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MG MGB Technical - Overheating

I've been gradually working on a general overheating problem I get, not unexpected particularly with the super hot Australian summer coming on at the moment.

For various reasons (in some cases just peace of mind) I have replaced the following with brand new parts...
Water pump
Belt for water pump
Thermostat
All hoses (for coolant system that is, not the fuel lines and other hoses).

Now the fan switch thermostat (the one that plugs into a hole on the side up top of the radiator) seems to be working only sometimes. It doesn't take much wriggling to pull it out of the radiator altogether (when the engine is cold of course).

The radiator is original for my 78 "B" Rubbernose. I'm not entirely sure how to check if it is any good, but I've run the garden hose through it with the overflow covered up and the water seems to flow through to the bottom quite easily.

I can pick up a new radiator for just a little over $300 AUD which sounds really good to me. It's an original (from the Heritage MG guys in Sydney) and yet I've seen the ones in the MG Owners Club catalogue for well over $1000. Anyway, I'm not sure if I need a new radiator anyway, but if the $300 is the real deal... well... I might do another "peace of mind" replacement.

What else should I be looking at to try and figure out this overheating problem? I'm told that a 2nd fan is really overkill and was only used on the V8 model MGs but removed on later models even. Not only that, a 2nd fan will blow the original look of my car.

Regards,
David
Sydney, Australia
D O'Brien

Hi David,
What are the symptoms? Are you overheating in traffic or at speed or both? How hot is it getting? There can be lots of causes and you can probably tie it down a bit.
Steve Postins

What are you referring to as "overheating"? If it's not boiling over, it's not overheating. You may be chasing ghosts.....
Rob Edwards

Start your car and let it idle up to operating temperature (when the thermostat opens), then turn off car. Feel the radiator for cold spots, idicative of a clogged or restricted radiator. Make sure the fins of the radiator are free from debri or bugs that would restrict the airflow through the radiator.

Under what conditions is your car overheating? Driving in stop and go traffic or on the highway? When you filled the cooling system after changing the hoses, did you have the heater on? Does turning on the heater help cool the motor? Is your car properly tuned? Does the fan switch on? Is the fan blowing air in the correct direction?
Kimberly

American spec (which went everywhere but the UK eventually) always had twin cooling fans on the 4-cylinder, as did the V8. UK 4-cylinders only had the one.

As Rob says, it is only overheating if it is steaming or losing coolant, but having said that if it regularly gets more than 2/3rds the way to H from N then I'd be a bit concerned as well. And having said *that*, I have seen my V8 in the red in 90+, but it still didn't lose any coolant, just get a bit rough which was probably super-heated carbs. I fitted a 25% uprated rad, but was disappointed in the difference it made. The '25%' refers to the number of tubes and because of the law of diminishing returns the extra row made much less difference than 25%. The biggest improvements came from making sure the fan was getting as much voltage as possible, it was losing 3v or more. I provided local grounds for each fan under the mounting brackets, and a direct 12v supply from the spare output spade on the alternator to the fan relay. Another big difference was a top-end overhaul and new water pump when I started getting the cooling system pressurised and blowing coolant out. After that the temp gauge was better behaved than it had ever been, so it seems to have had some kind of problem all the time I had it, even before it started blowing coolant.

The important thing is what is the temp gauge doing under various conditions, like when does it go up and when does it come back down. The coolant temperature always goes up and down on cars with electric cooling fans, and people have always been paranoid about it. Which is why on cars with ECUs the temp gauge is controlled from the ECU and doesn't 'see' these normal variations, only the gradual warm-up, and anything above the 'normal' range.
Paul Hunt 2

David. It would be nice to know what the actual cylinder head temperature was when the car is "over heating". That, plus the information the others have requested, would allow us to better understand your perceived problem. I have had several people complain of "over heating" as registered on their dash gauge. When we used an infa-red thermometer to measure the actual cylinder head temperature it was in the 180-190 deg F range. I suspect that old sending units, old wires, and old gauges result in more "over heating" problems than any other source.

