Urgent - 2007 Q7 3.6 cranks but does not start, no VAG-COM codes

[georgep1980]

Hello everyone. I need help with this as fast as humanly possible, I am hoping someone sees this and can give me some pointers.

My 2007 Q7 3.6 is cranking but not starting. This is the second time this happens (see below) but this time the car seems like it won't start on its own like before. Here are more details:
- VAG-COM does not display any codes when it just cranks and fails
- One out of 3-4 tries it does start but idles very rough and I can rev it up to 4000 RPMs but it immediately dies - in this case I do get a bunch of VAG-COM codes (misfires and intermittent signals for various sensors, none in the Engine module). The battery discharges quickly so I have to keep it hooked up to my friend's vehicle, I am assuming that's why I get the intermittent signals.
- I did change one of the fuel pump relays with no improvement
- I think I was able to read the fuel pressure (Engine/Measuring Blocks/Group 103) and it is pretty steady at 7 bar. This seems to be normal, correct? Does it also rule out the fuel filter?
- I cannot locate the engine speed sensor (crankshaft position sensor) G28. The fact that there are no codes thrown indicates this could be the culprit, correct? I purchased an AllData subscription and they do not have any information about the G28 sensor
- Could this be because of clogged injectors? I tend to think not, as this happened all of a sudden with no prior signs and it doesn't seem isolated to only a few of the injectors/cylinders
- This is only the second time this happens, both during extremely cold days. The first time the car just started and after a period of rough idling it began running smoothly after it spit a lot of black water out of the exhaust. I then thought the moisture buildup in the engine/exhaust froze and caused this. I now tend to think that was not the case.

I know this is a lot to process but any help would be appreciated. I found almost no information about fuel pump and crankshaft position sensor issues related to the Q7.

Thank you!

[Jason12692]

If the engine starts at all I would definitely rule out the crankshaft position sensor. If the injectors were clogged you would most likely throw a misfire code...also seeing how this is happening is extreme cold weather more than likely your fuel may have moisture in it. I would buy some heet or seafoam to help stabilize the fuel and see if that helps.

[Dtomasi]

Sounds like your not getting spark. I would check the coil packs. One might be dead causing the no start and rough idle issues and misfire codes.

[georgep1980]

Thanks for the suggestions, I will try them. Here are additional details:
- I did throw some Heet into the tank. Should I double up (any risk of damage with less than 1/2 the tank full?) or use Seafoam at the throttle body? Maybe both?
- the car has new coil packs installed by the dealer under a recall about 3-4 months ago as well as brand new spark plugs and air filter. I installed new spark plugs and the new air filter 3 days ago when this first happened
- I have noticed a lot of white smoke lately and it doesn't seem to be from coolant (coolant level unchanged and there is no milky stuff in the oil)

Thanks again!

[Dtomasi]

Is the 3.6 a direct inject motor? If so any fuel additive is not going to do much because the fuel is injected directly into the engine unlike port fuel injection. Not sure if the 3.6 had carbon issues but if it is a DI engine that could be the issue. Just about all DI engines suffer from carbon buildup.

What about your catalytic converter, have you checked to make sure they're not clogged? A clogged cat could also throw those codes.

[georgep1980]

Starter fluid did not help indeed. Car is at an independent mechanic now.

Question: if it is a carbon buildup problem, why would it happen all of a sudden? Shouldn't the car experience a gradual decrease in perfomance?

Thanks again!

[Dtomasi]

Yea, that's right but if you drive the car everyday it can be really difficult to tell the difference until there's so much carbon in the intake and valves that the car is no longer runs in the safe parameters of the ecu and it throws codes and runs bad.
Please let us know the diagnosis is once you find out. I'm really intrested to know what the problem is. I'm sure it's something simple.

[georgep1980]

I will keep everyone posted.

If it is carbon deposits what's the solution? Clean the injectors only? Valves?

Thanks

[Dtomasi]

It's pretty labor intensive. They take the intake manifold off and clean the inside of that thoroughly and then clean all the intake and exhaust valves. All hand scrubbed and vacuumed out. Some engines can take a full day disassembling, cleaning and reassembling. You don't have to worry about carbon buildup if your engine is not direct injection though.

[georgep1980]

How do I know if my engine is DI?

I am now told the exhaust system was clogged after the cats. Is there a resonator between the cats and the muffler?

[Dtomasi]

Here you go. http://www.caranddriver.com/reviews/...t-specs-page-2 it does indeed look like it's DI. So the carbon cleaning It might be something to consider if you start having issues and throwing codes.

Thats good news! A clogged resonator is a easy fix. Ask the mechanic what caused the clog, sometimes the CAT element can breakdown, break apart and get clogged in other areas in the exhaust system. There is a resonator between the cat and muffler maybe even 2. Looks like a easy fix either way.

[georgep1980]

It turns out this whole ordeal was caused by a 3-4' portion of the exhaust (including the resonator) that was frozen solid. I did notice a lot of white smoke lately and the icing on the cake must have been backing into my spot that was surrounded by snow/ice. The heat of the exhaust probably melted more snow that got in there and clogged everything up. I've seen pictures of the exhaust when they cut it, it's pretty incredible how that happened and how it caused this without throwing any codes...

[Dtomasi]

It turns out this whole ordeal was caused by a 3-4' portion of the exhaust (including the resonator) that was frozen solid. I did notice a lot of white smoke lately and the icing on the cake must have been backing into my spot that was surrounded by snow/ice. The heat of the exhaust probably melted more snow that got in there and clogged everything up. I've seen pictures of the exhaust when they cut it, it's pretty incredible how that happened and how it caused this without throwing any codes...

LOL You got to be kidding, well thats a first for me for sure. I glad it worked out for you. Hope they didnt charge you to much.