Our 2002 TDI has started surging at idle – especially in gear after a cold start, it seems to get better as the car warms up – and smoking (mostly HB's, vile habit). This has been going on for about a year and has slowly been getting worse. The car is an automatic, we got it in 2004 and as far as I know it is still on its original injectors, injection pump, and turbocharger.

The smoking seems to be continuous – I see a light stream of smoke coming out when I'm driving behind the car on the highway – and fuel economy is down. Occasionally there is a large puff of black smoke on acceleration, but it does not happen every time. It seems to be most likely to happen on the first hard acceleration of a trip or warm up cycle (e.g., when merging onto a highway).

Here's what I know:

  • No fault codes.
  • Injection timing is within spec (checked using VCDS, got a fuel temp value of 120 and a timing value of 60).
  • Turbo output seems OK. Testing with a Mity Vac I can see full range on the VNT actuator (it starts moving as soon as vacuum is applied and hits the stop at about 20" Hg). Logging boost while driving with VCDS shows that the turbo output tracks ECS requests – I don't know how to judge the accuracy of the tracking, but the turbo is definitely responding.
  • The intake manifold has been recently (< 2,000 miles) cleaned.
  • As far as I know the injectors and injection pump are original and have about 270,000 miles on them.
  • Timing belt was replaced about 1,000 miles ago.

Here's what I've tried, checked:

  • Cleaned all of the engine compartment grounds.
  • Replaced the air filter.
  • Replaced the fuel filter.
  • When I changed the fuel filter, I filled it with Liqui Moly Diesel Purge and ran another can of Diesel Purge through the system by pulling the lines from the fuel tank and running them into a bottle of Diesel Purge. The engine seemed to run better as soon as I did this. There was no surging or smoke running with no load through a range of speeds. When the bottle was empty, I reconnected the fuel tank and the engine seemed to be continuing to run well. It drove well and I couldn't produce any smoke, even on acceleration. I was ready to write Diesel Purge testimonials. All of this was on a warm engine and was less than 10 miles of driving. The next morning my partner drove the car and it was back to surging and smoking…

I'm thinking that the problem must be either the Engine Control Unit, the injection pump, or the injectors. Since they are all pretty expensive I'm hoping to narrow the problem down some before I start throwing money at parts.

After my experience with the Diesel Purge, I'm thinking that the problem may be fuel related – I've read that ULSD fuel doesn't lubricate the way older fuels did and this car is pre-ULSD so it seems like that could be a problem. I'm currently running Stanadyne Performance Formula which VW recommends, but I can't tell if it makes a difference or not. They also make a Lubricity Formula, but it is not available locally, so I haven't tried it yet. Thinking it is fuel related has me leaning towards either the injection pump or the injectors.

I'm hoping for some ideas/knowledge about what could be going on. For example:

  • Do the symptoms represent a failure mode of any of the components?
  • Is it likely that improvement from running on straight Diesel Purge could be had with an additive at normal dosages?
  • Suggestions for further troubleshooting?
  • Other possible causes of the problems?
  • Have you checked for any obstructions in the air intake system? Just a thought. – Pᴀᴜʟsᴛᴇʀ2 Jul 6 '16 at 10:45
  • A good one, thanks for asking it like that. I really haven't – while I've changed the air filter, cleaned the EGR deposits out of the intake, and sopped up the oil in the intercooler, I've not done a methodical check for obstructions. That's a good idea. – dlu Jul 7 '16 at 17:22
  • As you know, black smoke is indicative of unburnt diesel and is more or less normal for a moment upon hard acceleration. But what color is the smoke that you can continuously see when driving behind the car? Is it black as well? – Tedwin Jan 17 '17 at 18:08
  • Have you a possibility to compare the airflow against the amount of injected fuel? Have you made a measurement of the feedback line? – Martin Apr 28 '17 at 8:03

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