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MG MGB Technical - Car Suddenly Started to Overheat

A 150 mile drive went fine yesterday with 80 B the temp gauge out and back staying to the cool side of center. No problems even in the traffic leaving Chetek during the heat of the day, temp gauge was fine all the way to Stillwater. As I recall the temp gauge was fine sitting on the bridge in traffic. Then it was smooth driving through down town, until I started up the steep residential hills to home.

The temp gauge kept climbing up, to within a couple needle widths from Hot. I could smell something was going wrong.

I keep up the hills and pulled into garage, opened hood and got out a strong spot light.

No sign of any leakage anywhere around block, head - nothing, but it did stink like something was too hot. I even looked at catalytic converter with engine off of course, and it didn't look to be red even with light off and it didn't seem hotter than normal for a converter.

I looked the radiator over, everything front to back - nothing.

Then I noticed the antifreeze leaking out of the top of the overflow tank at the cap, not a seam - the cap.

I am letting car cool and left it at home and switched cars.

The only time I've had a similar situation was 20 years ago on a car that the temp needle went up and before I could pull over to the side of the road, the head gasket had blown.

I think my next check (Wed or Thur) will be to check fluid level in overflow tank, then if fine - start up the engine and see if I can detect pressure coming out of the head or elsewhere to see if there is sign the head gasket went.

So either the car just didn't like climbing the hill after being in traffic, the fan stopped coming on, the thermstat stuck closed or the head gasket is suspect?

So, part 2 - if I get a head gasket (the Peylon sp?) do I just buy the head gasket or get the set of gaskets ($20 vs. $70 or so)?

Your thoughts, and I won't see the car again until Wednesday to check anything out.
R.W Anderson

Did you notice if the Fuel Gauge was over reading? If the voltage stabiliser is SHORT Circuit ( if electronic) or stuck closed the applied voltage to the 2 instruments will be 14+- NOT 10 volts. The bi-metallic strip reg will cycle on and off to give the approx 10 volts required. A simple voltage check could save you hours and a pile of cash. Good luck. Ray
R. E Bester

Water level - not in the header tank but in the engine itself - pull the brass/plastic plug on the thermostat housing.

Chris at Octarine Services

I've been advised to first do a pressure test of cooling system to see if there is slow leak somewhere.

If upon inspection on Wed, I see the coolant level is way down, the lower coolant level may have been too low for the fan sensor (floating ball) to engage the fans.

I will also turn ignition on and jump across the wires at heat sensor at top corner of radiator to see if the fans come on.

Also, it may be a combination of a slow leak, fans not turning on and radiator over flow cap that is weak, not holding in the coolant.

Engine was not steaming upon original inspection. Coolant was just dripping out from under the cap.

More later.
R.W Anderson

personally I wouldn't bother with a pressure test for the coolant

if you've got just one leak slow leak you'd have to have had it for a very long time for it to make much odds to the level

as said check coolant level in engine, pressure cap condition and that it's the correct cap, checks fans cut in and are both going in the correct direction

check rad fins are clear and no obstructions, alternator belt is correct pressure

when was the last time the coolant/antifreeze was changed and flushed out

is the engine running too weak to cause the overheating?

check brakes are not binding

the smell was probably just hot coolant (hopefully that's all)

perhaps you were just labouring the car up steep hills in the wrong gear and/or going too slow
Nigel Atkins

Car has barely been driving in last many years, averaging about 50 miles a year for 20 years. It only has 20,000 on car. Water pump was changed many years ago, but only 2000 miles ago as was thermostat.

brakes are not binding as this car rolls with the push of a hand.

didn't labor up hill, kept at 2000 rpm or so going up residential street at 20+ mph, but it is a several block up hill run to get from river bottom to top of bluffs.

one person commented that it was 90s out that day, so quite hot, but it made the 75 drive home fine, it was the last 1 mile that things went wrong.

More soon.
R.W Anderson

How long has it been since you changed the radiator cap? The spring weakens with age. If there is not sufficient pressure, water will boil at a lower temperature. Make certain when you get a new cap that it is the correct pressure. MG's of different years take different pressure caps.

Often, this forum gets so involved with the technical, we forget to check the simple things first.

