Told you it wouldnt work get a twin piston and be done with problems (until it starts to leak!) or blip the throttle when down changing/stopping wont cure it idling badly tho as you may of noticed.

check the temp guage sender/wiring. Pull the plug off the sender and with the ignition on earth it on an exhaust stud (makesure it isnt hot lol) and get someone to check the guage goes right upto max!.

if that is fine, replace the sender, their only a few quid and takes 2 sex to do.

the sender is located on the right of the exhaust manifold, below the engine lift eye, black plug, brown wire iirc, small, plug just pulls off.

hth.