Horrible MPG, but car runs excellently? 2012 Jetta Sportwagen

[coolusername]

I have a 2012 Jetta Sportwagen TDI with about 73,000 miles on it. It's completely up to date on maintenance. Timing belt was replaced at 70k as a preventative measure, along with oil change, fuel filter, etc. DSG fluid also done. The car throws no codes and gets up to operating temperature perfectly fine. I have a PolarFIS, and from looking at it, everything seems to be behaving. The DPF is regenerating properly, with semi-frequent minor regens, and a full regen happening every 400 miles or so. The car drives absolutely perfectly, super smoothly. However, on a 200 mile drive of nothing but highway (live in CA, went to AZ border to pick up a 1996 B4 Passat Wagon TDI), going about 75 - 80 MPH, I only got 31 MPG, hand calculated with the trip meter. 31! All the little tricks in this forum don't explain why a clean, (relatively) new-ish TDI is getting 11 MPG below rated. Does anyone have any ideas what could be causing this? I talked to an ex VW tech and he said that it could be either my DPF or my CP4 pump is on its way out, but there were no metal shavings in my fuel filter bowl. I also get poor city mileage. Last fillup of all city driving (it's a hilly area so I don't expect the best) I got 24 MPG. Would really appreciate your guys' help.

 

[ToxicDoc]

We have a whole thread about how to proceed:

tips for fuel economy

I've put this together in an effort to help some folks diagnose what may be a loss of fuel economy (MPGs) with their TDIs. Keep in mind, many of these items can work against your car in concert to add up to a sizeable increase in fuel consumption over time, so please don't dismiss any one item...

forums.tdiclub.com

 

[coolusername]

We have a whole thread about how to proceed:

tips for fuel economy

I've put this together in an effort to help some folks diagnose what may be a loss of fuel economy (MPGs) with their TDIs. Keep in mind, many of these items can work against your car in concert to add up to a sizeable increase in fuel consumption over time, so please don't dismiss any one item...

forums.tdiclub.com

I have followed everything in that guide, and nothing has changed. I actually just got 27 MPG on my latest tank (hand calculated with the trip odometer). This was 90% highway driving.

Let me address each individual point in the (dated) FAQ:
- My tires are almost brand new, got them only 5,000 miles ago.
- My alignment is good.
- I have an Evolution Imports aluminum skidplate for peace of mind, it should be fine aero wise.
- My engine reaches operating temp quickly and reliably, no issues with the thermostat. I verify this by looking at the data on my PolarFIS.
- My timing belt was done by a mechanic I trust.
- To my knowledge my MAF / sensors are all intact, the car is lower miles so it shouldn't be an issue
- My brakes are fine, I replaced the rear pads just recently, and have tested that the wheels will roll freely with the parking brake disengaged.
- My turbocharger is perfectly fine, again by looking at data from my PolarFIS I can see that requested and actual boost are A-OK
- My driving habits have been complimented on as slow and smooth, that's not a problem
- I have no "excess weight" in my trunk, most of the time I drive around with just myself and no additional cargo.

So what gives? Why does my Jetta Sportwagen get such horrible mileage compared to all the other cars on here (and Fuelly)?

 

[ToxicDoc]

You haven't followed everything on that guide, only the parts you know how to check. You should have timing checked

 

[coolusername]

You haven't followed everything on that guide, only the parts you know how to check. You should have timing checked

Okay, so I took my car to a shop, and they measured and found the fuel pump to be overdelivering fuel all the time. At idle, at certain RPMs, etc. Also, their resident TDI guy did say that it felt like it was "down on power" a bit, probably because it doesn't combust perfectly with the extra fuel. So it's highly likely the timing for the fuel pump is off, and that's why my fuel mileage is so low. The problem with that is that I found a post here on this very forum that states that common rail (which is the engine I have) timing is adjusted by the ECU, and the actual physical belt has to only be roughly in spec. My fuel pump is clearly not delivering fuel properly, is there a way to adjust this in software? Do I have basically re-do my timing belt less than 3k miles later to fix my poor fuel economy?

 

[ToxicDoc]

Sorry, this portion is beyond my knowledge set. Your question is one that your TDI guy should have answered. Let's see if one of the other members here can offer information.

 

[JELLOWSUBMARINE]

How often and how long are your active regens? What aree the high temps the polar reports? Any soot in the tailpipe?

