2.0 TDI Rough idle only when warm

[d1s]

I've got a 2004 2.0TDI (BKD Engine) which runs fine when the engine is cold, but develops a very rough idle when the engine has warmed up to around 90. I don't think it's the DMF as the roughness doesn't change as the clutch is pressed in.
I've just changed all filters and oil, which had no effect.
Any ideas what it could be?

 

[SAiLO]

Could be your EGR recirculating exhaust gases back into the engine or faulty injector wiring loom.

 

[fouryoghurts]

Have you got access to VCDS? You'll be wanting to check some choice live data, including EGR measuring blocks as @SAiLO says, before condemning anything.

 

[d1s]

My EGR has been blanked (after the rough idle developed) so I don't think it's that. Anything else I should check with VCDS?

 

[fouryoghurts]

I've just changed all filters and oil, which had no effect.
Any ideas what it could be?

To confirm, this includes the fuel filter?

You will want to confirm plausibility on coolant temp and MAF (tho not so much cos this is primarily for monitoring EGR blending) before confirming fuel pressure to spec out of tandem pump (which needs specialist kit). If MAF is over-valuing flow, you'll end up with a rich mixture (though here there is the potential for black smoke and increased mpg...and you would notice this during warmup). More likely is a CTS that is signalling erroneous voltage (on the high side of 'actual') to the ECM.

 

[d1s]

To confirm, this includes the fuel filter?

Yes the fuel filter was changed and I had the injectors cleaned.
The car also suffers from the warm start issue. Could that be linked to the rough idle?

 

[fouryoghurts]

The car also suffers from the warm start issue. Could that be linked to the rough idle?

All inline with defective coolant temp sensor. Be ideal if you can rule that out first.

 

[d1s]

All inline with defective coolant temp sensor. Be ideal if you can rule that out first.

Is there any way I can check if the sensor is defective without replacing it? Although I don't mind paying since they're only around £10

 

[fouryoghurts]

Yes. Check with VCDS for plausibility (most useful check is the value when cold - should be ~ambient, give or take). To check properly really needs an oscilloscope. But ye, I'd just be swapping out the sensor if I were you seeing as they are so inexpensive and a CTS going bad can cause no end of bother.

 

[d1s]

If the CTS would bad, would my oil temp gauge on the dash show a wrong reading?