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MG MGB Technical - Overheating

My GT overheats in traffic. A lot of water is pushed out of the overflow (up to a litre) which only compounds the problem. It's an 1840cc with big valve head, 9.75:1, hot cam, sports exhaust, HS6s etc.

The thermostat is okay. The radiator cap is okay. There are no blockages. I've fitted a new water pump. The radiator is new - I had a modern core put into the old tanks. The head gasket is okay (copper/asbestos). Compression is good and equal. I've an electric fan which is man enough for the job (the mechanical fan wasn't man enough, and I took it off to make room for the new radiator). Timing and fuelling is spot on (rolling road tuned). I've tried wrapping the exhaust manifold but that made things worse and (curse all of those people who reckon on it being a good idea) warped the manifold. The heater is not blocked, the tap is okay.

Anybody got any ideas? I was wondering if one of the bores was porous, thereby pressuring the coolant - bit that seems a nit of a long shot.

Neil

Neil

I would suggest a complete pressure test of the cooling system. Information can be found in this link. You can have defective a head gasket, cracked head or block and still show good compression with a compression tester.
http://www.dippy.org/svcman/sm07.html#TESTCOOL

Clifton
Clifton Gordon

You may have checked this already..but have you removed your freeze plugs and really cleaned out the water passages in the block....just a thought.

Id also make sure the fan is blowing the correct way... I dont mean to insult you - but that happens sometimes...just two thoughts....

good luck
Jeff

Did you pressure test the radiator cap to make sure it is ok? Does the cooling system hold pressure?
Is the thermostat installed correctly? If it's installed upside down, it won't open when it should.
At what temp does the fan come on? Is it blowing air in the correct direction? Is it coming on?
Are there air pockets in the cooling system?
Are the hoses good? Bad hoses can collapse causing obstructions.
Are you sure there are no blockages? Did the shop that recored your radiator inadvertantly leave a rag or a ball of paper in the radiator when they were painting it?
Is the water pump good? Did the vanes break loose?
Kimberly

had similar problems earlier this year and did two things based on feedback from this bbs...

1. burped the system to make sure all air is out of the system. I ran engine with radiator cap off until engine heated up and you could see bubbles escape from the water at top of radiator, run until bubbles stop and the place cap back on. this will be messy so keep some rags around

2. check your timing... this was really my culprit... I had too far advanced which was causing overheating... believe it or not, set back two degrees did not lose any power and overheating stopped.
edward

Kimberly. Please be so kind as to contact me off line. Either at the server listed (rather poor, but catches all of the spam from this site) or through my website at www.custompistols.com/ (much better server).

Neil. You do not list the year of your car which can make a significant difference. My website, listed above, has a couple of tech articles which may be of use to you, one on the cooling system and one on over flow tanks.

All of the previous suggestions have been quite excellent. They, and the tech articles, should provide you with sufficient information to perform some tests and get back to us with what you find.

One thing that would help me is to know how you are filling your radiator. With the older style radiators, there is a certain amount of "blow out" if they are filled to the base of the filler neck. The coolant expands when it gets hot and, if filled to the bottom of the filler neck, some will be blown out of the radiator. When the system cools, the valve in the pressure cap opens (because there is a vacuum) and air is sucked back into the system. At that point, the system seems to stabilize and no further coolant is lost.

Might I suggest that, as a part of your experiments, you route the coolant over flow line into a bottle? This can be something as simple as a two quart/liter bottle cable tied to a secure attachment point with the over flow line from the radiator stuck into the open end. This will catch the over flow (good for the environment and local pets). It will also allow you to see if the ejected coolant is being sucked back into the radiator as the system cools. This is the basic theory behind the later model radiators, not having a filler neck and pressure cap, but having an expansion tank.

With the results of your tests, we should be able to make a more accurate interpretation of what is happening and be better able to advise you on how to proceed.

Les
Les Bengtson

The point Les makes about expansion is very important.
When people are used to modern systems they often think the water ejected under expansion represents overheating when it does not.

You say the thermostat is OK and the radiator cap is OK. How have you tested them?

