matter if it vanishes when fully cold. Also the system shouldn't lose coolant unless it is overfilled or faulty, except possibly in very hot conditions at high loads. The MGB ran at 10, 13 and 15lb on the Mk2 radiator, 10lb shouldn't be a problem on a Mk1, if it is then the system is probably leaking anyway, which *is* going to be causing coolant loss if not boiling. I ran my V8 at 20lb for a while when diagnosing a cooling system problem. All was well for several months (it was a long diagnosis) until shortly after switching it off one day the bottom hose exploded in spectacular fashion! This was plain rubber with no reinforcing, so not the strongest, and the rubber hoses have been pretty poor quality for some time. When the problem was resolved I measured the pressures at 3psi in normal running, only getting up to 11psi when the electric fans cut in by which time the temp gauge is mid-way between N and H, and dropping to 6psi when they cut back out but the temp gauge is still a little above N.
Paul Hunt

Paul, The non-overflow caps I have used simply have a thin metal diaphram at the uppermost point where they " seal" the radiator upper elbow to the cap. I haven't been able to use an overflow system because the radiator shop that recored my radiator mutilated the upper area where the cap would form its seal. The point I was making about changing to a higher pressure cap is that we are dealing with 30+ year old components which may or may not have been maintained over this time period and may fail when subjected to higher pressures. Ray
RAY

RAY, most radiator shops should be able to replace the filler neck on your radiator, maybe not the one that screwed yours up, tho. I would think it's a job that's possible with the rad still in the car, too.
Tom

The rubber hoses were replaced with the re-inforced style -- the ones with some kind of webbing material. The radiador is OE -- that came with the car before rebuilding it. Water pump is new.

My biggest fear is blowing out the radiator by going to a 10 lb cap. That's probably an unwarranted fear, but it's there just the same.

Did the factory go to the 10 lb cooling system because of over flow problems with the 7 lb cap? That the engine was just running too hot? And, how common is this problem - over-flowing with a 7 lb cap.

Would using a radiator flush to clean out the cores in the radiator be advisable -- if so, does the entire cooling system need to be flushed out with clean water before anti-freeze for the winter?

Just stuck on a simple problem. Your posts and comments are greatly appreciated.
G L Gimbut

The cap pressure was raised to 10lbs. to raise the boiling point of the coolant. This was a common practice in the 70's as increased emission standards were introduced. The hotter running engines put out less pollutants and this was an easy fix. The problem you face, though probably minor, is that the heater core, which is most likely original, is the weak link in the system. After all this time increasing the pressure on it could cause it to begin leaking. This happened to me and the heater core is a bear to replace. I've been running the 7lb. cap on my 67 B for many years and though seeing some high temps on the gauge under extreme conditions have yet to overheat. RAY
RAY

Subsequently it was increased to 13 and then 15, so 10 shouldn't be a problem. Whilst I have read of the occasional heater core leak, it certainly isn't a problem that afflicts the later cars with 15lb caps.

But in your original post you wonder if your 7lb cap is working properly. A friends 80 MGB with a 15lb cap used to suffer from the same thing - vomiting shortly after switch-off. I pressure-tested that at about 4 or 5 lb! A new cap solved the problem, so maybe a new 7lb cap should be the first step. However when I bought a new cap for the V8, again 15lb, it seemed to go on much easier than the old one. I pressed the two caps together seal to seal to compare the spring tension and the *new* cap moved before the old one, i.e. it had an incorrect (too weak) spring. The springs are colour-coded so that they should be used with the correct cap, but obviously mistakes can happen.
Paul Hunt

Thanks to all for your advice and thoughts.

With your help, I'm sure the problem can be fixed.
G L Gimbut

A while back I wrote this article and parts of it are applicable to your problem.

Cooling Systems Problems? - Possible Solutions
MG “T” Series, MGAs and early MGBs

MGs in general are notorious for having cooling system problems. The original design was, at best, marginal, even for the cooler climates of the United Kingdom. These cars when driven in the southern third of America or in modern day traffic, anywhere, will likely experience overheating problems. So, what can be done to improve the overall cooling efficiency of the system? Before you begin to make improvements you must first verify that car’s radiator is clean, the hoses are leak free, the fan belt is tight, all ensuring that the stock cooling system is operating at maximum efficiency. Additionally, be sure that the timing and carburetor air/fuel ratio are set correctly as these too, will influence engine-operating temperatures. The improvements that can be made are as follows:

