An idle regulator is one of the components you would find in something like a Bosch mechanical or mechano/electronic injection systems such as L or K Jetronic. It is an independent subsystem, usually controlled by it's own sensor, which works during cold starts. If you had said you had an early Porsche 924/44 then I'd have said the same thing. The Toyota uses a generic all electronic system developed by one of Suzuki's/Toyota's suppliers. It is non-OBDC, but still has a diagnoistic port. If your mechanic doesn't know this, I suspect it's time to change mechanics

A diagnostic test (depending on the vintage of the ECU) uses one of two possible test strategies:

- switches out the ECU and makes individual tests of all the system sensors (resistivity, conductivity, etc.)
- assumes the ECU does this testing itself and reads out any associated fault codes associated with a sensor test failure.

I would have thought a '91 non-ODBC ECU from Toyota would be the first case (I may be wrong). If so, a garage test will test out the temperature sensor directly. They should then be able to tell you what's up.

I would do two things in your shoes, maybe with the help of a spanner-savvy friend if you are not sure yourself:

- check the coolant level in the header tank, and also the colour. If it is low, top it up with clean anti-freeze rather than just water. If there is evidence the coolant is "muddy" (corrosion) or has an oily sheen (burst head gasket causing oil contamination) then you need to have it looked at. The first case is simple: have the engine flushed out and the coolant completely replaced with fresh. You should be doing this once every 1-2 years in an alloy block engine like yours anyway. In the second, then you really need to get a compression check done and have the head gasket replaced if necessary. A telltale sign of this condition is that you get a lot of steam out of the exhaust during startup and for quite a while afterwards. Don't leave this too long, as it will result in a dead engine.

- if the coolant level and colour looks OK, locate the temperature sensor for the engine. It should be near the thermostat towards the top of the engine, and there will be a lead running to it. Remove it and check if it is coated with residue. Prior to removal, also look around the body casing of the sensor to see if there is any corrosion or anything indicating the sensor casing has failed and it is leaking coolant. Replace it after cleaning and see if it makes a difference.

Either of these is pretty simple and easy to check before spending a lot of money at a garage. Don't be scared of it!

My current bet is on low coolant, with corrosion fouling of the sensor caused by exposure to steam above the coolant.