When I was setting the adjustable temp switch on my aftermarket electric fan I used a jam thermometer in the top of the radiator, with the cap off. This emabled me to discount any inaccuracy on the temp gauge fitted to the dash. The jam thermometer was also used to test the thermostat in a pan on the cooker.

David
David witham

Hi Neil

I found that moving the front number plate up to the front of the (rubber) bumper so that the under-bumper cooling orifices were better exposed considerably helped the cooling on my 1981 GT.

An overheating problem on another car was solved when I compared some thermostats in a saucepan on the stove and discovered that while they all opened at around the expected temperature, some of them opened further than others. Don't forget that anti-freeze is toxic !.

HTH... Don
Don

What is the temp gauge reading? If this is reading normal and you are getting constant cooling loss then you have some other problem in the engine or cooling system, not overheating. Of course too much coolant loss *will* lead to overheating, but that is an effect, not a cause.

Whilst the ejection of excess coolant as a result of overfilling is not overheating, if the engine *is* reading very high on the gauge - and I mean way into the H zone, not just a bit above N - then it could well build enough pressure to exceed the cap at which time it will start hissing and steaming and losing coolant that way, and that *is* losing coolant as a result of overheating.

For some time I had coolant loss on the V8 (followed by overheating if I left it long enough). I was getting bubbling from the top hose which was pressurising the system and pushing coolant into the expansion tank, and eventually out of the overflow. First thought was a head gasket but a test kit showed no combustion fumes in the radiator. Eventually I took the heads off to see if I could find anything wrong. All I found was the bypass pipe (which allows circulation round the engine while the thermostat is closed) blocked and I thought that must have been it. However then I realised that I still had the problem in winter, when I had the heater on, which duplicates almost exactly the circuit of the bypass pipe, so it couldn't have been that. As I had a new water pump to hand anyway I fitted that, and the problem was solved (along with some other cooling system oddities I'd had since buying the car). I can only surmise that the water pump was sucking air, probably from a faulty seal, even though it wasn't visibly leaking.
Paul Hunt 2

Wow - thanks for the responses. I'll try to respond to the points to see if we can narrow the problem down...

I've not done a pressure test; water is coming out of the overflow, but I'm not convinced that all of the water is disappearing that way. I'm going to try to rig up a catch tank.

Block was acid/caustic cleaned. Fan is blowing the right way. Radiator cap is okay (tried several). Its not possible to put the thermostat in upside down, but anyway I've tried several. Hoses are all new, no problems. No airlocks. Radiator is not blocked (new radiator was to try to fix this problem). Timing is cock-on. The car is a 1970, but the engine is an early 18V; it does not have an overflow tank (cap on the rad). Water will only come out of the overflow when the pressure lifts the cap off the seat in the rad; as it cools, the pressure drops, the cap seals and hence water cannot be sucked back in. Temp guage is reading just under normal on a run, heats up, drops as the thermostat opens, heats right up in traffic, almost in the H zone. Water pump has been replaced too!

Paul, where do you get a kit for testing the water? The head gasket is okay, but I'm wondering if there is some porosity there.

Other thought is whether all B series heads are compatible with all B series blocks as far as water passages are concerned.

Keep 'em coming, please!

Neil
Neil

Neil,

On my friend's 1970 B which we took to Gatlinburg I rigged an overflow bottle by taking the overflow hose and routing it to a 'cleaning wipe' vinyl canister with a secure cap. The hose goes thru the cap where the wipes used to exit to be ripped off their spool. While the new radiator cap has rubber seals at both the pressure lip and the outer lip (a feature which has significantly reduced the coolent dribbling down the front of the radiator) it will indeed suck the coolent out of the overflow bottle on cooling. Since rigging the system this way, air bubbles in the coolent have been reduced. We still have a small issue with a leaking core plug, but that will wait for this winter. Her car never exceeded midway between N and H on the temperature gauge in traffic, up hill.

With respect wrapping the manifold, Jet Hot is the product I had applied to my TD manifold. It is a ceramic coating on the inside and outside of the manifold which precludes the excessive surface temperatures which can warp a manifold, and reduces under hood temperature.

