Very hard to start when cold

i have fitted a new thermostat heat working fine. but temperature gauge not working

you did replace the one for the computer, not the one for the gauge or light, right?

computer trying to compensate still thinks the turbo is there might need the non-turbo computer

cold starting is assisted by the use of a choke or fuel enriching device
if it has carby , adjust the automatic choke device
if it has injectors replace the ambient temp sensor

You have a bad battery cable or one that is corroded or loose. My '93 f-250 7.3 non-turbo did the same thing.The give away to the problem is the radio presets are disappearing. Check that the cable ends are CLEAN and TIGHT. If that isn't it, the cable itself may be bad look for swolen or cracked insulation or even heat damage on the cables.

Hard to say really, but the two cylinders with low compression are side by side, and I would have to guess that a head gasket may be the culprit.

YOUR PROBLEM COULD BE POWER BRAKE BOOSTER LEAKING OR THE NON RETURN VALVE. IDLE SPEED LOW.HAVE IDLE SPEED ADJUSTED.IT SHOULD BE 875 RPM.

Right way to do this is to match the post position & case size, then buy the battery with the highest cca you can afford. Don't use less than the one that's in the car now. Turbo does not require more cca than non turbo...it creates no extra drag on the engine when starting. Higher cca will help with cold starts and hot weather, generally not being "worked hard" will also extend it's life.

The bad ecu blew the isc motor

Welcome to the forum. Your question has to many variables. Cold weather out here in Cen Cal this time of year is mid 40s. Is the problem when the vehicle is cold as in ovrernight but starts ok when it is warm? Does it crank OK but seem like it's not getting fuel? If you hold the throttle slightly open, does it start OK?