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MG MGB Technical - comedy water and oil gauges?

Hi guys. The water temp and oil press. gauges on my '69 BGT seem to have most unusual characteristics - the water temp climbs quite quickly from cold to nearly all the way to hot, before dropping back to just under normal where it then stays. The oil gauge seems to hover around 50 to 60 psi, but doesn't drop at idle. Is this normal? Cheers, Rich
r j symons

The temp gauge is reading things well - this is the pattern, on start up, thermosat is closed, engine heats up and then gradually, thermostat opens and keeps the tem[erature constant (or tries to keep it constant) from here on in. The water will get heat quite a bit during thermostat opening until full flow is assumed.

I would have thought your oil pressure would drop a little bit at least on idle...
Hal Adams

Ok, thanks Hal. I guess that it's due to the sender being in the head as opposed to in the rad like modern cars. It's just that I've never seen a gauge do that before!

Also, sometimes the engine tends to run on for a few seconds after switch-off. Any ideas as to why?
r j symons

Hi Rich.
Try running you car on a higher octane fuel and you will probably find the engine dose not run on.

Regards Patrick
P.T. Tighe

Check the oil gauge with a reference mechanical one. Then there are procedures in the shop manuals to trouble shoot it to the gauge or sender. Normally I suspect the sender.

You should have a pressure drop at idle on a hot engine. How much depends on engine bearings/oil pump/relief valve/idle speed and oil temperature.

Overall things seem to be good.
Robert McCoy

Your engine running on is more of a problem to me than your gauges!

As Patrick indicates, cause is not helped by the lower octane fuels we have, so run the car on as high an octane as you can and make sure your timing is spot on is all you can really do. It is not good for the engine to run on, so do keep it in good tune.

If it persists, you can buy an anti run-on valve kit, or easier which a lot of people do, is to leave the car in gear with the brakes on before you switch the engine off, then, just as you switch off, gently let the clutch out - effectively stalling the engine before it tries to continiue running.
Hal Adams

Thanks for all your input, folks. Forgot to mention that both gauges are non-electrical. Looks like I'll be using super unleaded or adding a drop of octane booster from now on. Cheers, rich
r j symons

Also, the design of the Westlake cylinder head's combustion chamber lends itself to run on. There's a point, in the middle of the chamber, that gets very hot and can act as a spark plug when the engine is shut off. Grinding down this high point often cures the run on problem. RAY
rjm RAY


Running on after switch off is a well known minor problem on B. In america we name it dieseling. I owned a '68 GT calif. spec. and it was almost impossible to stop that. But search archives on that topic Furthermore than modern low octane fuel, it is also cause by rich mixture or bad carburetters adjustments or too much advance or carbon built-up. It could be any of thoses enumerated or a combination of many. It is not destructive to engine, except people look at you stangely and believe that car need an exorcise...

Jean Guy Catford

Really, the way to go is an anti-run on setup found on N.A. smog cars. They are bullet proof and shut the engine down instantly. VEM
vem myers

This thread was discussed between 11/10/2012 and 21/10/2012

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