I suspect that my "Cold Start Switch", which appears to be similar to the Coolant Temp sensor next to it, is not working.
In case I have got their descriptions wrong, I mean the first one which goes to the ECU may not be working, while the other which goes to the gauge, is fine. I can test the Cold Start switch easily enough, but was wondering if it would be easier to have both the gauge and the ECU use the same sensor. At least then it would be evident when the sensor failed.  Right now my engine is running 10:1 rich and not boosting. Together - they are a symptom of the ECU thinking the engine is cold. 
 Now I suspect wiring the 2 "circuits" together would change the resistance and voltage, but my electronics knowledge is only fair. Does anyone have a definitive answer ?  
Thanks, as always....
 
 

Gen 2 3sgte engine? If so then it has 3 temp sensors in the water neck. One is a cold start switch, another the gauge cluster temp sensor and the last one which is green in colour is for the ECU. 

Aha, thanks for correcting. I think mine which is early 1990 might not have the 3rd switch. I will check. Either way
the one I need to test is the ECU one, and my question is about driving the gauge of it as well, so it is obvious to the driver when it is no longer working. 

You can test the green ecu sensor with a multimeter quite easily. The sensor itself is cheapish from Toyota and even cheaper for an aftermarket one.
 

 

As said, generally, water sensors are cheap to replace. You can also remove and lightly sand the surface of the sensor that's inside the coolant neck and they come good in most cases.

Mr2 3sgte BGB for how to test the sensors. Available online to download, or can provide.

First I have heard of your diagnosis; water sensor fail = no boost (exhaust gas spinning the exducer to inducer) as well as ECU activate cold start injector (tiny cc) providing an a/f variance to 10:1.

Hope you get it sorted or someone has more of an answer to help

Ok I measured the resistance of the temperature sensor at the ECU and it is fine. Impedance within  the BGB ranges for Hot & Cold.  Ok so I am at a loss trying to work out why my car is running rich. 
A dyno run about a year ago said 10:1 and 5psi, which is very common according to Dr Google.
The 5psi is likely to be an old wastegate spring and the TVIS bleed-off not working. 
Since then I have recapped the stock ECU, fitted an Omori boost gauge and it shows I am getting 9psi, which if true, says the TVIS is working. I am not so concerned about the boost pressure for now, but still the smell etc says still rich. I am at a bit of a loss as to why. I have static tested the MAF and the temp sensors and the O2 sensor.
 I am thinking I'll try to disconnect the Idle Cold Start, and put it on the dyno again. Might get a smoke test done, but I don't think I have any boost leaks....
 
 Does anyone have an original dyno report from when the 3SGTE was brand new around 1991 ?  Perhaps they have always been too rich (ie < 12:1).

Why a mr2 runs rich... General...
Signals to ECU; Incorrectly set adjustment screw in the air flow meter maf, incorrectly adjusted tps...
Service items; spark plugs, leads, distributor, timing, fuel injectors need servicing, fuel pressure regulator / return fault...
Annomilies; cold start injector stuck open, faulty valve / carbon issue, camshaft clearance
TVIS is checked by a boost gauge on the tvis at 4,100 rpm (from memory) - check bgb. Not checked because the engine makes boost.

Yes engines run rich with wide open throttle but it is adjusted by signals to ECU.

Dyno graph from 1991 - lol - I remember those years at dyno workshops and technology wasn't as it is today as it's the time before most people had dial up Internet and were using dos commands. Dyno graphs available online of stock mr2 from more recent years. However if someone has it would be amazing.

Unfortunately at the age of a mr2 there are many things that can be needing attention for optimal running (like new which was 25+ years ago).

Remove one of your spark plugs and take a picture of it so we can see what it looks like.  Better to take a picture of all four plugs would be interesting.

Thanks Tony,  I can discount some of the possibilities already checked or replaced. My injectors were cleaned a while back, but to be honest I wouldn't know if they were original size /flow or not. Maybe the little report has that and hopefully I kept it. I am suspect of the Cold Start Injector - would like to disconnect the fuel supply and test. Maybe put an on/off valve inline for when it next goes on the dyno.  Fuel pressure being high...from blocked return ? hadn't considered that.  TPS adjustment ? didn't know it was adjustable. While very pertinent to partial throttle, perhaps WOT discounts that as the culprit. Could be more than 1 culprit of course. 
 I know some people put after market ECUs in so that they get control of the fuel map etc, but I wonder how many had problems such as you've listed, and the better approach would be to sort the causes before moving to aftermarket.? Then again the aftermarkets probably have greater diagnostic capability ......
 
 Thanks Kojab,
 I've seen the plugs, all four are black. Definitely not a one cylinder problem, or distributor, leads etc.
I am new to turbos / 3SGTEs but been playing with NA engines (motorbikes) for decades. The car does not cough and splutter, it runs well, but it should run leaner  - I'd be happy with AFR in the 12s, very happy if 13s.  I believe the usual 14.6(?) for NAs is not appropriate / safe for turbos...