As to the thermocouple in the radiator. I had to replace mine three times, in five years, on my original 79. On my daughter's 77, the new thermocouple blew out of the gasket while I was driving the car in traffic. Great adventure. Others have also had this happen to them. The factory changed the design of the thermocouple, adding a retention device, for the 1980 model due to this problem.

On my cars, I have had a patch soldered over the hole for the thermocouple and added an aftermarket fan switch with thermocouple which has done away with such problems. There is much in the archives on this subject as it has recurred a number of times over the years.

Les
Les Bengtson

For securing the switch into the radiator, many people have used a wire tie.
Kimberly

It seems fine as long as I'm moving along and not doing a lot of stop/start traffic.
It climbs up to just short of the redline pretty quickly after just a few minutes in stop/start traffic. Once I start moving again it takes about 4-5 minutes to drop down to a little over halfway on the gauge. This happens most mornings on my way to work, and the last 10 minutes of the trip involve a few traffic lights. By the time I park I am at the "high" point on the gauge again.

Not sure if this matters a great deal, but lately I've been running a slightly hotter plug than normal. I normally run the NGK BPR6EY (the Y is the "v" groove tip variety) but recently I couldn't get them and had to get the BPR5EY. The "5" means they are one step hotter than the "6" version according to the NGK charts.

Anyway, today I picked up a new thermostat (the one under the cover at the front of the engine head) and the gasket for the cover. Also grabbed a new fan switch thermostat (the one that plugs into the top of the radiator). The guys at Heritage knew what I was doing and they talked me into taking the new radiator "just in case" and I can return it if I don't end up using it. All the hoses are less than a year old and in great condition. I replaced them when I replaced the water pump and thermostat cover earlier this year. About 6 months earlier I replaced the heater control valve and the hose connected to it.

I'm hearing Paul when he says most people are paranoid about the fluctuations on the temperature gauge. Problem is I can't afford an engine rebuild if something went wrong because I wasn't careful. Yes, this is the definition of paranoia, so call me paranoid. Today's exercise has only cost me $30 which is less than I spend on an oil change. If I add the $305 for the new radiator it's still cheap enough I'm happy to do it for peace of mind.
D O'Brien

Sounds like your fan is not coming on. Cross the two wires that go to the switch in the radiator and see if the fan will come on. Make sure the fan is moving air through the radiator.
Kimberly

I would vote for the fan not being switched on as well. If it does work when you short out the thermo switch then as an experiment you could run the wires inside the cockpit and put them thru a manual switch there, at the the traffic ligts turn on the fan, you may be able to hear it and see the voltage jump a bit, does the gauge behave itself now?
Stan Best

Thats what I did years ago,I have a switch on my dash that overrides the fan switch.Also make sure both fans are working I finally put a aftermarket fan and got rid of the stock ones I had to keep taking them aoart and cleaning them.My car will creep up to the red in traffic but has never actually overheated I think the gauge is off .
Pat in tehachapi
patrick bailey

Ditto to a fan problem. When the fan cuts in there should be a significant change in temp gauge needle behaviour. It may well keep climbing in very hot ambients, but it should be *much* slower than before it cuts in. The stock switch should cut in about 2/3rds the way from N to H and cut out again about 1/3rd the way. The SU Burlen replacement for the V8 tends to cut in only half way up, which sounds better, but it tends to keep them running until the needle has come all the way back down to N which is not so good.

I have an illuminated switch for my fans, so I can see when the thermo switch has operated as well as being able to override it. But since doing the work I describe above I have never felt the need.
Paul Hunt 2

On our 80 B and other late model MGB's I make a kit that I install on the cars. I use a 7/8 inch expansion plug to plug the hole at the radiator after removing the sensor. The coolant flows into the radiator thru the top radiator hose. Should the coolant get below the sensor, then ambient air temperature is what trips the switch instead of coolant. Generally, the air temp has to get a little hotter than the water to trip the sensor.

