My Marea starts badly when cold. I have to crank about 6 or 7 seconds before it takes off. Then the rpm starts fluctuating wildly between 500 and 1500 before it stabilises at 850, unless I help it by giving some gas.

I have removed and cleaned the idle valve on the throttle valve body but it won't help. Spark plugs are brand new.

The coolant tem. sensor should also be fine as the temperature gauge on the dash reads ok.

Any ideas what I can check more? One thing, the exhaust manfold is cracked causing leakage, could this be the cause? Does the lambda sonde measure the % oxygen or its amount? If it's the % then I think that a cracked manifold shouldn't have any adverse effect.

As far as I know there are separate sensors for the temp gauge and ecu. Ecu has the two wire and gauge has one wire where body acts as ground. If you got a scanner you could look at the temperature the ECU reports, and input airtemp and such at the same time.
Only times Ive had trouble starting and revs not picking up/revving/almost stalling have been under -20C conditions without heater. Solved that by putting thick cable from negative battery to the starter mounting bolt.

My temperature sender mounted on the thermostat has three leads

Well, different models have different sensors due to mounting. Wiring diagrams still show separate circuits for ECU and instrument cluster.

Ok, so I understand that there is also an ECU temperature sensor which controls this issue..

"The coolant tem. sensor should also be fine as the temperature gauge on the dash reads ok."

Just pointing out that. Might not be an issue.
Bad fuel pressure? prime pump couple times before you start.
Also when you disconnect sensors, the ECU starts using preprogrammed values or calculates them from another sensors (like throttle position from intake pressure, without pressure information it guesses them from rpm/mixture/such), so you could try finding the bad one by that way if you dont have a scanner.
Cant really think other stuff if the idle control is cleaned.
Also make sure the pipes from airfilter to engine are not cracked/split.

Hi,

"As far as I know there are separate sensors for the temp gauge and ecu. Ecu has the two wire...."


Excuse me for my ignorance but where is the ecu sensor located?

I also hearing sissing sound at the throttle valve body, seems like a (vacuum) leakage. Is this normal, I mean am I hearing the air being sucked in the throttle valve bypass or could I have a leak on the body?

Meanwhile I have checked the fuel pump, which seems to be priming ok (though I've noticed I don;t hear the noise each time I set the ignition on). Primed couplle of times beforer cranking but the problem persists.