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Discussion Starter #1 (Edited)
I have a T5 2.5 TDi 130 4motion 2005 that suddenly started to spit black smoke clouds some months ago. I took it to the garage (in two different places actually). They plugged in the computer but couldn't find any fault. Their conclusion is that it's just normal, it's an old lady, maybe i tanked up with dirty fuel, some diesel additive could help etc. I refueled many times since then, with or without additive, and this didn't really help.

After some good reading on this forum, I understood black smoke can have many origins, but a common one seems to be a leak in the intercooler or in the boost hoses that connect it to the turbo and the engine. So my question before I take it to the garage and pay for a related check: would a boost leak be fully consistent with the following symptoms?

1) black smoke appearing under load (gentle or hard acceleration, steady uphill), especially from 1800 to 2200rpm for any gear, but much less or not at all at higher rpm. If it were a boost leak, why would the air/fuel ratio become OK at high rpm, when the air pressure is a priori higher, leading to more lost air?

2) more smoke in 2nd or 3rd gear than in upper gears. The smoke in 4th to 6th gear is slightly less abundant, or maybe more grey than black (sorry for the lack of precision, these are wing mirror observations)

3) the engine judders slightly when accelerating in 2nd or 3rd from very low rpm (only up to about 1500rpm).The judder is always accompanied by a thick cloud of black smoke. This could be misfiring, but I can't tell for sure. This judder problem is quite new (2 months) compared to the first appearance of black smoke (8 months).

4) i can't feel a loss of power when accelerating. But the fuel consumption definitely went up

Something else about the age of my engine: the car clocks 170k miles but a new engine was installed at 130k (by the previous owner; due to some factory defect in the original engine, VW accepted to replace it together with free wheel).

Thanks for your insights!
 

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I would take it to a VW specialist, or someone who can do a log to see if actual boost matches specified boost. if it doesn't, you need to start checking for boost leaks, blockages, damaged turbo etc
if it doesn't, it could be anything else tbh, any number of sensors could have failed to lead to overfuelling, timing errors etc.. but I'm sure there will be fault codes logged for that.
 

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Discussion Starter #3
Thanks fenwick. So if i understand properly, VW specialists do have access to more data than standard mechs? Because when I took the van to the garage, they plugged their computer and I could see on the screen all sorts of numbers like air pressure, flux and so on, together with acceptable ranges, and all was within specs. And certainly no fault codes. But can it be that not everything was logged because this was a non-specialist software?
 

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When I had my 2006 that suffered with black smoke, it was cause due to one of the boost pipes leaking. On your age engine Im guessing the clips that secure the boost pips are the U style. The lobes on the pipe wear down and mis shapen caused them to slip past the clips. Common locations are the first pipe after the turbo and then where the pipes join the plastic connector on the exit from the inter cooler. Check and make sure all these connections are fully secured.
next could be a faulty intercooler if it’s leaking
 

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VW specialists do have access to more data than standard mechs? Because when I took the van to the garage, they plugged their computer and I could see on the screen all sorts of numbers like air pressure, flux and so on, together with acceptable ranges, and all was within specs. And certainly no fault codes. But can it be that not everything was logged because this was a non-specialist software?
possibly. some just have basic tools that look for codes, often which are then hard to interpret. Did they take if for a drive and measure the boost pressure whilst it was smoking?
 

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Discussion Starter #6
possibly. some just have basic tools that look for codes, often which are then hard to interpret. Did they take if for a drive and measure the boost pressure whilst it was smoking?
no they just looked at it when it was parked and pressing the accelerator. In these conditions it's not smoking much, so the test was probably irrelevant indeed...
 

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Discussion Starter #7
so i brought the van to a VW specialist today. They found a leak in the intercooler. Tomorrow I'll have it replaced, and i hope that damn smoke will be gone!
thanks for your advice
 

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I thought that would be the case. First garage knew f*** all then. you should really tell them, how else will they learn :ROFLMAO:
 

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Discussion Starter #9
all fixed now, no more smoke! The hole in the intercooler was pretty big: the mech showed me a video in which he leak-checked the boost circuit by overpressurizing it with some smoky gas, that was unmissable. On top of that, a large area around the leak was covered with spraying oil. Hard to believe the previous mechs didn't smell something wrong there, so he told.

just a last comment about why I observed smoke around 2000rpm but much less above: it's because the engine is designed to deliver most power around 2000-2500rpm, hence the computer controlled boost is made to peak around that value and then decrease.
 

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Great detective work there ursuletz.

The amount of lost air through a hole is dependent on the amount of pressure difference across the hole so it's related to the boost pressure, I bet the size of the split also grew with boost pressure. The amount of air going through the intercooler to the engine is a function of engine power (speed and boost) so at 2000rpm boost is high and power is low so the relative amount leakage to useful air entering the engine is high. At peak power the boost is dropping but the engine is taking its maximum airflow so the leak is small as a percentage and the fuel injected is close enough to not smoke.
 
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