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MG MGB Technical - Temperature Creep

1979 stock MGB. Just when I thought all the problems had been fixed.....noticed over the last two weeks that my temperature gauge is slowly creeping toward the red zone - a little further each time I drive it (our weather is getting cooler - and the car ran at 1/2 during the hot summer). Fans are working, belt is tight, sending unit replaced. Could it be the water pump failing? For what it is worth my fuel gauge started acting up about the same as the temp guage. Sometimes it reads full, then 3/4, then full again. When it get to half tank it just drops to empty (with gas 1/2 tank of gas). Thought I would mention for troubleshooting purposes. As usual, appreciate all the advise!

Mike
Mike D.

Mike, Sounds like your voltage stabiliser may be bad. I don't know where it is on a 79, it's on the firewall near the pedals on earlier cars. Look for a varying voltage on the light green and green lead attached to the stabiliser. It should vary from near zero to near 12 volts around one time per second with the ignition switch on. Clifton
Clifton Gordon

If the problem started fairly suddenly, it could
be the thermostat, or failed pump as you suggest.

Also, if the distributor advance suddenly broke
then running retarded will cause you to heat up
fast. Also running lean (sudden air leak?)

The worst-case scenario is a failing head
gasket (try a compression check, maybe a
"block tester" (blue fluid) or a cracked head.

I would expect a silted-up radiator to cause
gradually hotter running conditions, not the
rapid change you have seen.

Does the problem occur at all speeds? Freeway
speeds? This would give more of an indication
also. This is certainly a frustrating problem
to diagnose.

One more obscure cause -- check the air temperature
regulator on the air cleaner. If the flapper is
stuck in the position away from the manifold,
then the engine will heat up fast.

Oh yeah -- a clogged catalytic converter (check
for vacuum dropping to zero when revving the
engine with a gauge on the inlet manifold)
will cause overheating.
Ronald

Mike, Paul Hunt's web site has a good trouble finding quide for the gauges and the stabiliser.
http://www.mgb-stuff.org.uk/wn_techlinksframe.htm
Click on Hammer and Spanners, electrics and then gauges. You will note Paul states the voltage from the stabliser varies about 2/3 second, so disregard my quote on that.
Hope this helps. Clifton
Clifton Gordon

Ron - I don't have a cat. converter (guess I shouldn't use the term stock) and all the pollution stuff is gone. Problem occurs once warmed up, the temp doesn't go back down while driving at any speed. Distributer is working fine. I replaced thermostat last year (185) but that doesn't mean anything....Is there anyway to check the water pump? Don't see any water in the oil.

I will check the V-stab in the morning.

Mike

Mike D.

Mike, I would do a compression test first, for
peace of mind if nothing else.

The temperature gauge or sender may be suspect,
and as pointed out by Clifton, the voltage
stabilizer could be bad. Can you cross-check
the temp. gauge with a thermometer?

I think the blue fluid block tester is really
worthwhile to (almost) conclusively rule out a
combustion leak. NAPA stores have these.
The tester goes in the expansion tank on the 79,
since the radiator itself doesn't have a cap.

Backflushing the radiator and block wouldn't hurt,
how does the antifreeze look? Still sounds like
the voltage stabilizer or thermostat are
the most likely culprits.
Ronald

You can usually (but not always) see a leak when the pump starts to go. There is also free play when you grasp the front pulley and move it side to side.
Terry

I agree with Clifton's postings on this problem. Since there are two gauges that have started acting the same way at the same time, I would bet the farm on the voltage stabalizer. A thermometer in the radiator fill neck should confirm whether or not there is really a temperature problem. Cheers - Dave
David DuBois

Mike,

I would go with Clifton's suggestion. This has happened to me twice on my 79B - it's not that the voltage stabilizer is particularly unreliable, it's that I've owned the car for 19 years, so some problems come around again!

The first time, the fuel gauge variations were, like yours, the most obvious change, so I assumed the sender unit was bad, changed it, no difference. Then I noticed the temp gauge, like yours, was subtly changing with time, and eventually came down to the voltage stabilizer as the only common item outside the wiring. The second time I went for it straight away! Having said that, the possibility of a bad ground is also there.

Good luck!
Kevin Kelleher

This thread was discussed on 28/08/2002

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