LS Sheldon

I'll buy a new 16 # cap ASAP. I like to eliminate the small things first.
R.W Anderson

quite agree Lee if only because I only know the simple stuff, did mention cap and if correct

sorry but 2,000 revs up hill at 20+ mph on a day in the 90s sounds to me very much like you might have been labouring the car

the fan(s) must have been working because at that speed you'd barely have sufficient airflow on a flat road on a cool day

very low mileage like that will create more problems than higher mileage, at that average the stat was changed more that 20 years ago

please say you've changed the tyres at least and that the car has been attended to at least once every year in those 20 years

when the temp went high did you try putting the heater on full?

(if you have one and if the valve actually opens)

sorry to sound awkward and you might be right but >>brakes are not binding as this car rolls with the push of a hand<< doesn't mean they couldn't be binding slightly, if you have them off the ground and try spinning them you'll be able to tell better
Nigel Atkins

I always recommend the Driver's Handbook (not a workshop manual but what you got in the glovebox when the car was new)

it will tell you all about your car, how it operates, its service requirements and schedule and how to do the work

(other suppliers are available)

and I forgot to finish list before

change of coolant, gearbox oil, diff oil, engine oil, grease, etc.
Nigel Atkins

In between the various recent posts I've made I may have forgotten to mention that I purchased this car about 2 weeks ago and have put about 200+ miles on it.

The car is about as close to new as one can get. And I am quite aware of the harm done to a car that sits for years. I've been through a lot of inspections of cars (gems) sitting in a garage for years over the recent months. When I saw this one, I knew I was buying it.

And buying a car that has seen little road use, I am slowly going through all the fluids, filters, etc.

So, no I haven't changed everything YET, I'm working on it. I have no idea if the tires are original or replaced somewhere along the way. They are Dunlops, not sure what came on the car. And tires look spotless, not a sign of wear, no cracking - nothing - and they hold pressure perfectly. So I won't be pulling them off just because they may be old.

The whole purpose of the recent 150 mile drive was to see how everything worked.

Once I get past the overheating issue, and tend to some other maintenance, I will be draining trans, rearend, etc., and putting new oil in everything.

But these things take time.

I have friends that suggest pulling everything off and starting over; carb, head, exhaust, ditch the emissions system, new dizzy, etc.

We'll see how the maintenance service goes, but for now I'm wondering about the cooling system. No sense flushing the cooling system until after the head gasket has been replaced, and any leaks addressed.

Eventually all fluids, filters, lube points, etc. will be addressed. Today I ordered head gasket set and plugs for when I remove the air pump. Next up a thermostat and radiator cap, but that can wait until tomorrow.

R.W Anderson

Water pump quit?
Art Pearse

Oh, and I ordered an owner's manual 2-3 days after buying car and just received it.

I haven't even thought to see if the water pump quit.

One more thing to check.

Oh, and I plan to flush the brake fluid out too.
R.W Anderson

> So I won't be pulling them off just because they may be old.

There should be a date stamp on them somewhere. Find it and see what it says. If they're older than 6-8 years old, pull them off. Just because they /look/ ok doesn't mean they're safe!
Rob Edwards

My 1980 BGT had four new tyres in 1991 at about 26,000 miles. After only another 1500 miles by 2007, I bought it.

After about a year, a leaking valve caused me to use the spare. I forgot about it, it had good tread and no apparent damage. A couple of months later, it blew at 70mph. It had spent the previous 17 years (at least) in the dark in the boot. It was a Dunlop with no obvious date marks, so I assume it was an original 1980 tyre.

Geoff Ev

although it might not seem it we're agree with each other

you're driving the car to iron out its wrinkles, got the Driver's Handbook and will carry out a staggered 36k-mile service/check up including a few other changes not listed there - normally these are what I suggest for any classic that is new to the owner (numerous of my posts include this) but we were on the overheating first

the things that we disagree about are the tyres possible (I normally use Geoff's as an example) and here's a good web site for tyres and other stuff, a bit down the page it tells you how to date the tyres -

and I'd never suggest changing a head gasket unless necessary and certainly not at this stage

on the waterpump you'll soon know if it's not working by how very quickly the engine warms up to hot

the 36k-mile service/check up plus other bits will help to find issues and prevent problems, if required a thorough clean and flush of the cooling system is cheap quick and easy to do and I'd put it at the top of the list (if braking is OK) if you need to do it again later for some reason so what another flush with get out the crud missed first time

again as it's quick, cheap and easy to do if required I'd change the engine oil and filter and air filters just to help and again act as a flushing as I'd change the oil and filter again after say a 1-2,000 miles as by that time you should know if anything needs doing to the engine

if you weren't labouring the engine it could be that it's running weak for some reason so a check of tappets, then points and plugs followed by adjusting the timing and last carbs mixture, again you may need to adjust again in a few thousand miles

driving the car regularly is what it needs building distance and speed as things get better and they will just by driving the car let alone the maintenance work

finally, if you read that Driver's Handbook you will know learn and know more about the car than some long term owners same applies if you regularly drive the car
Nigel Atkins

