It simply changes its resistance based on the coolant temperature. If its reading out of whack the ecu will think the coolant temp is different that it actually is. Adding more or less fuel then it should.
It would prob cause a fault code if unplugged or completely faulty (break in wiring)

Bgb is easy to find. Just google Mr2 bgb and the first few web pages will have it.

Now you mention it
When I take the oil cap off it does smell a bit like fuel in there,
Is there anyother signs to show a possible oring failure

They look fairly black besides the electrode section
Do my spark plugs show it's running ok?

Don't really want to pull it apart to replace injector rings
But the head is coming off next month for a full reco and some bigger cams and valve springs etc so may wait till then

tested coolant temp sensor, tested within spec.
going to check base timing.
very sure it may be off as it is gutless taking off.

what is the correct heat range for my gen 2 motor
perhaps the may be the issue?
I think I have bkr6e iex

Doug, the motor had about 230k kms on it when those plugs were taken out, and they'd been in for about 40k kms.
But it's now up to about 265k kms and still running the same - great.
When I bought the car I must have been lucky and got a good one.  

Problem has been solved, ended up being a combo of timing not properly adjusted, and a partially blocked fuel filter.

Fuel filter was replaced when i brought it as the one in it looked VERY old still toyota genuine probably replaced once..
replaced with Ryco- than full fuel cleaner run through, 10xxxkms later used a toyota injector cleaner.

so id say it put alot of crap from the tank in to the filter? as i just replaced it friday with a genuine one then had timing checked and found exhaust half a tooth out and it reading 8 degrees
timing fixed and adjusted to 12 degrees.
runs like a dream now.