 

[coolusername]

How often and how long are your active regens? What aree the high temps the polar reports? Any soot in the tailpipe?

I think I know what you're getting at (DPF is on its way out), and I think you're right. Tonight my mechanic swung by and checked the fuel pump timing. It was one tooth off, which is close enough where the computer would self-adjust. Timing is not the issue. My mechanic says that the issue is probably the DPF. I mean, my car regens every 200-300 miles, each regen lasting for about 15 minutes on the Polar. EGTs go above 1000 I think? Maybe peaking at 1200? As for soot in the tailpipe, I'll have to check when I next have access to the car (currently out of town). Regardless, I'm glad I've still got another five months on the emissions warranty. Does anyone have any tips about getting them to pay for a DPF replacement? The problem is that I don't have any codes, and the DPF hasn't plugged once. It's technically running fine.

 

[coolusername]

How often and how long are your active regens? What aree the high temps the polar reports? Any soot in the tailpipe?

Okay, so it's been a minute, and I've retyped all my issues and everything I've tried up. TL;DR: I have no reason to believe there's an issue with my DPF. EGTs peak at I wanna say 1400 during regen? Warning, there's some repetitiveness:

Hey guys, I’ve been chasing down an MPG problem for the past few months and need some help. Car is a 2012 Jetta SportWagen TDI with 75,000 miles. I have Dieselgate warranty until May 2023. I am all up to date on maintenance, including oil, oil filter, timing belt (replaced due to age), DSG fluid, fuel filter, air filters, the works. Problem is, I’ve been getting horrendous MPG. Just today, I drove about 14 miles lightly in the city, then 40 miles on the freeway with the cruise control set to 70 mph. Drove like a granny. The lie-o-meter told me I was averaging 36 MPG. Hand calculated, I got 27 miles per gallon. My best ever tank was 32 MPG, worst tank ever was 24. Here’s all the things I’ve tried / investigated:

• New tires w/ fresh alignment, inflated to 35 PSI
• New fuel filter (there are no metal shavings in the filter bowl, clean as a whistle)
• New air filters
• Under car aero is OK, I have an Evolution Imports aluminum skidplate
• I don’t think my parking brake cable is stuck, I was able to turn the wheels freely when I had the car on a lift to clean out my sunroof drains
• Engine reaches operating temp as independently verified by my PolarFIS, so the thermostat is perfect
• Computer requested and measured boost are in sync as independently verified by my PolarFIS, so the turbo is functioning normally
• No soot buildup in the tailpipe, it’s reasonably clean
• I investigated my DPF with VCDS. Differential pressure sensor read super small values (like 10 mbar when idling), so flow shouldn’t be an issue. Monitoring with the PolarFIS, the DPF regenerates like it should. The DPF does regenerate faster than what I’ve heard is the norm (it regens every 300 miles for me, I do a lot of city driving). Additionally, I had a measured oil ash volume of 90 mL, which is a little high for this mileage, but I think that’s completely within spec. Can’t find anything wrong with it

I’ve tried everything I possibly can, but the car runs excellently. The engine is smooth and quiet, it’s snappy, there are ZERO diagnostic codes. No check engine light, no OBD2 codes, no VCDS codes. It just keeps getting abysmal fuel mileage, and I think it’s been slowly getting worse these past few months. Would love some input, and possibly suggestions of additional tests to run. Thanks!

 

[IndigoBlueWagon]

I'd reset the belt correctly and see if FE improves. I've never heard of any mechanic dismiss the belt being one tooth off as OK: That will negatively affect FE, especially at low engine speeds where the ECU does not attempt to correct timing.

 

[ToxicDoc]

Was your mileage ok before the timing belt change 3K miles ago? If so, it sounds like you have identified your problem. I'm with IBW, fix the timing before doing anything else. You can't make an accurate diagnosis with something that wrong being present.

 

[coolusername]

Was your mileage ok before the timing belt change 3K miles ago? If so, it sounds like you have identified your problem. I'm with IBW, fix the timing before doing anything else. You can't make an accurate diagnosis with something that wrong being present.

Sorry, I forgot to mention, but I did fix the timing. The fuel pump is now perfectly in sync. Regardless, with this car, the ECU is supposed to self correct if it's slightly off. And I think my FE was still horrible before, but I got the belt changed right after I got the car, so I don't know.

