Ok I recently posted about how my engine temp tends to drop below normal when cruising at speed. I did as was suggested and blocked the front of my oil cooler but it didn't really make that much difference.

However I have discovered something bizarre.

If I cruise along with the windows rolled up the temp reads normal.

If I roll down the drivers window within 10 - 15 seconds the gauge drops to below normal.

Window up - back to normal within 10 - 15 sec.

I did this a number of times so it was no coincidence.

I've got one theory but I'm interested in any suggestions.

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68 Morris Cooper S Mk1 (*ex 78 1275 LS 4th last built, 70 Morris 1500 OHC & 70 MiniMatic)

is the Heater tap open or closed

If open i suggest the extra airflow through the heater when the window is open

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I had this problem after rebuilding my Cooper. I sussed it out after a little while once I took the voltage sensor out of the equation.

The gauges operate with a heated bi-metallic strip. For a given temperature at the engine, certain resistance is created by the temperature sender which allows a certain amount of current to flow through the gauge. This heats the bi-metallic strip which then deflects the needle.

There are little holes on the back of the gauge provided for calibrating at the factory. If air blows through the holes, and then into the cabin because of engine bay pressure differential, it cools the gauge and the temperature across the strip drops.

You need to check the seal at the back of the gauge over the holes. Put a piece of masking tape over the calibration holes and see if the problem disappears. I think air can still get in however through other perforations in the case. I'm not certain at this time (I'm at Jervis Bay and nowhere near a mini) if the gauges have other ways in on the back besides the calibration holes.

There is/was also a rubber seal inside the chrome bezel which seals the glass. Often when the gauge gets pulled apart to chase the dirt out, the seal is not replaced. This allows a lot of air to bleed through and alter the temperature indication. Black silicone is good here.


Out of interest on the heater's effect on jacket temperatures, I've done some informal tests of engine temperature with the heater on and off and with the windows open. I've got a digital temperature gauge slipped under the top hose at the radiator. On a given day, with an engine temperature of 84 deg C, turning on the heater and heater tap gives a temperature drop of only 2 to 3 degrees at the top of the thermostat. This would increase as the coolant temperature increases, and no doubt help you on a hot day to keep the engine a little cooler, but for a non-overheating engine at operating temperature, the change is very small.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

I had a fuel gauge problem similar to this-
The faster I went, the lower the fuel level would read. Then, you would stop at the lights and watch the needle come back up!!
Sooooo, for a test I ran it up on stands at 100kmh.. and the needle stayed steady.
Conclusion- road vibes were disturbing the 1940s (bimetal strip) technology s0-called `voltage stabilizer'... so a new electronic Wombat special voltage regulator fixed it.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Holy Crap were you sitting in it thinking , SH!T i hope this thing don't jump of
these stands and take off out the door ? ( big balls if you ask me )

Wouldn't this be more about the inertia 'sloping' the fuel in the tank, and the gauge float being on the low end of the slope?

Smac,
Nope, the problem disappeared once I fitted an electronic voltage regulator/stabilizer. Been good ever since (5 or 6 years).

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Yes I was thinking along those lines perhaps there were leaks around the firewall and the draft increased the affect of the heater on cooling so I switched off the heater and still the same thing.

I already have one of wombats electronic stabilizers so the fuel and temp are always very consistant.

Then I was thinking along the same lines as Mick, that the extra draught may be effecting the gauge itself as I was thinking there is no way the engine could be rising and falling 10 degrees in 10 seconds.

Thanks, I'll take a look at the back of the guage for openings.

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68 Morris Cooper S Mk1 (*ex 78 1275 LS 4th last built, 70 Morris 1500 OHC & 70 MiniMatic)

Mick you were spot on.

I removed the black instument cowl and checked the back of the temp guage, there where no obvious openings, however, the cream central binnacle had a hole in it right near the temp guage that was unplugged. Would normally be for the panel light switch on early LHD cars.

I covered it over with tape and also placed a extra square of underfelt on the engine bay side to stop draughts.

Took the car for a drive and problem solved.

Thanks heaps

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68 Morris Cooper S Mk1 (*ex 78 1275 LS 4th last built, 70 Morris 1500 OHC & 70 MiniMatic)

Wow, good sleuth work. I would never have guessed it was all that sensitive in a million years.

Yes amazing, and I guess a testament to the quality if the smiths gear to still be working so well all these years later.

Also I've said it before and Doc has also including in this post, if you want accurate temp and fuel gauges in your Mini get yourself one of the electronic voltage regulators Wombat makes. A consistent voltage to the gauges makes them very accurate and consistant.

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68 Morris Cooper S Mk1 (*ex 78 1275 LS 4th last built, 70 Morris 1500 OHC & 70 MiniMatic)

HA!!!

I've always had the same problem with my fuel and temp gauges - I'll have a squiz

I reckon cars without any firewall blanket would be even more effected esp. the fuel guage. Back of the instruments would get quite cold at speed in winter.

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68 Morris Cooper S Mk1 (*ex 78 1275 LS 4th last built, 70 Morris 1500 OHC & 70 MiniMatic)

The firewall blanket doesn't help really...you only need a little bit of air bleeding through the gauge to upset it, and more than enough air can get around the blanket easily enough. Seal the gauge up as it should be and the problem will go away.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

yeah!!! mine do the same whether the firewall blanket is there or not - there's still always a breeze coming through the speedo