VW Sharan Diesel Issues: High Fuel Use & Rough Idle

This sounds familiar, I had a similar issue with my 2011 VW Touran diesel about a year ago. I have some experience working on VWs but usually leave complex electrical issues to professionals. The symptoms match what I experienced, check engine light, increased fuel consumption, and rough idle. In my case, the problem turned out to be a loose wiring connection to the manifold pressure sensor. The workshop diagnosed it using their diagnostic computer and found the faulty connection was causing incorrect readings, making the engine run rich. The repair was straightforward once identified, they repaired the damaged wiring and plug connection, which cost 95€ including diagnostic time. After the fix, fuel efficiency returned to normal and the engine idle smoothed out completely. To help diagnose your specific case, could you share: Current mileage on your Sharan, Whether you notice any other warning lights, If the engine knock varies with engine temperature, Whether you have any error codes from a diagnostic scan These details would help determine if your issue is similar to what I encountered. The increased fuel consumption definitely suggests something affecting the engine management system.

Thanks for sharing your experience. I had my last service at 109522km in June. The symptoms you described with your Touran match my current situation perfectly. My poor performance issues started after some heavy rain, which makes me think water might have affected the wiring. The malfunction indicator light has been inconsistent, sometimes flashing, sometimes steady. The knock sensor issue seems likely given the timing. Right after starting the car each morning, the knocking sound is quite noticeable and fuel efficiency takes a real hit during the first 10 minutes of driving. I will get it checked at a workshop since the symptoms point to an electrical issue affecting engine management. Better to address it now before it leads to more serious problems or further increased fuel consumption.

Thanks for those additional details about the symptoms following heavy rain. That definitely reinforces my initial suspicion about an electrical connection issue, especially given how similar your symptoms are to what I experienced. After my repair, I learned that water intrusion commonly affects the wiring harness connectors in these VW engines. When my Touran had this problem, the mechanic found corrosion in several connector pins near the manifold, causing intermittent ignition timing issues and incorrect fuel mixture. The flashing malfunction indicator you mentioned is particularly important, in my case, this indicated the engine management system was receiving inconsistent signals. The morning knock and poor fuel efficiency during warmup are classic signs of the engine running with incorrect timing due to faulty sensor readings. One thing I noticed post-repair: the workshop applied dielectric grease to the connections to prevent future water-related issues. It might be worth asking your mechanic about this preventive measure. Since the fix, my engine knock disappeared completely and fuel efficiency returned to normal levels. Given our identical symptoms, I strongly suspect you will find a similar wiring or connector issue rather than a complete knock sensor failure. This would explain the inconsistent behavior and why the problems worsen after rain exposure.

Thanks for the detailed information. I just booked a workshop appointment for next week. Your experience has been really helpful in understanding what might be wrong with my car. I checked my onboard computer and the current mileage is now 110750km, so around 1200km since the last service. The auto repair shop said they will do a full diagnostic scan, focusing on the wiring connections and knock sensor given the symptoms. They estimated about 90€ for the initial diagnosis. The poor fuel efficiency is really noticeable now, about 7 liters more per tank than usual. The warning light behavior matches what you described perfectly, sometimes steady, sometimes flashing. And yes, the engine knock during cold starts is getting more pronounced. If it turns out to be a wiring issue like in your case, that would be a relief compared to a full sensor replacement. I will definitely ask about the dielectric grease treatment you mentioned to prevent future problems with water exposure. Will update once I get the diagnostic results and repair completed.