ex-e46

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 could not hook up a fuel pressure gauge to test the actual fuel pressure (cannot find the proper fitting). However, I disconnected the fuel line by the engine and turning the key to position 1 or or attempting to start the vehicle does make it spit gas at fairly high pressure. I understand I need to get a reading of the actual fuel pressure as both too low or too high pressure can make the injectors not open, but does this 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!
George

B490MTL

There is a good chance your ECU is fried, if the Vagcom is not able to reach the Engine adress, your ECU is dead. Look at the plenum to see if there is any water. You might be able to salvage the ECU if not find a dealer that will install a used one. 1hr of programming plus the ECU. Used ones go for around 300$, brand new is about 1000$.

ex-e46

I am able to get in the Engine control module, I should have been more clear. I just get no DTCs if all I do is let it crank and stop. When I can get it started for 1-2 seconds and rev it I get some misfire codes and other unrelated low voltage or weak signal codes in other modules.

ex-e46

Here's an update: 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 mean this leaves the G28 sensor or clogged injectors as contenders?

Thanks again

B490MTL

Even if the injectors are gummed or the fuel filter dirty it shouldnt make the engine a no-run... An indy should be able to clear it up

ex-e46

B490MTL, what do you mean by an indy? Independent mechanic? I probably sound ridiculous to the other forum members

Anyway, here are additional details:
- 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)
- I put 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?

Thanks again!

Clues

I would suggest replacing the battery. These cars are so electronically dependent that any voltage irregularities will often cause some mechanical issue. Cold weather is notoriously hard on a batteries - especially older ones. My neighbor's Range Rover was experiencing major engine issues which turned out to be a symptom of a dying battery - VAG-COM showed no errors and luckily an astute tech suggested the replacing the battery based on the age of the oem battery and the cold weather... Good luck.

B490MTL

Indy=independant mecanic, you are right. Heet or seafoam wont do much to your engine since it is direct injection. The battery would be a good place to start to see if it's still good, but aside from that or the ECU, can't really tell. The Q7 is really more of a hit or miss vehicule, when you can't narrow down the problem, you just change parts until you find the bad one.

ex-e46

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...