No engine power after replacing battery and alternator
 

My alternator failed and my battery was on its way out as well so I replaced both. Now when I start the q7 it idles rough almost a gallop, sounds terrible when in drive and will not drive at all. It's making zero power and will barley move the truck when in gear. I've put a few scann tools on it and I get no codes. Anyone have advice or real world experience?

You need a VAGCOM speaking scan tool for this car, and any VW owned brand for that matter. Regular OBDII type scanners can't see the internal trouble codes on the control modules this car stores for you.

That's where you start.

Also, anytime you disconnect the battery on this car, it will create massive fault codes in most every control module on car; these must be cleared using a VAGCOM scan tool before any troubleshooting or accurate vehicle assessment can be attempted. If the BEM system (battery management) is still hooked up to the negative terminal on battery, then you also need to program the details on the new battery to allow the BEM system to charge it properly. Honestly, probably just unplugging that monitor wire from the negative terminal is the way to go, as these cars develop parasitic battery drains from door handles and are very sensitive to low voltage sensing, etc.

Get yourself a VCDS VAGCOM from Ross-Tech if you plan to DIY maintain/repair this car; it'll save you lots of time/frustration/$$$ over buying other VAGCOM tools.

I used a vag compatable scanner and it shows no codes besides low voltage(battery dead/disconnect) on most every module but after clearing them they have yet to come back but the symptoms still persist.

Did you remove any intercooler lines when you replaced the alternator? I ended up removing my alternator through the bottom, and I had to pull a soft hose on the right intercooler in order to allow enough clearance to move the part through there. When I put it back, I *thought* I had it on right, but the seal was deformed due to oil exposure/expansion, so it wasn't allowing turbo pressure to build properly, which put the car in limp mode. However, I was getting a code for that (don't recall what it was, but it didn't give me an "aha!" moment, that's for sure.

The good thing is this: When something goes wrong immediately after you perform work, it is nearly always causality, so retrace your steps. More than likely, you're going to find that you either didn't get something back together properly, or you accidently broke something "insignificant' at the time.

If it is your intercooler system not sealing, I found that pulling the seal, cleaning it in detergent really well, then warming it up with a heat gun (like 500 degrees "warm" to force out any oil residue) and immediately immersing it in ice (i was "lucky" in that it was below zero outside, so I just tossed it in the snow for a couple minutes) shrank and firmed it up perfectly. I added a little Sil-Glyde to it to lubricate and protect it, slipped the parts together again, and it's been perfect every since.

You sir or madam are a genius, the upper intercooler hose blew off. Guess i didn't seat it when I pit it back together and never noticed. Thank you so much!

Excellent! I'm thrilled that it was an easy fix!