I also totally agree with the timing check. Sometimes, an engine is happier off spec than on.

warmly,
dave
Dave Braun

Neil,

Like you I thought the system could not suck back in. But one day Paul Hunt pointed out, on theis board, that there is a little spring loaded valve in the centre of the bottom of the cap. This prevents vacuum damage to the radiator.

It is only over heating if you have steam and water gushing out. I would expect the needle to be on the hot if it was over heating. Are you sure it is not just clearing some expansion space in the top tank.

After a run to warm the car up park it on your drive just ticking over. The temperature should rise untill your electric fan cuts in. On the cars originally supplied with electric fans there are a couple of very small dots on the gauge. The needle should be between them when the fan comes on.

David
David Witham

Les Bengtson-

My computer that I use for email is down now. You can send me a private message at www.britishcarforum.com or www.mgexperience.net
My username is Kimberly.

or you can send me an email at kpetr@pacbell.net
I should be able to send and receive email by the end of the week.

-Kimberly
Kimberly

Thanks, I'll take the cap off to convince myself that water can be sucked back in and then rig up a bottle to see if all of the water is coming out through the overflow.

You're right, it's not really overheating. The question is really about where the water is going. From the info I can find on the web, cooling system capacity is 12 pints or about 7 litres. Density of water at 4degC is 1, at 80degC is 0.97 - so raising the temperature of 7 litres of water by 80 degC would cause it to expand by 7/0.97=0.2 of a litre or about a third of a pint. I'm losing a litre, maybe two.

Timing is cock-on for maximum power on the rolling road. I have a variable thermo-switch on the electric fan.

I've used exhaust wrap previously and would absolutely NOT recommend it; it seems to lower under-bonnet temperatures (I wanted to get colder air in the carbs) but probably would not have any effect on cooling. Does warp manifolds though. I haven't tried ceramic coating, but I now think that the manifold is a good way of getting heat out of the engine anyway. The heat has to go somewhere - if you wrap/coat the manifold, then some heat is going to go down the exhaust pipe, but some is going to stay in the head.

Don - there is some good news about anti-freeze. I swallowed some a couple of years back (a long story involving an exploding radiator cap). Advice from A&E (courtesy of the 'national poisons centre') was that the anti-dote was alcohol: they prescribed a large whisky...! I took my medicine like a man.

Neil
Neil Lock

Neil - someone brought it over from California for me, I tried but could not find any source of anything like it in the UK. If you fit a catch bottle to the overflow then with the correct cap it *should* suck the water back in when it cools, but it shouldn't be necessary.
Paul Hunt 2

Neil, you are way over-thinking this. For one thing, you aren't filling your radiator at nearly freezing, probably more like 25degC, secondly you are neglecting the affect of the anti-freeze on the coolent.

at 27degC (80degF) 50% ethylene glycol solution has a s.g. of 1.077, and at 93degC (200degF) the solution has a s.g. 1.038.

Which, if your analysis was valid would actually account for a larger volume of lost fluid.

I added the recovery bottle because I wanted to avoid sucking air into the cooling system, and I wanted to avoid spilling the coolent on the ground. The level in the bottle minimizes the air gap which would occur allowing air into the system, and minimizes the amount of fluid needed to replenish that lost due to pressures exceeding the 10 psi allowed by the pressure cap, and automatically provides a constant level of fluid and allows for simple adding of fluid when the coolent bottle is low.

In the 'olden' days when our cars were designed, people didn't mind adding fluids, and didn't worry about pets drinking the puddle on the ground, or the runoff into lakes and streams. Things change, the coolent is more expensive, and a recovery bottle is very simple and effective.

You aren't overheating, you are simply venting fluid, but eventually, adding some becomes needed. Open your heater valve, fill your system, burp your lower radiator hose to vent air from the system, and add a recovery bottle and all will be right with your B!

warmly,
dave
Dave Braun

Oh, Neil, and about keeping heat in the head. If the exhaust gases are flowing through the manifold without transfering much heat to the manifold because of inner and outer ceramic coatings, the exhaust gases will remain at a higher temperature, and the heat will be disapated fruther down stream through the metalic walls of the muffler and pipes. But they will still be leaving the engine. Since the manifold will absorb less heat, it can't transfer heat it doesn't have to the head by conduction. Since the exhaust gasses are constantly being created and sent on their merry way as the engine runs, the head should actually be cooler, not hotter.

