I have a diesel powered Audi A4 B7 (2006) 2.0TDi (the 170BHP variety) with a manual gearbox, which I've owned for around 18 months. The car has just short of 100k miles and has a full dealer service history.

The car was given a service just under 12 months ago, and it was a major service. It's next service is due shortly, but due to the age of the car, I'm considering doing this myself. I'm by no means an expert, but have done numerous DIY jobs on cars over the past 20 years.

The car isn't reporting any problems (and my OBD2 scanner reports no faults logged on the ECU)

The car is starting to become difficult to start when the engine is cold. My first thought was that it could be the glow plugs, however after some reading, I believe that glow plugs are only required when temperatures are approaching freezing, which isn't the case here in the UK at the moment. This morning, the ambient temperature was in excess of 10 degrees Celsius, and the car took around 5 seconds of starter motor activity before the engine fired up. Once the engine was running for a few seconds, the engine ran as normal.

Later in the afternoon, when leaving work (the car had been stood for over 8 hours), the ambient temperature was around 20 degrees Celsius, and the car took several minutes to fire up, with around 5 attempts with the starter motor for between 5 and 10 seconds per attempt. Again, once the engine fired up, it ran as normal. Given that the engine/ambient temperature was higher in the afternoon and it took longer to start, I'm increasingly thinking it isn't glow plugs.

The normal engine running condition is fine, but whilst the engine is cold, it does smoke quite badly (not enough to fail an emissions test), and the idle speed can be erratic when cold. I reported this to our Audi dealer when they serviced the car last year, and they assured me that there was nothing wrong with the car (I didn't buy the car from the dealer, so they have no reason to lie).

There is a known problem with the injectors on this engine model, but my Audi dealer was able to check back on their computer and confirmed that all the injectors had been replaced under a recall due to the fault (by a previous owner). I asked them to double check, and they told me they checked one of the injectors, and it was indeed an updated one that didn't have the known fault. - again why would they lie, they'd be losing a sale.

The car is used daily and probably does around 1,000 miles a month, and despite my concerns about the erratic idle speed and smoking when cold, it's been very reliable until this recent problem starting with a cold engine.

Could the car simply need a service (oil change, oil filter change, new air filter, new diesel filter, etc) or could there be a more sinister problem looming?


This morning the car started perfectly first time. Last night I filled the car up with Diesel, and the car did around 20-30 miles before being parked up for the night. Could this be related in any way?

  • It's been suggested to me that the EGR might be clogged up, so I'm purchasing a diesel fuel additive designed to clean the EGR, valves and injectors. Fingers crossed, this will solve the problem.
    – user1682
    Aug 31, 2014 at 13:13
  • Any new updatrs on this? Did you fix the problem? I have the very same problem. Thanks
    – user7441
    Nov 6, 2014 at 14:28
  • @MarkoGrohar Nope, still suffering. I've used diesel fuel additives that are supposed to clean the fuel system, and Wynns stop smoke oil additive at the last service (about 3 weeks ago), but still having the exact same problem. Once the car is up to temperature, it runs perfectly. I'm currently suspecting worn valve stems, but that's just a hunch/guess, as I'm a computer programmer, not a mechanic :)
    – user1682
    Nov 7, 2014 at 17:39
  • I never did get to the bottom of this problem and I've since traded in the car. I guess I'll leave the question open should somebody manage to find a solution. The problem seemed to be worse when the engine temperature was transitioning from cold to hot (but not when cold or hot).
    – user1682
    Feb 4, 2015 at 0:06
  • My audi has the same problem would be bit hard to start on cold mornings and bit smokey but be's easy to start the rest of the day I was told I need my EGR value clean so am getting it done in next few days will let use know if it help my car or not
    – user14108
    Jan 7, 2016 at 13:23

4 Answers 4


Couple things you might want to check.

Coolant temperature sensor. With a cold engine the ecu should try to dump more fuel in for starting. If the sensor isn't working it might send the wrong amount of fuel and inhibit starting.

Idle Air Control valve. This going bad could cause similar issues.

  • +1; Thanks Mike, I've been reading up on this, but need to research it a little more. I'm going to start by checking the ohms readings for the coolant temperature sensor and see what I find, although the readings on the dash for this look normal.
    – user1682
    Jun 8, 2013 at 11:18
  • 2
    A diesel does not have a throttle, why would it use an IAC?
    – Allman
    Jan 8, 2016 at 13:55

There are three relays located under the ECU, within what should be a water tight ECU housing. Apparently this is a common fault, and the relay contacts will often corrode.

One of the relays is for the glow plugs, another relay is for the radiator cooling fan.

So it's quite likely that this is the problem.


I had a similar problem with my navara bakkie - appears it was the fuel filter allowing air into the system. Replaced this and changed the clamps on the fuel lines where they join the fuel filter. Seems to have worked.


If your engine struggles when it is cold, but starts fine when hot and the car runs fine afterwards with no loss of power, I would check the following:

  1. Check your battery with a voltmeter from the posts. If it reads 12.7 V or above, then the battery is good. If it under 12.3 V, then get a new one .

  2. It is very likely your starter motor is not turning quickly enough. If your car is 10 year old or older, check your starter motor.

These are the main two reasons why your car is not starting when it is cold.

You can check your injectors, but if the car runs fine I would be surprised if this is your problem.

The other one cause which I hope you never have is the HP fuel pump not working correctly while cranking the engine with the starter motor. This means an expensive repair because the hp fuel pump usually works with the timing belt and a new pump is expensive. If this is the problem, I would recommend a second-hand part if you have an old car, as they are usually good, but they can go wrong like everything else.

I hope this helps.

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