Thermostat and Engine Mounts produce chugging and smoke – Diagnostic Consultation on an Audi A4 1.9 TDI (2007 – 164,260 miles)

⚠️ Vehicle not running right? Black smoke, noisy, chugging and overall poor performance?

🚗 Here we have an Audi A4 1.9 TDI (2007 – 164,260 miles) booked in for diagnostic consultation and potential Engine Carbon Clean.

ℹ️ The owner booked it in as it was ‘chugging’ when coming off the throttle, smoking black on acceleration and generally not running at its best. Having been looked at elsewhere previously and no codes being found we were then called to take a look and try to work out what was occurring.

🔎 Initial inspection found little of note in the engine bay and there were no codes present. Dashboard temperature appeared OK at idle. There was a small amount of jelly wobble at idle up to 1,200rpm which suggested engine mounts were a little worn.

Chugging when coming off the throttle can sometimes be down to insufficient flow through an EGR, but having had a new EGR not that long ago it would be unlikely to be due to the EGR itself (perhaps pipework or inlet manifold).

The owner took us for a quick spin whilst we rode shotgun working the diagnostics.

📊 The EGR flows (desired vs actual) looked good and EGR movement was fine so that eliminated the EGR as the issue. Whilst monitoring the live data we saw the coolant temperature had dropped significantly whilst driving. The cold air coming in over the radiator had cooled everything down and the thermostat wasn’t closing properly – the culprit had been found. We brought the vehicle back and when idling the temperature climbed again (giving a false impression everything was OK).

👨‍🔧 No clean on this one. The original issue is a thermostat not closing properly meaning the engine is running cold when driven. Running cold will produce more emissions, hence the black smoke and over time this will have left a lot of buildup around the system adding to the black smoke issue. A cold running engine will vibrate more and with the partly worn mounts would produce the ‘chugging’ reported by the owner.

Next steps for the owner are
1) replace the thermostat.
2) get us back again to get the engine cleaned up to get it running smooth and quiet again.
3) replace the engine mounts once the clean is done.

☑️ Full live data Diagnostics
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