1. The most effective, but most costly, modification is to re-core the radiator with a modern high-efficiency 3-row tube design core replacing the stock two-row design. This greatly improves heat transfer and will, in itself, likely solve your overheating problems. Most radiator shops can re-core the “T” Series radiators. The downside is that it will require the removal of the radiator and will cost upwards of $400.
2. If your car’s only problem is overheating when operating in slow or in bumper–to-bumper traffic but otherwise okay, then a replacement fan may solve your problem. My understanding is that the 7-blade plastic fan (Moss p/n 434-340 - $76.46 at LBC) that is used on later model MGBs will fit the “T” Series cars. I do not have any personal experience with this fan, but it appears to be a sound approach to solving this specific problem.
3. The most cost-effective modification that you can make is to convert the current “open-to-atmosphere” cooling system to a pressurized coolant recovery system. Please see Building a Pressurized Coolant Recovery System” below. Water boils at 212 deg. F or 100 deg. C (Higher, with antifreeze added, but for the purpose of this paper, we will assume that water and the water/antifreeze mix coolant boils at the same temperature).

If, under all normally encountered driving conditions, your car operates at less than 212-degrees F, the open system is just fine. However, if on occasion, you exceed 212 degrees by even a small amount, the coolant in your radiator will boil, the steam generated will vent to atmosphere and when the engine cools it will contain less coolant and, hence, less cooling capacity. As this cycle is repeated, boiling will occur earlier in the cycle and with a greater coolant loss. If you do not top-up your radiator at frequent intervals, overheating will surely occur.

In an even worst scenario, assume that your cooling system is marginal for your car’s typical operating conditions. On a very hot day, you are driving on the freeway at 70 MPH and the coolant temperature increases to 212 degrees. The coolant will begin to boil, and steam will be created. Steam, being a poor heat transfer medium will exacerbate the cooling problem and, to make matters worse, the steam generated will be vented, resulting in less available coolant and a lower radiator cooling capacity. If the car is operated in this high speed, high ambient temperature, steady state condition, engine-operating temperatures will continuously increase and continuous coolant loss will occur. Unless you notice that the car is overheating and pull over to let that engine cool you will shortly exceed the critical temperature and the engine will seize. A very costly repair!!!

How can a pressurized coolant recovery system help? This type of system, in use since the 1960s, performs two functions. First, it pressurizes the system and second it allows for coolant that would normally be lost to be recovered. By pressurizing the system, to say 7-PSI to13-PSI (typical radiator cap ratings) above atmospheric pressure, the water’s boiling point will be increased to between 233 deg F to 246 deg. F. Additionally, when starting a cold engine and as it warms and reaches its normal operating temperature, the coolant in the radiator will expand. The expanded coolant will create positive pressure in the radiator and open the radiator cap’s bypass valve and coolant will flow into the car’s radiator coolant expansion tank (aka - radiator overflow tank). Later, when the engine is shut down and allowed to cools, a strong vacuum will develop in the radiator that will open the secondary bypass valve in the radiator cap. This permits coolant to be sucked back into the radiator, thus replenishing the coolant level. This cycle is repeated trip after trip.

With occasional minor overheating, the increased boiling point of the coolant will eliminate the generation of steam and, hence, minimize permanent coolant loss. The result is less frequent radiator top-ups and the reduced probability of future overheating problems.

If a temporary problem is present or if ambient temperatures are extremely high (e.g. a worst case scenario), the pressurized system will allow the engine to operate and an elevated temperature above the coolant’s normal boiling point. If the engine can reach an equilibrium temperature of 220 deg F or less, the engine will operate without catastrophic damage. Ideally, an engine should operate in a temperature range of 180 to 200 degrees. The occasional operation of up to 220 degrees is not desirable, but a far better alternative to overheating and an engine seize up.

4. To help detect a real-time overheating situation, you should consider adding a visual and/or audible alarm system. On my TD, the radiator has a 3/8-inch NPT (tapered) female fitting installed on the engine side or the radiator. I used this fitting and installed a temperature sensor normally used operate a modern-day temperature controlled, electric cooling fan. The sensor has an internal switch that closes at 204 deg. F. The sensor is wired to a dash light and a mutable alarm. If I am not attentive to the temperature gauge, the high-pitched alarm will certainly correct that situation. There are, of course, other ways to accomplish this idea. The objective is to install a sensor/switch in the cooling system and wire it to at, least, an audible alarm.