I take the two green wires and add two female wire ends to them.

I purchase a thermostaticlly controled switch with a metal tube and sensor bulb on the end of the capillary tube. It has two male posts to plug the two wires into. Install the wires in the direction which runs the air thru the radiator. They can be reversed.

Mount the unit in an open area near the expansion tank and fusebox.

Run the capillary tube along under the drip rail of the wing underneath, above and behind the expansion tank, across the radiator securing panel, tiewrap tube to the upper radiator pipe. Use a foam pad about 1/8 inch thick to pass the capillary tube over and insert the end of the tube into the upper radiator hose. Carefully slip the hose over the radiator pipe while maintaining the foam's security to the radiator pipe.

"Bed" the capillary tube into the middle of the foam and secure the hose with a hose clamp. The foam will prevent any leakage.

The thermostatically adjustable switch can be adjusted to turn the fan(s) on from staying on constantly to turned off completely and any temperature range in between.

Once I have the switch adjusted to kick the fans on when the gage is nearly straight up the middle, I take and mark the adjuster and its base as the point of choice. Then I can adjust it if from that reference point as necessary.

Yes, those hotter plugs are heating that motor up. The Bosch Platinum's that I use say WR7DP on the sides for summer use (cooler). For the winter I go to one heat range hotter.

It is inperative that you bleed the air out of this system on the 77-80 MGB's. I do this after installing the above mentioned switch by opening the heater valve and remove the cap from the top of the thermostat housing. With radiator cap off the header tank I insert a funnel into the hole on the thermostat housing. I begin filling, allow the coolant to sink in and continue. When the coolant starts to fill up the expansion tank, I install the radiator cap. Then I continue filling the housing, I pump the bottom radiator hose to force the air bubbles up thru the housing. It is also important that you have a 1/8th inch hole drilled into the thermostat if you do not have that little valve that the original thermostats came with. This allows that air to escape. Keep filling and pumping until all air is out and only coolant comes out of the housing.

After this is done, install the plug on the housing, start the car, let the temp come to operating temp and adjust your switch.

I have been running this switch for over ten years with no problems.

I also have an aftermarket fan that pulls the air thru the radiator. Without checking off the top of my head, it is either a 12 or 14 inch fan. Quieter than the original and cheaper.

I do have one extra fan in the front that is original, I keep it unplugged except for those really hot days, then I plug it in for those 100 degree days in the dog days of summer here in the south.

Mark
M Whitt

I don't know about the RB cars, but on my CB GT there is a piece of foam that seals between the radiator and the diaphragm.

My car was always too hot - a standing joke in the MGCC as the bonnet was always up at events - until someone spotted the missing foam. Problem solved. I think the fan was recirculating the hot air from the engine bay without the foam in place. A £1 cure.

Neil
Neil Lock

My suggestion for the next set of ghosts to chase

1) check timing and mixture (retarded timing or lean mixture will make it run hot

2) make sure the fan is kicking in - maybe replace with an electric puller fan on the engine side of rad

3) make sure the rad is full of water (if the water level isn't reaching the fan switch, it may not kick on correctly)

4) bad or early year temp sender - replace with correct one (cheap, so, might be worth doing anyway). The temp senders do degrade over time and the early year sender gives a high reading on the later gauge.

I'm sure most of this has been covered. More extreme measures include louvres in hood, vent holes in the wheel well, etc.

J. Lawrence

An interesting point, Neil. After 15 years without one (and no overheating) I did fit one this year (I already had the diaphragm to bonnet seal), and it did seem to run slightly lower on the gauge even in the very hot weather we have had. Mind you, the adhesive between the two strips of foam was very poor, I caught it before it came completely detached but I'd have been pretty miffed if I'd lost one of them on the road.
Paul Hunt 2

This thread was discussed between 29/11/2006 and 01/12/2006

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