E34 525auto heating prob & petrol v diesel

[bimmerholic]

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I currently have a E34 525iSE Sport saloon 'K' reg (1993) auto, petrol M50 engine that has done 220,000+miles & still a great & fast drive, magic. Unfortunately only does about 20mpg around town. On very odd occasions (maybe once every 2/3 months) (including a drive of approx 100mls) the temp guage decides to rise rather quickly towards the red but I can control it by putting the heating on & you can see the needle drop with no problem. I had a new viscous fan put on but it happened again on the test run! It normally only happens the one time before going a month or two; so they put the old one back on! I have to top water up occasionly but this also fluctuates between a cup & a litre every few weeks.?

I would be interested to know the performance difference between a petrol & diesel E34-5 series, for instance does the same size engine compare or do you have to go larger & what are the BHP/Torque figures. I would only have a few £grand(sterling) to spend & wonder what is the best comparison, hopefully affording one of the last E34's 95/96, M/N 'reg'? My preference would be auto with cruise & a/c etc

[632 Regal]

air in the cooling system?