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MG MGA - Coolant temperature
|This week I had a leak from the bottom hose of the radiator that drained away a lot of the coolant that I did not detect. I first noticed it when the the temp gauge went off the scale past the 230 mark. Having replenished the coolant to get home I drained the radiator and replaced the hose. I normally run with 25% antifreeze but this time I refilled with plain rainwater from a water butt. What surprises me is that my engine now runs at least 10 degrees lower temp from its usual 190 degrees down to 180 degrees. Why should this be so? Is it that the thermal conductivity of water is better than antifreeze or has the high temperature water boiling disturbed something such as the stat or temp gauge?|
|J H Cole|
|Maybe the boiling process washed out some of the crud that was partially blocking the flow?|
|Ah, a technical question about coolant mixture. The short answer is, I don't know why you see a 10dF drop of coolant temperature, because changing from 50/50 coolant mix to plain water should not make that much difference. I can tell you exactly what the difference should be (if you have enough patience to tolerate the explanation). And I suppose this will have to make a new web page.|
The Constant pressure heat capacity of ethylene glycol is 2.42 J/g K.
The Constant pressure heat capacity of water is about 4.20 J/g K.
Specific heat of water is defined as 1.0 as the baseline for the specific heat unit.
Specific gravity of water is defined as 1.0 as the baseline for the specific gravity unit.
Ethylene glycol is about 10% heavier than water (depending on operating temperature).
So specific gravity of ethylene glycol is 1.10.
This makes specific heat for ethylene-glycol to be 2.42/4.20x1.10 = 0.634.
This means that ethylene glycol (common antifreeze) carries 37% less heat per unit volume.
This is not nearly as bad as it might sound.
When engine coolant is a 50/50 mix (by volume) of ethylene-glycol in water, the resulting specific heat of the mixed fluid will be about 0.82. For this fluid to carry away the same amount of waste heat from the engine, the temperature rise in the engine and temperature fall in the radiator must be 1/0.82 = 1.22, or 22% greater change of fluid temperature. Again, this also is not nearly as bad as you might think.
This does not mean that the radiator needs to be larger. Cooling capacity of the radiator depends on heat transfer from internal fluid to external air. This heat transfer function through the radiator wall from liquid to air is similar regardless (almost) of what fluid is inside the radiator. Also the radiator will need to dispose of the same amount of heat, regardless of what fluid is inside. As the fluid temperature increases, the external temperature difference increases, and the radiator becomes more efficient, requiring slightly less temperature rise to dispose of the same amount of heat.
As a starting baseline with plain water in the cooling system, assume temperature is 190dF at top of radiator core and 160dF at bottom of core for a 30dF temperature difference (temperature rise in engine and temperature drop in radiator). Also assume that ambient air temperature is 100dF, and that the thermostat (perhaps a 180dF opening thermostat) is wide open (no restriction to flow). Also assume the entire system is in a condition of dynamic stability with temperatures neither rising or falling with time. That is, coolant temperature has risen and stabilized at 190dF (gauge reading near top radiator hose) to carry away all waste heat as required.
Then change the internal fluid from plain water to 50/50 antifreeze mix. Without changing coolant flow or air flow, temperature rise in the engine will then need to be 1.22x30 = 36.6dF, so you might "expect" to see a 6.6dF increase of coolant temperature near the engine fluid outlet (thermostat housing or top of radiator). But this is not what happens.
Keep in mind that total waste heat is still the same, and ambient air temperature and air flow has not changed, so the difference between air temperature and radiator average core temperature must also be unchanged. The trick here is that with higher temperature at top of the core heat transfer to air is increased slightly, so more heat is transferred from the top half of the core. This leaves less waste heat needing to be transferred from the bottom half of the core, so the lower core temperature will be slightly lower, and fluid outlet temperature will be lower. This may at first seem to be counter-intuitive, but that is what actually happens.
In the end, for the required 6.6dF increase in fluid temperature "difference" between inlet and outlet, temperature will be about 3.4dF higher at top of the radiator (as seen on the gauge), and about 3.2dF lower temperature at bottom of the radiator (in the lower radiator hose). So the end result of change from water to 50/50 antifreeze is about 3.4dF increase of temperature as seen on the gauge. Not so bad, huh?
In another scenario, if the system is over-cooled, or if ambient air temperature is lower, it may operate with the thermostat only partly open to restrict fluid flow (in order to maintain proper minimum engine operating temperature). In this case when you change from water to 50/50 antifreeze mix, the thermostat will open more to allow more fluid flow. When fluid flow through the radiator increases about 20% it will carry away the required amount of waste heat with very little change of fluid temperature, perhaps only 1 or 2dF change showing on the gauge. The condition of greater temperature change on the gauge (due to fluid change) only happens when the system is at full dynamic heat transfer capacity with the thermostat wide open.
