Hi,

I've recently bought a low mileage 2006 4.2V8 S-type.

After the revs settle down on startup when cold the idle is rough and vibrates the whole car. The engine has cut out once.

When it's warm there are no issues and drives well.

I've also noticed the temperature gauge shoots up to midpoint on start up and then settles to where I'd expect it to be after a few moments.

I'm planning to change the thermostat and the coolant temperature sensor but I'm not sure where to go with the rough idle.

Seems like this would be an idle control valve but I understand the S-type doesn't have one?

Thoughts would be appreciated.

 

Welcome to the forum. Congratulations on your purchase.

I'd suggest holding off on the thermostat for now. I think a new temperature sensor would be a worthwhile gamble.

Please keep in mind the temperature gauge is deliberately misleading. From approximately 180-230F, the needle is centered. This is considered the 'normal' range.

If you're seeing an indication within that range when the engine is cold, that would most likely be a defective sensor. Once the coolant warms slightly, the indication seems to right itself. Seems like the sensor puts out an erroneous signal when cool.

The powertrain control module receives this same bad signal and thinks the engine is already warm. For a cold start, the computer behaves a little differently to keep things stable. For a warm start (real or not), some of those protections are inhibited. That might explain the rough idle, with the engine still cold but the computer thinking otherwise.

If you do feel compelled to also throw a thermostat at the problem, be sure to test the new one before installation. I've found thermostats to have a very high failure rate right out of the box. It's got so bad I won't install a new one without testing it first.

 

Thanks, I'll give that a try first!

 

I'd at least use OBD to check as many sensors as I think I understand are reporting plausible values and change as the engine heats.

At least IAT & ECT.