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MG Car Repair/V8 temperature gauge


QUESTION: I have just put my MGB GT V8 back on the road after 28 years.

On my first run out, after about 15 miles the temperature reading went from normal to the red line.
I stopped the car and checked under the bonnet. The engine didn't appear to be boiling and the electric fans were running.
on checking the temp gauge the temperature had dropped again to just above normal.
I hesitantly set off again and the same thing happened again after a couple of miles; the temperature dropping quite quickly after stopping.
I was on my way to show a friend the finished car and turned round twice to limp home but the temperature seemed to settle, so I continued on.
The temperature creeped up a couple of time again, but not into the red
After visiting my friend and allowing the car to cool, I checked the water level all seemed ok !
Before I set off home, I took my friend for a quick spin and within 2 miles the temperature again went into the red and the falling quickly back to above normal after stopping.
I set off home heart in mouth but the temperature only went up to under the red line once then dropped to just above normal where it stayed for the rest of the journey home; about 45 miles.
Could it be an air block or do you think the sensor could be at fault ?

kind regards Steve

ANSWER: Hi Steve.

This is probably a blocked air relief hose.  By the side of the cooling fan switch unit (on top of the engine in the middle of the V) you will see a small loop of rubber hose.  It is held on with two small Jubilee clips.  Take it off and make sure it is clear.  If in doubt, replace it.  A short length of fuel hose will do.  

While it is off, take a small electrical screwdriver and clear the brass pipes to which the hose is attached.  These often clog up.  You may need to use a 3mm drill.  

Put everything back together and check that the radiator is filled to the brim, and the overflow tank is less that half full, but not empty.  If a top-up is needed, use anti-freeze, not plain water.

Start the engine and check the rubber hose.  It should get hot quite quickly, which shows that everything is now working correctly.

---------- FOLLOW-UP ----------

QUESTION: Morning Barrie,

I did as you suggested and checked the air relief hose. The hose was clear ( new, as are all the other hoses; replaced during restoration ).
Both brass pipes seamed clear, but would only take a 2.5mm drill; so after a bit of jiggery pokery with a electrical screw driver a 3mm drill bit would go in. I reassembled the car and checked the coolant levels; radiator was full ( new rad ) and expansion tank was half full.
I started the car, but I wouldn't say the air relief hose got hot quickly, it just seemed to warm up as the engine temp gauge lifted. the main top hose to the radiator stayed cool until the engine temp reached normal when it got hot as presumably the thermostat opened, the air relief hose by this time was also hot; the electric fans hadn't come into play at this stage.
I then took her for a run to see what would happen. The temp gauge lifted to between normal and the red after about half a mile, then settled down to just above normal.
Then almost to the mile ( 17miles ) the gauge started to creep up to the red; I'd been doing a steady 55 / 60.
I eased back a bit and the temp gauge gradually fell back to just above normal. I stopped for 5mins and topped up with petrol, then set off back home. Once again, after a couple of miles the gauge went up to the red line, easing back slightly  on the accelerator the temp gauge dropped quite quickly to just above normal; where it stayed for the rest of the journey home ( 15 miles )
Any thoughts ?

Regards Steve

Hi Steve.

I assume that you have already replaced the radiator cap.  The seals on them do fail leading to a non-pressurised cooling system that boils at 100C instead of 120C.

A temperature gauge that goes up fast is normally an indication of a thermostat that is stuck shut or a pocket of steam in the system.  That small pipe is called a steam pipe in the parts manual.

I am slightly suspicious that your thermostat is sticking.  For the sake of 5 I would replace it.  When fitting, make sure that the bleed valve is uppermost.  It may be a hole, it may be a groove in the rim, but they all have a bleed valve somewhere.

Second thought.  It is very easy to fit the cooling fans incorrectly.  It is important that they blow through the radiator.  I had one V8 that had one fan blowing and one sucking.  The effect was as you described - the car would cycle through hot and normal states.

Third thought.  I see you have a new radiator, so this is most unlikely.  If a car has been standing for 28 years the radiator could be either partially blocked or lacking in cooling fins.  I have seen old radiators which have lost 25% or more of their cooling fins due to corrosion.  They still hold water but the car overheats more easily.  You have fitted a new radiator, but it is always possible that you have a faulty unit - that would be very difficult to verify. Also, a standing engine can silt up internally, so a good flush might help.  Did you flush the block out when fitting the new rad?

Let me know how you get on.  

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Barrie Jones


MG Sports Cars from 1949 to 1980 - Including MGB, MGBGT V8, TF1500, TF, TD and modern Midgets. I also specialise in SU carburettors, Lucas wiring, suspension, steering & brakes.


I have owned my 1955 TF1500 since 1966. Technical specialist for the TD and TF with the MG Car Club T Register. Also owned 20 MGBs and currently own an MGBGT V8. Written 3 books on MGs and produced a DVD on how to strip and rebuild the TD/TF gearbox.

MG Car Club.

Barrie's Notes on the 1953-55 MG TF (author). /=====/ Barrie's Notes on the MGB (author). /=====/ Barrie's Notes on the MGB GT V8 (author). /=====/ The Essential Buyer's Guide to the MG TD, TF and TF1500. (author)

M Phil, C Eng, BSc (Eng).

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