So for the question, I don't know why the fluid temperature might be 10dF lower after you changed to plain water. I suspect we don't have complete information about other changes of conditions. Maybe ambient air temperature dropped. Maybe you blew some bugs off of the radiator core to increase air flow. Maybe you had coolant on the fan belt causing slippage and low fan speed before the fluid change. Anyone else want to offer some theories?
|Impressive Barney - I need time to take this on board, I did some of this stuff 40 years ago. A few other points: my coolant also had 'water wetter' in it but I don't think its absence could have caused the temp drop - if anything a rise. I wondered on a pragmatic basis whether pushing the temp needle off the scale might exceed some elastic limit - is it a bellows type gauge?|
|J H Cole|
I expect your hose leak helped shift some crud languishing in the bottom hose area, now you have a better flow rate?
|Internal mechanism of the temperature gauge is actually a pressure gauge, nearly identical to the oil pressure gauge. This is why the analog temperature gauge is often in the same instrument housing with the analog oil pressure gauge. See here: http://mgaguru.com/mgtech/dash/dt105.htm|
The gauge operating device is a Bordon tube. The flattened tube formed in a slight arc will expand (puff up thicker) when it is pressurized. Due to difference if inside and outside radius, this will cause the arc to "uncoil" slightly. Resulting motion of the free end of the coil is linked to drive the indicator needle.
Pressure to drive the temperature gauge is derived from a volatile fluid (ether) that boils to develop vapor pressure when heated. The oil pressure gauge goes to 100 psi while the water temperature gauge goes to 230dF (also 100 psi inside the tube).
This device is very tolerant of slight overpressure, so it can be driven past end of scale with no damage. If it was driven to perhaps 150 psi or higher it might stretch the tube beyond the elastic limit point where it would not return to the original point when relaxed. This would make the gauge read higher, not lower.
|If the ether in the temperature is all vapour, then stretching the tube lowers the pressure a little (but I'm not sure that it would be noticeable). |
If there is still liquid ether in the tube, then the vapour pressure is unchanged whatever the size of the tube and bulb.
|That is correct. There is liquid ether in the thermal bulb on the front end of the pipe. As it heats up the fluid boils, and some of it changes from liquid to gas. When pressure matches the driving temperature, the fluid stops boiling and retains a static pressure (sort of).|
In reality the process is more dynamic. Fluid in the tube is constantly cooling and condensing while fluid in the bulb continues to boil enough to replenish the pressurized gas, and the system as a whole maintains constant pressure and a constant amount of liquid and gas at any given temperature. With increasing temperature the fluid boils more to create higher vapor pressure.
The system always has some liquid ether in the bulb to boil up and maintain the vapor pressure. If it ever springs a leak, and enough of the fluid escapes to reduce amount of liquid to nil, then pressure drops and the gauge stops working.
|I have two 1600 MGA's (coupe and roadster) although I have owned six in all. Having had quite a few cooling problems, I now have the radiators rodded out and cleaned about every five years. I get the bottom tank removed so that my (pristine) top tank is not dis- figured. This seems to overcome the problems, probably brought about by poor airflow through the core. In competition the factory removed a few grille slats (even with a new, or at least good, core!) and MGB's had an air dam in front of the grille.|
|Despite the presence of anti freeze the water in my radiator was rust coloured. When I drained the water from the radiator it was slightly disturbing to see that a red sediment quickly formed in the bottom of the tray. This I take it is rust corrosion from the internal waterways that I had hoped the antifreeze would stop. I regard this corrosion as a 'ticking' time bomb that is likely to cause problems in the long term. I had hoped that once the available oxygen in the water was converted to iron oxide no more corrosion would take place but am not sure if this is the case. Is it still worth using rainwater instead of tapwater or if it can be afforded distilled water? A recent contribution raised the issue of a non water based coolant but it seemed a rather exotic solution to me. Does anyone know about the anti-corrosion additive that is put in domestic heating systems using wet radiators?|
|J H Cole|
|I agree with Barney that high temperature won't affect the temp gauge accuracy. My experience of this was arriving via an awful traffic jam in the centre of Le Mans to be in the 'Parade des Pilotes' on a sweltering Friday afternoon before the 24 hour race with dreadful vapour-lock going on and the gauge at 230degF. On switching off the gauge went up to 100psi on the oil gauge!!! ...and then dumped loads of coolant out of the overflow. It was later topped up from a drinking bottle and subsequently the temp gauge carried on giving the same sort of readings it always had done.|
|P N Tipping|
|Engine over-temp can have a negative impact on the oil viscosity, all motor oils have a temperature breakdown point. Many years ago the thermostat failed on the freeway and I over temped my '67 GTO. After prying open the thermostat with a screwdriver to get home, checking coolant, letting it cool, the oil pressure read and stayed low upon starting. I had it towed home and drained the oil. It came out with all the viscosity of water. We changed to Rod and Main bearings and there was a slight scuffing of the bearings. This was many years ago and I expect (hope) oils have changed and improved, but it may be a good idea to check the condition of the oil when over-temp happens, preferably before re-starting. The short term high oil pressure could be an indication of this condition. Old electronics saying: when in doubt, change it out! Most of us that grew up with cars of this era develop the (paranoid?) habit of watching the oil pressure gauge with hand to key upon starting anyway. Every time! Good habits. Russ|
(Started out to make a comment or two, wrote a textbook! Should be useful.)
