Hi all, and Happy New Year!

I have a 2015 Q7 Premium Plus 3.0t with around 133k miles. It's had some mods since around 90k miles. It's now dual-pulley stage 2 with JHM tune, RKX intake with K&N drop-in filter and Magnaflow cat back exhaust.

For the last couple years, I had been getting 18-19mpg combined. But more recently and after adding the crank pulley, I have been getting 14-15mpg. I'm pretty sure my cats have failed.

I'm trying to figure out what to do. I'm planning to drive this vehicle to well over 200k miles and getting this mpg isn't going to work, plus I have paid a lot of money for performance mods that I'm not seeing the full benefits from.

Doing research, it seems like replacing cats on my car isn't an easy task. Has anyone done it?

Where I live we don't have DEQ or vehicle inspections so I could go catless or high flow cats. So these are the options I am researching -

1. Pull my stock cat pipes and gut them. Then I would figure out spacers for the o2 sensors. Pros - low cost of parts, no cats to worry about. Cons - sounds like my setup catless would be loud as hell so I would either need to add a cat or a muffler.

2. Replace cats with test pipes from CAP. These would be $680 for the pipes and o2 spacers plus any tariffs. Pros- lower cost of parts, no cats to worry about, best performance. Cons - will be loud as hell and may need to add a cat or muffler. Tariffs might make this option more expensive.

3. Replace cats with Magnaflow bolt in replacements. Pros - these are probably very high quality. I wouldn't have to add any mufflers. Cons - These would be a little over $1300 shipped then I would have to install or pay to have them installed.

4. Replace cats with JHM High Flow cats. Pros - these move the cats down-stream from where the factory cats are. Assuming I wouldn't have to add any mufflers especially if I bought the baffled version. Cons - these would be ~ $1270 shipped for the non-resonated version, ~ $1410 for the resonated. I also saw online that there was some question of the quality of the cats they use.

I would appreciate any insight and opinions on what to do. I need to do something.

Thanks!

 

Did you retune it after changing out the crankshaft pulley, since you said the reduced fuel economy occurred post-install of the pulley, that is a logical conclusion, given that the ECU has to adapt to the data it's receiving from myriad sensors to adjust the fueling. If you didn't make inquiries about updating your tune with the pulley install, then that may be all yours needs, IMHO.

There's plenty other reasons for fuel economy to take a notable nose-dive though, which don't concern the cats needing replacement, so don't guess at what's going on; look at all the live data from sensors feeding the ECU data about airflow, exhaust sensors, etc., in your VAGCOM scan tool, like the Ross-Tech VCDS, etc.

IMO, it could also be other basic stuff too, like on the gasser engines, the plugs really need to be changed at/before the 50k mile interval spec, and the coil packs may last up to 100k miles, or even beyond, or they may not. Old spark plugs will most definitely impact/ kill your mpg, and eventually your engine too if you leave them in too long, especially if your engine is already consuming oil in a moderate to heavy manner between oil changes do to the carboned-up piston rings. Carbon deposits on the valves is another known issue on your gas, DI engine that can/will impact fuel economy, amongst other longer-term problems and failures that may result.

I personally wouldn't jump to the conclusion that the catalytic converter(s) are bad simply because there's a lot of other known issues with less expensive wear bits, including your more aggressive tuning mods, fuel quality, how/where it's being driven, etc, that all work against you having the same gas mpg you had before you began tuning the car for significantly higher power output, which likewise can impact your fuel economy.

On my own Q7, Stage 1 gives more power/torque output, but mpg stays basically same or even slightly improved (TDI), but Stage 2 is all about the $hits & Giggles factor, and so the fuel economy observed/calculated drops off, just for some round-out perspective related to tuning impacts on fuel economy, which you should expect, especially if the new pulley wasn't compensated for in the tuning you've done.

However, you can also do live data monitoring via engine module to monitor your O2 sensors, and see what's happening there; hint: O2 sensors performance may be another reason for an observed/calculated decline in fuel economy. The good news is O2 sensors performance can be live-data-logged, and diagnosed easily and often times, if they failing/ performing poorly, they can be cleaned as a DIY process at home (look it up) rather than needing to buy new O2 sensors, which can be moderately expensive sensors to purchase.

I've personally not had good experiences with K&N oiled filters, after using them in various naturally-aspirated vehicles. Primarily this concerns the MAF (airflow sensor) on the cars I've installed on, but IIRC your gas engine uses a manifold type airflow sensor. On my TDI motor, the MAF sensor is literally located right next to the airbox outlet though, so I'd never mount an oiled filter in mine. I put those on several cars in past and while they seemed great at first, I eventually pulled them out of all my cars, when began having airflow sensor/contamination problems across my 'fun-cars'. Your mileage may vary.

Check all the maintenance basics, and be sure your tune reflects you changed out the crank pulley. That's likely all you need, not that the hotted-up gas motors are known for their fuel economy.

 

Hey. I have a 2015, 3.0t, stage 2 using single pulley and APR. I have a custom mid muffler back exhaust, RocEuro modded CAI, and so many more mods.

