TDI Misfiring when cold

**Jamiem** · 17-10-2010,09:34 PM

I've got a problem which appeared gradually over the past few weeks. Normally when the engine is cold, it will run just fine for the first 2 mins or so, then it will randomly misifre for 3-4 minutes, both on idle, and when driving throuhout the rev range. After this period of a few minutes, it will clear, and then won't do it again at all until the next time it's started from cold.

I've fault scanned the car with VCDS, and nothing is showing up at all. The car is a remapped 130 PD, been fine for the last 17k miles since I bought it in March, It was serviced 8.5k ago, and is coming up on its next service withint the next 3 weeks. I serviced it using genuine parts, but did not change the fuel filter as I didn;t have a 13mm allen head socket. I'm hoping its this, but I also rekon it could be the MAF sensor. Is there anything im likely to see in the measuring blocks in VCDS if I try and trace it to say the MAF sensor?

Any help appreciated

**Crasher** · 17-10-2010,09:41 PM

The AMM will not cause a misfire and I don’t feel not changing the fuel filter will either. I am surprised you have not had a fault code relating to an injector open circuit so I suspect you have an injector failing but you will have to wait for a code to know which one.

**caldirun** · 18-10-2010,07:02 AM

Originally Posted by Jamiem

I've got a problem which appeared gradually over the past few weeks. Normally when the engine is cold, it will run just fine for the first 2 mins or so, then it will randomly misifre for 3-4 minutes, both on idle, and when driving throuhout the rev range. After this period of a few minutes, it will clear, and then won't do it again at all until the next time it's started from cold.

I've fault scanned the car with VCDS, and nothing is showing up at all. The car is a remapped 130 PD, been fine for the last 17k miles since I bought it in March, It was serviced 8.5k ago, and is coming up on its next service withint the next 3 weeks. I serviced it using genuine parts, but did not change the fuel filter as I didn;t have a 13mm allen head socket. I'm hoping its this, but I also rekon it could be the MAF sensor. Is there anything im likely to see in the measuring blocks in VCDS if I try and trace it to say the MAF sensor?

Any help appreciated

Try changing the injector wiring loom in the head, very common problem, lots of posts on here re how to.

**Jamiem** · 18-10-2010,08:10 AM

Thanks for the replys

Thid morning it was worst than ever, and didn't stop running on 3 until I turned it off and back on again. Then it was absolutely fine. It's really odd that it's not throwing any codes up, or engine management light, and it still allows me to engage cruise. It did however totally destroy the fuel economy. At a point in my daily commute were it would normally be at around 40mpg, it had averaged 22!

I'm goin over to a mates tonight who has a snapon modus, hopefully that might pick something up. I dont mind paying to sort it out, but I don't want to start throwing parts at it hoping that it will cure it. I'll do a search on the wiring loom and see what thats all about.

**Crasher** · 18-10-2010,09:11 AM

I am very surprised it is not showing a fault code for an electrical fault on an injector, normally number 1. I would also suggest changing the injector harness to see what happens.

**martin1810** · 18-10-2010,09:27 AM

Mechanical PD injector faults often don't throw a code. I wonder what the cam lobes look like.

**Crasher** · 18-10-2010,01:21 PM

True but mechanical PD faults (worn injector arm lobes) don’t come and go.

**Jamiem** · 18-10-2010,01:32 PM

Just been out in it at lunch time. Driven for 2 miles or so and it was misfiring badly all the way, then I shut it off, waited a few seconds, restarted, and running like a dream again. It has to be electrical I would have thought. There's an injector loom on eBay new for £35, anyone know off the top of their head how much from a dealer?

**Crasher** · 18-10-2010,01:35 PM

About £50.

**caldirun** · 18-10-2010,05:24 PM

Originally Posted by Jamiem

Thanks for the replys

Thid morning it was worst than ever, and didn't stop running on 3 until I turned it off and back on again. Then it was absolutely fine. It's really odd that it's not throwing any codes up, or engine management light, and it still allows me to engage cruise. It did however totally destroy the fuel economy. At a point in my daily commute were it would normally be at around 40mpg, it had averaged 22!

I'm goin over to a mates tonight who has a snapon modus, hopefully that might pick something up. I dont mind paying to sort it out, but I don't want to start throwing parts at it hoping that it will cure it. I'll do a search on the wiring loom and see what thats all about.

I have suffered 2 bouts of wiring loom problems, started to show when cold but got worse and happened all the time, sometimes there are no codes logged, turning the engine off did work most of the time, even when I did not detect misfiring the economy was very poor. If a code came out it was always injector 1. I tightened the push on connectors the first time but it only cured it for 12 months then it came back so I changed the loom (simple job if you warm the old wire support so you can bend it without shattering it, the new one will bend no problem)