Here is an interesting problem. I usually wash the car by hand and have experienced a problem in hesitation and rough idle? It clears up after driving it for 10-20 minutes. I also noticed it happens if I drive the car in the rain? No error codes are displayed?

I was dealing with a coil problem but that situation is corrected. New coils and cat. Car runs nice now in good weather.

Any ideas welcomed to help pin point. You can eat off the engine...

 

My thoughts after opening the bonnet on one of ours and having a peep.

There are 3 electrical connectors just aft of the radiator panel plastic cover.

The TPS
The Airflow meter.
The Intake Air Temp.

I would unplug all 3 and clean and DRY the plug and socket, then a small spray of WD40 or similar, prior to replugging.

Not much else smacked me that would get damp in the driven rain, and maybe also during washing.

 

The S-Type is notorious for collecting water in the trunk. Try lifting the trunk floor and looking in the bottom of the spare tire well. Some electronic modules are located there, but I don't remember which ones or if they'd have any effect on your engine.

On my '02, I was having trouble with water collecting back there, but it didn't cause any engine problems. A new trunk lid seal took care of the leak. Not sure this could be your problem (maybe you've got more water than mine?) but unless you've got an angry trunk troll demanding payment before letting you pass, it's free to look.

One other thought: Whenever I take my car through the carwash on a hot day, the outside air temp display reads approximately 20 degree F lower for a few minutes until airflow dries it out. I don't know if this is a faulty sensor or a bad connection, but it always goes away. The only downside is this causes the climate control to reduce the AC output, thinking it's a cooler day. It only happens at the carwash, and never in the rain. (Living in western Oregon, I've experienced all sorts of rain...) I don't believe the outside air temp sensor is connected to the engine control system in any way, but you could have a similar situation with a different component.

 

The only sound bit of advice i can give you, is a similar fault i had with my 1st s type.

When washing, or parked the car facing down hill in the rain, the wiper grille is exposed to water seeping in, onto the coil pack, resulting in rough running until dried out.

The garage who works on my cars told me this, i'm no mechanic, but it made since, when he explained to me.

Another thing that happened the same time was, the front windscreen fogged over very fast too !.

Hope this helps, Shane.

 

All, thank you for the insight! Ive been traveling.

I have not noticed it in a while but have noticed it only under these conditions;

1 - When I wash my car

2 - When I go through a car wash

I have not checked but will do both and check these suggested areas! Right now she is running nice. At some point though I would like someone else to drive. I just don't find it very responsive on the throttle. Sluggish on the low end. I have to move the pedal a good inch or more to make her move....unlike my Volvo S80....This car is pristine in and out and you can eat off the engine! But easy on the eye and a head turner....

 

Be sure to check fuel trims as hesitation can be a leak. Also can mess up throttle response because the PCM simply can't get it to do what it expects let alone instantly.

 

Are all the cowl drains clear? Is water standing anywhere in the engine compartment? If you unplug any electrical connectors, use electrical silicone grease on the contacts when you plug them back in. Are your hood gaskets, front and rear in place and in good condition? The gasket over the grill goes first but is important. It is only a foot long or so. After a wash, park the car and open the hood (bonnet) and look for the water. There's your sign.