ETA: sorry I distracted myself away from the tyres -
even if the sidewalls of the tyres look OK and they have plenty of tread left without a flat spot from standing because of their age and/or lack of use they could have 'gone hard' and so will not be effective as they should be at braking, steering, handling and comfort and noise

you're doing the right thing with your car but you might be too concerned about some things and not yet aware of other issues

obviously you need to be cautious of the car initially until time, distance and maintenance has passed a bit but you need to stretch the car a little in what it can do to find if there are issues and to 'loosen' the car up more

time to mention pumping new grease into all the points especially the front suspension (and force out all the old grease this time round)
Nigel Atkins

"too low for the fan sensor (floating ball)"

I don't think so! It's a bi-metal strip. I've had coolant pushed from the radiator and hence engine into the expansion tank on my V8, I could see air bubbles coming into the rad through the filler hole on those cars. That could have been a head gasket but wasn't according to a combustion leak detector, in my case it was probably the water pump sucking in air, and I've also heard of it happening when the bottom hose clamps were a bit loose, but not so loose as to cause a leak.

If the pump is just not circulating coolant as it should then you would get a steady rise in temperature, but it wouldn't lose coolant from the rad unless it got high enough to boil. Incidentally these should have a 15lb cap, a pals 1980 was regularly vomiting after switch-off and his 15lb cap was only holding 3lb!
Paul Hunt

First off, have the cooling system pressure tested. At the same time, have the radiator cap pressure tested. The same tool is used for both jobs, using different adapters for the radiator and cap. This will locate any leaks, including the water pump, but actual operation of the pump can only be tested under a load. RAY
rjm RAY

Okay - time for an update.

I still need to remember to look for a date stamp on tires, but friend who went with me to buy car recalls the seller saying she replaced tires. Now to figure out when.

I replaced the thermostat and expansion tank cap yesterday. I am curious about old thermostat so I've saved in and will do a test in pot of boiling water, but I need a high temp thermometer first.

I know the purists won't like to hear this, but since removing the thermostate requires disconnecting the air pump and wrestling it off to the side; I decided to remove the air pump, the rail going to the head, as well as the gulp valve.

This is about the time Nigel's and other's concerns came to my immediate attention.

While removing the rail I discovered that the large nut going into the check valve at top of rail was finger tight, not much of a secure air system there.

While removing the gulp valve I discovered that the hose to the valve and the hose to the L elbow that goes into the manifold were so dried out they split on contact, or may have been split before from heat and stress.

My 72B had the ventilation from the side of the block venting through the PCV in the same location on the intake manifold as my 80B has the gulp valve, so I wanted to make sure I wasn't compromising the ventilation while taking things off the car.

This is when I discovered that the venting hose coming from the side case to beneath the carb had broken off at the carb. It wasn't broken earlier, but it was an obvious heat and age break, so I replaced that hose too.

So, once I got these hoses, parts and various vacuum hoses re-connected or bypassed and coolant topped off; I started up the engine and watched for leaks. All appeared fine, and I just watched the temp gauge and when the gauge hit center the fans turned on. Temp never went past center.

No leaks so I figured it was time for another road test. 20 miles later, temp gauge was still to left (cool) of center. Another 20 miles in to work this morning, with same result.

I have no idea what caused original overheating, most likely suspect is thermostate, as everything else is working. Or it was just the 90 degree day and 150 miles. I'll know when I test the old thermostate.

But no time to test it yet, I need to buy more hoses; and I think an alternator belt too while I'm at it.

Oh, yes I have pushed grease through all 6 of the front fittings until new came out.

I haven't figured out how to check fluid in steering boots yet, as on my 72B I just do this from above by removing the cover and adding oil very slowing dripping it in from above while turning wheels back and forth. The 80B has that plastic shroud blocking my access to the cover.

