So my 2012 TDI Q7 (original owner, 245,000KM) won't start and is doing something pretty weird. The best way to I can explain it is...it's puffing white smoke\steam out of the driver's side of the engine like a steam train. A puff every second or so (in a constant rhythm: puff...puff...puff as the engine turns over) while attempting to start the car. I have a video but can't figure out how to upload (wrong format apparently). Screen grab attached. Anyone ever seen anything like this before or have any ideas??

It appears to be coming from somewhere down near the exhaust manifold and puffing up around the fuel filter and battery terminal posts. I managed to grab a screen shot mid "puff".

 

Howdy Rick, and welcome to the 4L Q7 forum!

Why do I feel like you are possibly leaving out some useful details about what's been going on prior to the no-start situation you currently have.

Have you put a VAGCOM compatible diagnostic scan tool on your Q7? It sounds likely your glow plugs aren't working, and that should be pretty clear by the strong diesel fuel smell under hood from the unburnt fuel, but surely could be several things going on there, since it puffing smoke of any color at you means that something is leaking...either exhaust manifold gasket or could be injector seals, as there are relief holes drilled in the block on a TDI, etc.

A diagnostic scan will quickly identify and/or rule out specific causes for you just by showing you all the internal fault codes the car keeps records on, and if the CEL/MIL is staying lit on dash, or your glow plug lamp is flashing at you, as those are all clues you need to scan the car ASAP. Just buy a cheap VAGCOM to read/clear codes if you don't DIY maintain/repair the car, or get a capable VAGCOM like the Ross-Tech VCDS that will allow you to properly DIY and repair /diagnose the Q7. It's the 'gold standard' and it'll save you so much money and time on managing your Q7.

Here's some info you may find helpful:

https://learndiesels.com/the-causes-...t-to-look-for/

 

Glowplug failure causing diesel fumes under bonnet? 🤦‍♂️ Seriously, please fathom out how an internal combustion engine works before posting.

Nothing (except the exhaust tailpipes) is supposed to vent combustion gases to atmosphere.

Any combustion gases escaping upstream of the cats or dpfs will stink like a 1970’s diesel.

If you’re getting fumes and smoke under the bonnet, something is leaking. Top culprits are the egr system, the crankcase ventilation system and the injector leakage vents. Based on the image, it’s likely you have an injector seal leaking. This will be accompanied by on oily mess above the exhaust manifold in line with the offending injector.

And now a sanity check on the topic of vcds and how it can or cannot identify faults… the engine is fitted with a number of sensors (and sometimes actuators) whose feedback to the ecu fine-tunes the combustion process for optimal efficiency/performance. Each of these sensors has plausible limits which help the ecu determine if a fault exists. The ecu then interprets this information to hazard a guess at what has gone wrong. So a basic open circuit condition or a questionable resistance condition on a simple device like a glow plug would point to a failed plug (or faulty wiring), but when the implausible feedback comes from something like an oxygen or temperature sensor then the root cause is less straightforward. Furthermore, a component can fail but continue to operate with plausible limits so interrogating the ecu is frequently pointless and the leap to vcds encouraged on here is probably why a number of common issues remain without a definitive cause or solution. The fact is that a serious obvious fault will trigger a dashboard light so the need for ecu interrogation in such cases is obvious. But for under-bonnet fumes it’s far more intelligent to check the multiple of physical items such as gaskets and diaphragms whose failure will eventually trigger an ecu fault, but by that point you’re probably not just spending out on a cheap gasket. Over-reliance on vcds is worse than just not buying it at all.

I am not saying that your glow plugs are not faulty. I am saying that faulty glow plugs do not necessarily register as an ecu fault until they are properly dead, and in conjunction with smoke and fumes means another more serious issue exists.

Under bonnet items that vent to atmosphere include axle breathers, brake fluid and coolant expansion. Everything else goes out the back unless a gasket or seal has broken.

 

Speaking of the crankcase ventilation system.... where is the breather on the TDI (gen 2) located? Try as I might, I just cannot find anything definitive that points to it....

 

Here's an excellent engine video on Gen 2 TDI 3.0 (which I found and used to get my coolant leak in engine valley warranty covered on our '15 TDI, as they were denying it). You can see the vent located on passenger side (LHD Q7) at top of valve cover towards rear of engine, as the engine spins at beginning of video...it's disc-shaped with the vent hose clearly connected to it, etc.

You should feel lucky, LOL, on my Gen 1 TDI motor the oil separator/breather is embedded deep in the extreme rear of the engine valley with zero access, so you better really, really have a strong motivation or reason to change that sucker on the Gen 1 TDI or do it while the valley is already opened up. I sent the part back on my Gen 1.

 

So Rick, what happened with your TDI situation; it'd be great if you'd close out your original inquiry with your actual findings/resolution required, etc. Exhaust seal leaks /injector seals leaking are somewhat common on these TDI motors with age/wear on the components, but the car not starting was, to me anyways, the key symptom you detailed.

Thanks for following-up with your resolution!

 

Oh, that's interesting. I see what you're talking about in the video.... but it doesn't look anything like that on the vehicle. As I watch a little more of the video, there are other distinct differences in how the video presents versus what the parts look like in real life, too. Curious. I guess I'll just have to play with it once I get my parts in and start digging. My turbo is failing, so I'm going to be pulling apart the top end again in a couple weeks to get at that, and I figure I'll try to hit a few other items while I'm in there (yet again). This will be the third time in as many years! I had it apart in 2022 for the oil cooler leak, 2024 for the EGR cooler, and now again in 2025 for the turbo. Goodness.

 

SSP479 shows the updated breather system. Oddly, it’s referred to as the engine management system…“The engine management system has been moved from the inner vee of the cylinder heads into the cylinder head covers together with the coarse and fine oil separators. Both crankcase vents lead to the pressure control valve and from there to the intake side of the turbocharger.”

So basically each cam cover has an incorporated oil separator and control valve, with hoses running from there to the turbo intake. If you remove the air intake hose that runs from air flow meter to turbo, you’ll see the crankcase vent hose connection underneath. You can probably follow the hoses from the connections at the rear of each cam cover.

 

Thank you! I found a copy of the self-study program 479 and will take a close look!

 

Happy to help.
In that the older in-vee breather design is a big job to repair if it ever gets blocked, it would be very useful if the newer design could simply be retrofitted but I suspect the injector clamping differences would make it a can of worms.