Wrapping a manifold and locking heat inside can destroy a manifold because the heat transfer still takes place from the inside out, but builds up because the heat is prevented from transfering by radiation and convection by the wrap. Hence, a hotter manifold and head.

Cooler head, cooler carbs, cooler fuel, cooler underbonnet temperature... what's not to like? And the coating on the inside and outside of the manifold protects the metal of the manifold, preventing warping and degradation. If I end up going through more stainless steel exhausts as a result, I'll let everyone know!

warmly,
dave
Dave Braun

Dave,

Not sure what you mean about "over-thinking". I was trying to calculate how much the volume would change from cold to hot so that I could get a feel for how much water would be pushed out of the overflow due to expansion. I chose 4degC because water is densest at 4degC, also the range 4degC to 100degC is large so I would end up over estimating the amount of volume change rather than underestimating. My point is that I only calculate a volume change of 0.2litre, whereas I am losing a litre or two of water every trip. Look at the max/min marks on a modern car and see how much the level moves between hot and cold - not much.

I'm going to rig up a big bottle and see how much I catch. If I catch less than I lose then the problem must be internal.

Actually, your figures for ethylene glycol suggest a similar, if smaller amount of expansion which makes the loss of 2 litres even more mysterious.

I'll bow to a greater knowledge of thermodynamics with respect to the manifold temperatures. But the heat has to go somewhere. I'd reckon on the manifold being cooler than the head in the area of the exhaust ports because (a) the ports are nearer the heat source and (b) the manifold has a greater surface area to dissipate the heat. If the manifold is cooler than the head then it must take some heat from the head. If the manifold was coated/wrapped only on the outside, then the manifold could take some heat from the gases but not transfer it to the air under the bonnet - hence it gets really hot and the warping. If the manifold is coated inside and out then it will still heat up as a result of the physical connection with the head I suppose which will transfer some heat, but where's it going to go? To the exhaust pipe? Not if the exhaust pipe is now hotter than the manifold. I don't get it!

When I was running wrapping, the under-bonnet temperatures were way cooler as you say. But the wrapping cost £30+ and a new manifold another £100. I'm talking about a tubular manifold, not a cast iron job.

Neil
Neil Lock

A few years back, I took a plastic transmission bottle with a handle on the side and used nylon zip connectors to tie it to the radiator diagonal supports. I cut a hole in the screw-on cap about the diameter of the overflow hose and shoved the hose in.

I filled the bottle about 1/2 full. I haven't had to ad water for close to a year and over 6,000 miles.

It's one of those items for which there is no downside. It's also extremely simple and cheap to implement.

Interesting story: The radiator overflow concept was patented many years ago. When US Ford started using the bottles in the late '70's they particularly stated that no fluid was to be put in the bottles and that they were solely to prevent overflow and anti-freeze pollution.

The patent holders sued Ford and won even though Ford claimed it was used for a different use than the patent.

It was a little obvious. The overflow bottles held more than a quart and typical overflow from a full cold radiator would only be a few ounces. The dealers regularly filled the overflow bottles halfway.

With an overflow bottle you can give your car a anti-freeze booster by filling the bottle with pure antifreeze. Over a period of time the overflow and flow back will mix and give a fresh dosage of anti freeze to the system.


Barry
Barry Parkinson

If it's just expansion loss that is occuring that will only happen for the first two or three heat cycles after over-filling, thereafter you won't lose any more. But if you keep putting the excess back then it will keep coming out. If you don't top it back up again, and it keeps pushing fluid out, then you do have a problem. In both cases how much the volume increases due to expansion is irrelevant.
Paul Hunt 2

Gone a while away from this BBS..., I find it back very relevant. Paul Hunt says: " I can only surmise that the water pump was sucking air, probably from a faulty seal, even though it wasn't visibly leaking."
I can confirm that too, replacing a falty leaking water pump solved my problem too.
Renou

This thread was discussed between 28/06/2006 and 06/07/2006

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