I've often wondered why some cars show red water disease (and other corrosion effects), while others do not, even when conditions appear similar. It represents some complex chemistry, created by multiple factors. It would seem to be pretty specific, since most cars do not show it nowadays. So, I have a sort of running mental file on the subject.
"Red iron" is a particular state of iron oxidation; other states are oxides but not red. While it requires oxygen, it doesn't take much, and the amounts required are easily sucked into the system by normal expansion/contraction as temperature varies, or even by diffusion through hoses etc.
"red iron" is generally formed in an acid solution with high oxygen content; acid mine drainage is the most spectacular example.
A sealed recovery system will help reduce oxygen input, but the main benefit is to reduce addition of "new" liquid. If the older non-recovery system is at the correct level, expansion space in the top tank contains air, but the liquid goes about its business below the air, with little turbulence. However, if the system is low, air is whipped into the coolant, so the recovery system helps here too.
Modern coolants have additive packages that deal with "most" "average" conditions. "Average" conditions are constantly changing with material changes and environmental understanding. If your conditions are unusual, it might not work. Additives wear out chemically, hence the need to supplement and/or change periodically, and this interval will depend on all the factors discussed here. And should you have a "source" which is a large scale user of coolant, be aware that they sometimes create their own additive packages based on testing; these may be very specific to some condition you do not have. I discovered that there is a lot of testing done by fleets that can make or break an operation - orders of magnitude differences in cooling system life and maintenance costs. Heavy diesel trucks have water filters with additive packages which get changed, and additive supplements added, either by conservative (expensive) schedule or ideally by testing - system life may vary from short (months) to effectively infinite if it is correct.
You nay be able to find test strips for various coolant checks from industrial engine places, and maybe increasingly in automotive service suppliers - certainly PH tests alone would tell you if things are horrid; you'd want slightly alkaline, but I can't recall the specifics - maybe PH7-8.
Pure water is a very chemically active and pretty good solvent for all sorts of stuff, including especially metals. In some circles, this is called "mineral hungry" - it WILL dissolve SOMETHING. So, it is wise to feed it something you can afford, rather than your engine & rad. Don't use distilled water, unless you have the right additives.
Rainwater may be far worse, partly depending on how and when/where you collect it. Acid rain has been measured at PH around battery electrolyte, downwind of power plants after a long dry spell; I have seen limestone pavement fizzing like AlkaSeltzer at the beginning of rain storms. Near the sea, salt levels can be extreme. Ferric chloride is used as a very effective etchant in many processes, All of these are made worse if collection is off a roof that has not been thoroughly washed clean by heavy clean rain; places where potable water is collected off roofs typically say a half hour of heavy rain is left for washing before collection starts, but those numbers were from before heavy industrialization.
"Tap water" can be very good or moderately bad. Calcium/magnesium are good anti corrosion additives, and "in process" steel parts are sometimes kept in long term storage in lime water. Too much of these can result in scale, a topic of great importance in the industrial boiler field. If your tea kettle and coffee machines don't scale up too quickly, you are not badly off. I have a tea kettle that has not been cleaned in over 20 years and has only enough scale to discolour it a bit.
The biggest problem with tap water may be chlorine, in a municipal supply, or salt plus lack of minerals in softened water systems.
I LOVE my well!
Combustion leaks, however small, introduce all sorts of contaminants into the cooling system. Mostly you are worried about acidic gases lowering the PH of the coolant.
Sludged up areas cause anaerobic corrosion of ferrous alloys. This is the black oxide form of iron, which is known to eat right through areas where coolant flow is blocked, especially low core plugs, and the cast iron around them - it will eat right through solid cast iron, leaving it looking fine but dark - trouble is it is all crumbly. Same stuff is the black part of the heavy rust that once was your frame or sills.
Forms of anaerobic corrosion also affect other metals; "crevice corrosion" is the kind that eats the ends off stainless steel brake lines; it is caused by low oxygen and a bit of electrolyte, as salt. This also does things like eat the tubes of condensers from inside.
All the above relates to chemical reactions between different materials in electrolytic solutions - it is all "galvanic electrolysis". This can be on the scale of one iron atom next to one carbon atom in the case of the anaerobic black death of cast iron I mentioned, or it can be the brass /copper rad versus the iron block, or an aluminum rad with a copper heater and an iron block with acid rainwater.