I have contemplated JHM, just have not pulled the trigger,oney has been going to other mods on other cars. That was a tangent. Haha.

I don't think cats is the source of issues.

On my stage 2, now that I am RTO, I am seeing and paying more attention to my MPG, I am seeing nearly 19mpg. When I am on trips, I can push to about 21-22mpg.

If your cats are clogged, based on expierence on a 2001 Audi A4 1.8T, so too long ago... I tripped errors, power issues, etc...

With the Q7, I have a dry cone air filter, and when gas mileage is down, I pull it off, purple power and use 140 degree water to clean the filter. Reapply, and I am back to normal again. Sometimes I trip a code/CEL, do this process, and we are good.

If you gass mileage is down significantly, I would start with all of the regular maintenance, flushes, spark plugs, packs, etc...

If you are still down in MPG after a 100 to 200 miles, then you might have something else going on.

For an example, MPG was tanking, but no CEL for a almost 200 miles... My intake runner manifolds finally failed. I was averaging 14mpg also. Drive from Pensacola to Atlanta and just kept noticing my gas mileage getting worst. Finally limp mode and CEL. But it took a while and my mechanic and I were puzzle until we got the light.

Not sure if this helps. We are not comparing apples to apples, but here is some of my personal experience.

fYU, if you intake manifolds are going out, super charger has to come out, so get everything done while it's out
😁

 

Thank you for the detailed response. The reasons why I think that I have failed cats were the MPG drop, reading that the cats on a 3.0t are prone to fail before 100k miles (I don't know if this is true), and sometimes my exhaust has a gunpowder smell which I researched that also pointed to failed cats.

I'm pretty sure the tune I have is for any "dual pulley" stage 2.

I'm running the original coil packs and my plugs haven't been changed for more than 50k, so that is something I should do regardless. And a good step to do first.

And doing some data logging sounds like a really good idea. I just need to figure out how to do it.

 

So did you end up doing any exhaust work? I'm looking to replace my cats and downpipes, likely with the JHM catted/baffled setup or maybe the IE B8 Performance Downpipes..

 

IMO, when talking dropoffs in mpg, you really need to look at cause & affect related to the timeline you did your mods on versus symptoms appearing, etc. Often it is as simple as needing to replace the worn-out plugs and/or perhaps switch to a more appropriate heat range on the plugs to better match/compliment your tune setup. Then there are the age related vacuum leaks, clogged airflow sensors, engine air filters, O2 sensors (control fueling feedback) trying to fail that haven't yet, etc., etc. The live data feeds and having a scan tool like VCDS are super helpful in checking up on these items, and there's always the fuel system itself to verify flow rates of injectors as they age, as they do tend to get gunked-up over time. I run a super-duper fuel system/injector cleaner when performing my annual maintenance cycle.

FWIW: I pulled all my fuel injectors at 120k miles for condition check and replaced their oil control O-ring and copper seal, as well as fuel return (tiny) O-rings. Turned out I had two with leaking oil control O-rings and couple with copper seals about breached and one was leaking exhaust/breached, and those with the copper seals going/gone had a fair bit of nasty buildup on/around the injector spray tips. While doing this you also should inspect the injector body and shape of spray tip for any abnormal bulges or the spray tip end 'mushrooming', which are signs of impending/future injector failure coming your way that can ruin your engine. The coil packs also have a maintenance schedule, as they lose efficiency over time, and the aftermarket coil packs often will cause problems for you, at least from what I've read on owners trying to 'upgrade' by using aftermarket 'performance' ignition branded coil packs...you know the ones.

Just make sure you aren't ignoring all the typical wear items on your car and especially those items that get overlooked when tuning an engine at home, like spark plug heat range/gaps, etc. Also, I've gotten brand-new, Ford branded iridium spark plugs from Ford for our F-150 that were supposed to be pre-gapped and need nothing, but yeah, apparently somebody dropped the box of plugs before they reached me, and that changed the gap on one plug, which then became an issue; point being check everything is correct before installing even when using the factory plugs/parts, etc.

Best of luck to you both!

 

mine is 3.0T and Stage 1 unitronic and gas mileage is horrendous
I did put new NGL coils, new NGK spark plugs, new filters, new PCV, carbon cleaned the intake, car runs with zero codes, but mileage.... combined city/open road between 12.5mpg to 14.5mpg

I can see 19mpg only when I lick the gas pedal on open road doing 50mph

 

I know this thread is Cat Converter, and definitely interested in hearing about the JHM. Not sure I read anything about IE B8 working on the 4L Q7, so I am intrigued.

In regards to Gas Mileage. 3.0T. APR stage 2 single pulley. All the maintenance you can think of, center muffler back custom exhaust, lowered, call it and I probably have had the work done.

City: avg 15.5 MPG
City Hwy: avg 19mpg
Hwy Travelling: 21mpg

For it being the fat pig that it is, not bad. If anything, kinda pisses me off that the our other cars get just as good of gas mileage as this thing does.