what could be wrong
 

i have uk twin turbo supra that has been parked up due to a faulty alarm this past month, the alarm has been took out but went to start the car for the first time and the car seems to be under fueling and the rev hand is dropping to 500 and less in revs i left the car running for 20mins but its still spluttering like its going to cut out... then i took the car for a short run down the road but from leaving the house to getting back there is wicked steam leaving the exhaust covering the road behind the car.. its steam and not smoke as i got the wife out to rev the car at the front of the house and its just crazy the area it covers when reved but when you lift your foot of pedal it goes back to spluttering... the car is well looked after and all coolant level and oil levels are good

any ideas?

Unusual to get quite so much steam, although these cars do steam a bit due to the size of the exhaust systems.

Before I get too worried I think i'd reset the ECU (remove the EF1 & EFI2 fuses for 5 mins, or disconnect the battery for the same amount of time) then take the car for at least a 20 min drive of different types of driving. Then see how it is after the drive.

This would allow the ECU to re-learn settings and also clear any moisture\steam. If there are still problems idling after that i'd look to clean the ISCV.

I'd check that all your plugs are firing correctly (IE plug condition and gap checked, and coil packs)