Hi Guys

We've had heavy weather here recently and i drove through a flooded road on the way home a few days ago and i got a bit of smoke/steam from the exhaust and the engine now idles slightly rough and i noticed an oily rainbow puddle (v light oil) coming from under my car this morning.

Any ideas how i should go about diagnosing the problem, checks i should do to eliminate possible causes. etc.

More Info:
I saw the flood before entering it and raised suspension to L4, water was around the bottom of the door/lower-mid bumper level as i went through. drove through at slow speed.

As i exited the water i could see some smoke/steam behind the car which dispersed about 20m further along the road. Engine response was normal as i drove the rest of the way home.

The rough idle is what i'd describe slightly noisier than normal and slightly bumpy. i.e normal idle is approx 800rpm and the rough idle might be surging to 900 with no throttle.with very light/partial throttle at a standstill ie holding at 1krpm the revs hold then surge to maybe 1.2krpm then settles then repeats. no set pattern to surging, erratic and mild. under throttle and on the move engine response is normal/maybe slightly sluggish, but engine revs normally to red and boosts in normal range.

Have checked oil and seems normal, i.e. normal oil level and not clouldy/milky in any way.

Car spec:
Gen 1 2004 Allroad 2.5TDi 180bhp engine.
Thanks in advance