Have just bought a '71 MGB with a 3.5 V8 from a Rover P6. Every so often the temperature gauge reading rises very suddenly (10 to 30 seconds) and goes completely off the clock - right into the oil pressure segment of the (dual) gauge. There are no obvious symptoms of overheating and the oil pressure remains rock solid. After a time the temperatutre reading falls almost as fast as it had risen.
The guy I bought it from reckons that it could be an over sensitive gauge (that the ether filled core type can do this) or that the electric fans (which are permanently on) could be blocking the airflow to the read(??). My local tuning guru reckons that the capillary gauges either work or they don't and that this is serious. Any words of wisdom from people with more experience than me?
Temperature fluctuation
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Ian Anderson
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More than likely a thermostat with no bleed hole. Or if it has a bleed hole its not at the 12 o'clock position. Or you have the right stat but one hell of an air lock.
This was my first symptom when I rebuilt my factory V8 MGB. The new stat I got had no bleed hole. I would get about 2 miles from home on a cold engine and the gauge did the same thing. Off the scale for a few seconds while I tried to pull over to see what was wrong and then all of a sudden it would drop quickly back to cool before settling down for the rest of the run.
Its scary when you see it happen eh
Fans running act as a block to air flow across the rad core?? What planet is this guy on
Your tuning guy is right. Capillary gauges work if they have a full charge of vapour. If they leak and lose the vapour they dont work at all. But as with all mechanical gauges they can drift out of calibration. This normally manifests as an over or under reading. But never a wild swing in display if the sensing bulb is in a temperature stable medium. My guess is that your gauge is representing what the sensing bulb is seeing. A sudden rise in temperature as the engine warms and hot water backs up in the manifold before the stat opens fully and slugs cold water back over the sensing bulb as flow goes from almost nowt to full bore.
If you have an electric fan kit fitted then ditch the 88-90C stat that you probably have in there and replace it with an 82C stat. Make sure it has a bleed hole in the flange and that its fitted with bleed hole at the top.
Make sure that the rad fan controller is set around 88-90C and you should be good to go.
Perry
This was my first symptom when I rebuilt my factory V8 MGB. The new stat I got had no bleed hole. I would get about 2 miles from home on a cold engine and the gauge did the same thing. Off the scale for a few seconds while I tried to pull over to see what was wrong and then all of a sudden it would drop quickly back to cool before settling down for the rest of the run.
Its scary when you see it happen eh
Fans running act as a block to air flow across the rad core?? What planet is this guy on
Your tuning guy is right. Capillary gauges work if they have a full charge of vapour. If they leak and lose the vapour they dont work at all. But as with all mechanical gauges they can drift out of calibration. This normally manifests as an over or under reading. But never a wild swing in display if the sensing bulb is in a temperature stable medium. My guess is that your gauge is representing what the sensing bulb is seeing. A sudden rise in temperature as the engine warms and hot water backs up in the manifold before the stat opens fully and slugs cold water back over the sensing bulb as flow goes from almost nowt to full bore.
If you have an electric fan kit fitted then ditch the 88-90C stat that you probably have in there and replace it with an 82C stat. Make sure it has a bleed hole in the flange and that its fitted with bleed hole at the top.
Make sure that the rad fan controller is set around 88-90C and you should be good to go.
Perry
Perry Stephenson
MGB GT + Rover V8
9.62 @ 137.37mph
Now looking for 8 seconds with a SBC engine