The temperature sensor switch that I used is NAPA P/N FS116 - $34.99. This switch has a 3/8-inch straight thread. It works, but you will be better off if you can find a switch with a tapered pipe thread. The mutable piezo alarm that I used was purchased from www.floydbell.com , P/N OC09-530-QO – $28.42.

(PS – I installed one of these alarms in parallel with the low fuel level warning light. I now have an audible and visual warning when the fuel level is low)

5. One last suggestion; there is a product called Water Wetter, made by Redline and available at Advanced Auto, Amazon.com and other places. The manufacturer claims that, when added to your radiator, your car will run up to 30 degrees F cooler. I don’t believe 30 degrees, but online testimonials claim actual cooler temperatures. It is most effective when used with water only (e.g. no antifreeze). It is supposed to contain anti-corrosive and water pump lubrication agents. I have no personal experience with Water Wetter, but for under $10.00, it may be worth a try.



Building a Pressurized Coolant Recovery System

The design objective is to construct a small sealed chamber that can be pressurized by the stock cooling system. This pressurized chamber will act, in many ways, like a miniature radiator. Also included is a radiator coolant expansion tank that will store the overflow coolant.

The items required are as follows:

1. Two 2-inch copper plumbing end caps.
2. 3-inch section of 2-inch copper pipe. (Items 1 and 2 are used to make the pressurized chamber and can be purchased at a commercial plumbing supply house.)
3. Standard, small or large size, new or used brass filler-neck for a copper/brass radiator.
4. Mounting bracket made from a 1”x3” strip of brass or steel sheet metal with screw mounting holes drilled about 1/4-inch from each end.
5. 7-PSI to13-psi radiator cap, sized to fit the neck.
6. New or used small size coolant expansion tank. (A junk yard is a good source)
7. 2-inch length of 5/16”-copper tubing.
8. Two lengths of 5/16”-rubber fuel line (length to fit) and 4-hose clamps and a length of rubber tubing for the expansion tank’s outlet to atmosphere


To construct…

1. Drill a hole, with a size that matches the diameter of the radiator’s brass filler-neck (item3) in the flat end of one of the 2-inch copper end caps. (A spade-type wood boring drill bit will work)
2. Place the end caps over the copper pipe and solder together.
3. Drill a 5/16” hole in the side of the end cap without the hole for the radiator neck, about a ½-inch from the end. (This assumes that you will be mounting the chamber upright. If you are going to mount the chamber on its side, drill the hole in the bottom of the end cap)
4. Insert the 5/16-copper tubing (item 7) in the hole about half way and solder in place.
5. If mounting upright, solder the sheet metal mounting bracket (item 4) to the bottom of the chamber and centered on the end cap.
6. Insert the radiator neck (item 3) through the hole in the top end cap and solder in place.

The pressure chamber is now complete.

Assemble as follows:

1. Mount the pressure chamber at a convenient place in the engine compartment. I mounted mine on the topside of the passenger side frame rail.
2. Mount the expansion tank in an inconspicuous spot. On the front and right side of the toolbox, the backside of the frame cross member, just below the radiator are good places.
3. Using an appropriate length of rubber fuel line, connect the end of the radiator’s stock 5/16” overflow pipe to the 5/16” copper tubing installed in step 4 above. Secure in place with hose clamps.
4. Connect the Expansion tank to the 5/16-inch inlet/outlet pipe on the radiator neck (item 3 above) using the appropriate length of fuel line and secure with hose clamps.
5. Fill the expansion tank to the cool level mark. Fill the radiator to the bottom of the filler neck. Screw the original chrome radiator cap onto the radiator and the new 7-PSI or 13-psi radiator cap onto the copper pressure chamber.


You’re Done!!! Happy Motoring!!!

PS – If you have any questions, please call me at 843-838-0822 – Frank Grimaldi
Frank Grimaldi

With a modern cap you don't need to go to the bother and expense of a pressured expansion tank, just a plastic catch bottle with the end of the overflow pipe dangling in the bottom will be fine.
Paul Hunt

Paul is correct. The double seal caps are readily available. You can scarf the end (cut it at an angle) of the tube to assure that it does a good job of siphoning. I used a 'Handi-Wipes' dispenser. the opening for the wipes was perfect to hold the tube in place. See image.

warmly,
dave

Dave Braun

This thread was discussed between 12/09/2008 and 23/09/2008

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