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MG MGB Technical - OVERHEATING issues

I need your help folks... My 70 B APEARS to be overheating from the gauge reading just by backing out of my driveway and traveling around the block a few times. I have tryed all the usual things ie... changing out the thermostat and gasket, the radiator all be it "used", cap and both upper and lower hoses and clamps, I replaced the 5 blade metal fan and checked for fin loosness all around, the temperature tramsmitter, I replaced the coolent with a new mix of 50/50 + an additive that claims to reduce the operating temp by 25%. I removed the licence plate from in front of the grill opening to allow as much air through as possable. I do have a pair of OE Lucas driving/fog lamps back mounted but they are far enough apart not to obstruct air flow much (I think). I installed a foam block above the radiator to force as much air through the radiator as possible. It does not puke out any coolent on the ground however a small bit does come out from the overflow tubing at speed. Here are my thoughts... could it be a faulty temp. guage as this is original to the car or a faulty water pump not moving enough coolent through the block however the pump is not leaking or making noises and the belt is tight with no slippage. Another option might be some sort of blockage within the blocks water passages. When I removed the Thermostat housing to replace the thermostat I noticed a whitish powder had formed inside the housing with a rusty deposit along the lip of the base leading me to speculate the blockage within the block. As I have stated I replaced the radiator however I only flushed it out with the garden hose and did not boil it to clean out the core but I knew the history of this radiator and the car it came out of and both were working fine when removed. I am out of ideas however I do have a few questions to ask, (1) How would I check the continuity of the wire going to the temp. transmitter? (2) and How do I check the gauge it self? (3) Is there another device inline that might have failed? ANY ADVICE or COMMENTS are appreciated, Thank You
Jeremiah Randolph

What is your timing set at? Too far advanced will cause it to run hot too.
M Whitt

I did remove and rebuild the 25D distributor about a year ago all new "tune up" stuff plus new cap, wires springs, counter waights, drive dog ect.. The center shaft was very dry and trying to seize up on me but I can not remimber what I set the timing at. I will check and get back to the board on this.
Jeremiah Randolph

Check the temperature sending unit in the block. Mine went bad and indicated I was running hot - but I knew I wasn't from other actual temperature readings. I went through four of the sending units before I found one that worked correctly. Try another sending unit.
Robert Browning

You could check the actual cooling system temperature with fair accuracy by placing a candy thermometer in the coolant in the radiator filler neck on a warm idling engine. It would be good if you had it at the point where the dash gauge is reading "H". The filler neck temperature should be close to the head temperature. I don't know the exact conversion, but I would expect if the gauge is on H it should be about 225°F or above. If you check the radiator coolant temp and the candy thermometer reads 195°F or less that would be normal, and I would suspect the gauge itself. Just be careful taking the cap off the rad, it might be better to run the car with the cap on loosely and then remove it without the pressure build up. Easier yet would be if you knew someone who had a spare gauge and wire you could try. I don't think you can rule the wire out so I would switch it too if you go with the spare gauge method. I used the candy thermometer on my MGA and it was within 3°F of the dashboard temperature gage. I would definitely verify the gauge before doing any more work. It sounds like you have covered just about everything with no improvement.

Good Luck,
Ralph
Ralph

Jeremiah,
Are you also having problems with your other electrical instruments? If the voltage regulator for the electrical instruments is bad, I suppose it could do this to you. Robert's comments are right on too. If this is a sudden occurence, most likely it is either an electrical problem given the short distances travelled you mention.

You might want to read the water temp at the radiator to see what it says. If it says your coolant temp is actually OK, then it is probably one of these problems.

A relatively rare occurence is that some of the wire bracing for the sand used in casting the inner passages may not have been cleaned out at the factory and finally rusted to a point where a chunk of it is now in some place where it blocks coolant flow. Pretty rare, but it has happened.
Bob Muenchausen

If it's puking coolant after a couple trips around the block, it sounds like the water pump is not circulating the coolant. This is just what my car did when I drove it about a mile without a fan belt. Pegged the guage and started puking. Luckily I didn't have to go any farther (I probably wouldn't have without damage).

I guess the radiator could still be plugged, you could have it flow tested or a tank taken off and rodded out. But I'd look inside the pump.
Tom

To answer Bob Yes I am having problems with all the guages now that I think about it, Fuel- is not going all the way up to full when filled, Oil Pressure is not reading at all or 0 to 10 psi most of the time Speedo- jumps the full sweep of the dial then just dies and then come back to life suddenly, Tach- does the same as Speedo, Temp- we have covered. Could all be related?
Jeremiah Randolph

I think you need to find out why you have little or no oil pressure on the gauge before you worry about the coolant temperature. Verify the the oil pressure with another gauge.

The heat and fuel gauges use a voltage stabliser and if it fails these gauges will not read correctly. The stabliser doesn't have any effect on your other gauges. Check the output voltage of stabliser, it should be less than 12 volts ususlly 9-10 volts but the reading may vary depending on the type meter you use (digital or analog).

Clifton



Clifton Gordon

"Temp- we have covered"

No, we have not. What temp face do you have - degrees or the C-N-H type/ What, exactly, does your guage show that leads you to believe you are overheating? Does the coolant boil?

Regards
Roger
Roger T

The standard voltage stabiliser switches 12v on and off about once per second (after a 2 or 3 second 'warming-up' period) and this *averages* about 10v, but you won't see this on a typical multi-meter. A steady 12v on the output of the stabiliser (light-green/green wire) indicates the stabiliser is not working, which could be down to a bad ground which it should get via its fixing screw. This will cause the fuel and temp gauges (and oil gauge on a 68 model) to read high. A low reading fuel gauge at full is usually caused by the sender having been changed, for a long time now they havbe had different characteristics to the originals. This also makes them read high at empty, which is far more of a problem.

A 70 oil gauge *is* electric but independant of the stabiliser. This must be investigated first with a mechanical gauge, the sender could be faulty, or it could be a bad engine.

Speedo sounds like a sticking cable.

Tach is probaly bad connections in the ignition circuit, possibly bad points.

All in all you have alot of remedial work to do!

If it isn't losing coolant or steaming it isn't overheating, the needle can get quite close to the H in extreme conditions, but still be OK. Measure the coolant temp in the radiator when the gauge reads high (remove the cap carefully with a cloth in case it *is* overheating and boils when you remove the cap). If that is OK then it is the gauge indication that is wrong, and the sender could be bad, as well as the other possibilities of the gauge, stabiliser, and wiring shorting out.
Paul Hunt 2

Thanks for the correction, Clifton and Paul. For some reason I was thinking that it also provided voltage for the oil press gauge. I changed over to a 1973 mechanical gauge on my 68 some years ago and I guess that is where my confusion comes from.
Bob Muenchausen

This thread was discussed on 27/07/2006

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