More later.
R.W Anderson

if rubber hoses are breaking with age just image what the tyres could be like even if they have been changed but how long ago was that and lack of use makes them worse - the date stamps can be only on one side of the tyre so they might be on the inside side, see previous link given for details -

as much many rubber products are rubbish now Iíd be looking at getting a set of silicone hoses with rounded-edged clips, personally one of the first things I do with a classic new to me is clean, flush, back flush, flush again the heating/cooling system and replace as required, pressure cap, fan belt, thermostat, hoses, water pump, heater valve, internal and external heater seals

I like to flush out the heater matrix, rad and engine separately and remove the engine drain plug and scrap out the crud from there

when testing the previous thermostat bear in mind they donít open and close in an instant at the stated temperature, sorry but I still suspect that you might have been labouring, a possibly gunked up, car up hills on a hot day that was the cause

did you check both fan were going in the correct direction to get air through the rad?

donít forget to check the Driverís Handbook before you start on the car, the servicing schedule should reference the pages with the info on how to do the work

how about (if you want to) your name in the user name and setting up a vehicle profile to be viewed as this can help to remind of the model year you have and any alterations from standard - plus of course I'm sure we'd all like to see a photo of your car
Nigel Atkins

Here is a low resolution version of photo.

R.W Anderson

The extremely lean mixture, caused by the split hoses at the intake manifold, could well have caused your engine to run hot. Replacing all of the hoses is a wise investment. Unless there are signs of oil having leaked from the rubber boots on the steering rack, the old ones are probably still doing their job and the system doesn't require servicing at this time. My '67 had the orignal boots start to leak in the early '80s. I replaced them then and have done nothing to the steering rack since. To this day, there is no play in the steering system whatsoever and the car still has the original kingpins. The main wear items, in the front suspension, are the lower A frame inner bushings. They consist of eight small rubber bushings that tend to have very short lives. When BL introduced the GT V8, they used much stronger bushings. They consist of four rubber and steel bushings that replaced the earlier eight piece design. When I replaced the lower inner A frame bushings on my car, at the same time that I replaced the rack boots, the old ones were in very bad shape. The V8 bushings that I installed then still look like new. A complete set costs less than $10. RAY
rjm RAY

yes RW that looks very smart indeed

good call on the running weak, we did cover it earlier in the thread but I didn't tie it in with the hoses as they split on contact after the event but I forgot about all the extra gubbins on the US cars, if it was also running weak then it would have been even more laboured going up those hills at 2,000 revs in 90 degrees

RW, once you've got the car fully sorted and driven regularly you'll know how to drive it for best (overall) performance and recognize when it's not running at its best

as I'm being my usual grumpy self - is it me or have you shut the seat belt in the door and if those are glasses on the roof then it's best to get into the habit of not putting anything on the roof or car as that's how rips and scratches can happen and it will show more on black paint
Nigel Atkins

Wow, lots of suggestions. To many to read so just a question.
Have you tried a compression test?
Barc Cunningham

I may eventually get to a compression test, while doing other things, but with 22,000 miles on car, there had best not be an engine problem.

I've now ordered upper and lower radiator hoses and in and out hoses for heater. I may have other hose material on hand, for other items as I work through all this.

I'm going to start a new thread on a related topic.
R.W Anderson

Oh, and how does one tell the difference between hoses drying out as suggested above from a too lean mixture, and the excessive heat coming off the catalytic converter?
R.W Anderson

with a car of that age and mileage I'm not sure how you'd tell what caused the rubber to particularly dry out

bear in mind many modern made rubber products are rubbish including coolant and heater hoses

cleaning, flushing etc. as in previous post and scrapping out the crud from the engine drain plug hole and thorough cleaning and flushing of the heater matrix and radiator will help the engine and car
Nigel Atkins

There are products which SHOULD be changed if you have no idea when they were renewed. Mileage is not an arbiter of whether a change is needed, TIME is, or unavailable data.!
Brake flexibles
All brake seals and fluid
AND TYRES!!. 10 years is an absolute max, personally I wouldn't push them past 6 or 7. Less if the car has been standing.
Other rubber product will cause great inconvenience if not major damage. New water hoses go on all my new cars if it hasn't already been done.
Allan Reeling

Depends on the quality of the rubber used more than anything. If there is any sign of surface cracking when you bend the hose, then change it/them, especially fuel hoses.
Paul Hunt

This thread was discussed between 07/07/2013 and 16/07/2013

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