You can then superimpose an overall electrical gradient across this mess by having stray currents from the electrical system floating around. The prime thing here is bad earthing of components.
Your engine can wind up earthing alternator/dynamo or ignition current thought the coolant to the radiator mountings. The results may be unpredictable but the following should be informative:
"From David Read, South Oz
BTW, on a ground related note ... I recently replaced an aluminium cored radiator with plastic tanks in my daughter's car.
The installation instructions with the new OEM rad. were very specific.
Connect a digital voltmeter between battery ground point and the coolant.
If the meter reads in excess of 50mV, go look for the poor ground and fix it otherwise there was no warranty! "
|On rereading the entire thread, it seems that Barney and I have created a pretty good treatise on the subject.|
One point apropos to your inquiry is this:
It is possible that having both somewhat overheated and run a least part of the system exposed to steam instead of liquid, you have dislodged a certain amount of normal surface rust coloured scale, that has now been drained out into your bucket. That could be a one time thing. I have found that even a few hours of plain water in a system will produce red iron water sometimes, variability likely to do with surface conditions in the system.
I will note that I have not ever had to "rod out" a rad that had been in my care for any amount of time, up to at least 30 years. And I have replaced one core personally, on a truck (73 Chev)I had just bought, with a modern paper thin rad. I have another similar truck (89 Dodge) now that could use one, but never an old Britcar, some of which had half the fins missing from external chloride corrosion.
In one of your above paragraphs, you caution against using distilled water unless etc. Could you please elaborate on this subject.
|George Raham [TD4224]|
|I would never put distilled water in the cooling system. Distilled water has a great affinity for absorbing things. It can absorb copper from the inside of the radiator, iron from inside of the engine water jacket, aluminum from inside the heater valve (or an alloy bodied water pump). Soft water can be corrosive, while some well water may contain enough minerals to build water stone in the system.|
It's all about alkalinity and acidity. Alkalinity should be between 80-120 ppm, and should be "adjusted" first. Ph is measured on a scale 1-14. Desired Ph level is 7.2-7.8. Lower than 7.2 can dissolve metals. Higher the 7.8 can deposit minerals.
You can read up on hot tub (spa) and swimming pool water treatment, and the chemical jargon can drive you batty. I installed a hot tub about 20 years ago, and made the mistake of filling it with water from the water softener. I then spend a week adding chemicals to increase alkalinity, and then adjust Ph level. I would have been better off starting with the mineral laden well water.
If you don't want to be guessing or constantly measuring the chemical properties of your coolant, then find some well water that is good enough to drink. This has already absorbed enough minerals to be pretty neutral, not wanting to absorb anything more. Put that in your radiator along with minimum 30% antifreeze solution. Without antifreeze you need some corrosion inhibitor package, and the instructions for those will drive you nuts as well (if you ever read it). If the well water looks clean and clear, it's just the ticket. If it looks dirty, murky or rust colored, that's too much minerals for the cooling system.
It is the first para under
"Water conditions" ...."Pure water..."
And all of "water conditions" nicely restated by Barney.
|Oh dear, here was me thinking that collected rainwater from a roof would be good for the engine. I think I will now drain it out and put tap water in the radiator. In Hampshire with the chalk our water is quite hard but it seems preferable to soft acidic water from what FR says above. We do get lime scale though and I found this out when taking off the thermostat housing and had to scrape scale off the aluminum sides. I'm assuming however that once the lime has crystallized out from one filling the process then stops. Can I also assume that when I put antifreeze in it will make the coolant a little more friendly and act as a rust inhibitor? The % used seems to vary amongst owners - is 25% regarded as OK for the UK? Barney why are you using as much as 50%? I will be interested to see if my engine temp rises after this is done. I've decided not to continue with the use of 'water wetter'.|
|J H Cole|
Well the discussion boiled over onto the Spridget board "Cooling", and I added this:
"Yes, good water = good. But, mineral deposits from Ca/Mg are easy to remove periodically with mild acids (vinegar etc), and will be minimal if the system doesn't require topping up a bunch. And beats having yr engine and rad et!
The cooling system should only see 2 gal liquid (half of it water) between changes, in a basically sealed system; your kettle might see thousands of hogsheads over years, and it is being boiled down constantly."
The lime itself will act as a rust inhibitor and acid neutralizer, but yes the AF has that too, as well as pump lubricants etc.
In our part of the world winter temps can be -30F, and summer over 100F as now, so we do not mess with wimpy antifreeze solutions! (50/50 is good for -37F). Also, 50/50 is easy to mix without measuring much and gives you some slack for residual water in the system and errors and emergency top offs.
This thread was discussed between 27/06/2012 and 03/07/2012
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