 

[ToxicDoc]

There is a limit to how much an ecu can correct timing, otherwise you wouldn't need to set it... That issue aside, any changes to mileage since the timing correction?

With the ash a little high, could you have a leaky injector?

 

[coolusername]

There is a limit to how much an ecu can correct timing, otherwise you wouldn't need to set it... That issue aside, any changes to mileage since the timing correction?

With the ash a little high, could you have a leaky injector?

No, no change in mileage since the timing correction. How can I test for a leaky injector using VCDS?

 

[borninabus]

my understanding is that the timing of the HPFP on CR engines is not critical like it is on VE and PD engines. maybe your mechanic doesn't understand (or is not effectively communicating) that? the engine could damage itself if the entire TB is "one tooth off", where as, the HPFP itself being "one tooth off" is not a big deal--or even a thing for that matter. that being said...my understanding could be wrong and anyone who knows better is welcome to correct me.

my suggestion would be to look at the torsion value of the timing belt which is called "idle stabilization" in VCDS for the CJAA engine. this is the "computer controlled" aspect of the timing. it's in the engine measuring blocks. sorry, i don't remember the number. factory spec is 0 +/- 3.

this is a huge rabbit hole that you can go down yourself by searching right here on this very sight, but i will break it down briefly: each engine's optimal torsion value was set at the factory by some mystery technique and that number was recorded on the timing cover. it's best to use that number to set your TV....but that's not always possible for a variety of reasons....so here's how i've done it.

this should be done on a warm, but not hot engine. fire up VCDS in idle stabilization measuring block and the engine. note the value. rev the engine. the TV, plus or minus, should move towards 0 by about .5. if it stays at 0 and never moves then the belt install AND/OR TV adjustment is way off and need to be corrected in the appropriate manner.

getting pretty long-winded here.....you can fine tune the TV by experimenting to find that happy place for driver, engine and MPGs. i have run mine from +3 to -3. i get the best all around results @ -1.7 and i will certainly reset it there after my belt is changed. changing the TV involves removing the upper timing cover and loosening the intake cam to make slight adjustments to its position. also very searchable.

lastly in answer to your question, the way to use VCDS to look for fuel pressure bleed-off is to fully warm up the engine and monitor pressure after shut down. the pressure should rise initially within 5 or so minutes after shut down and hold pretty steady for many hours dropping only slightly as the engine cools.

 

[coolusername]

Ok, it's been a few weeks with no changes. Car continues to run well. My best tank during this time, of 100% gentle, flat highway driving, was 31 MPG. The DPF regeneration interval continues to be short, I think the worst it was at one point was 180 miles between regens. Another question, is 180° operating temp something to be concerned about? That's what my Polar reads is where my car seems to settle in at. Is 10° off the "official" spec enough to worry?

P.S.: At this point I'm very close to just deleting everything, maybe that would solve it.

 

[DivineChaos]

Okay, so I took my car to a shop, and they measured and found the fuel pump to be overdelivering fuel all the time. At idle, at certain RPMs, etc. Also, their resident TDI guy did say that it felt like it was "down on power" a bit, probably because it doesn't combust perfectly with the extra fuel. So it's highly likely the timing for the fuel pump is off, and that's why my fuel mileage is so low. The problem with that is that I found a post here on this very forum that states that common rail (which is the engine I have) timing is adjusted by the ECU, and the actual physical belt has to only be roughly in spec. My fuel pump is clearly not delivering fuel properly, is there a way to adjust this in software? Do I have basically re-do my timing belt less than 3k miles later to fix my poor fuel economy?

Sounds like a bad fuel metering valve

 

[sauron18]

Delete everything and forget about it. Also get this GM Genuine Parts 12641035 Fuel Pressure Regulator Valve.
That is the upgraded metering valve for CP4 that Darkside sells for $100 more. I tested it in my CJAA stage 2 and the car felt better, started a bit faster and after 3k rpm i felt a little boost. You don't have to tune for it or do anything just install it. My suggestion is delete everything install a stage 0.5 if you don't want to tune the DSG or go for the stage 2 from Malone or any tuner of your preference.

http://imgur.com/QPDhPr2

 

[IndigoBlueWagon]

A full delete is a recipe for disaster in California. Even a tune will get you in trouble these days.

You need to find someone who can accurately diagnose this car. Unfortunately that's probably not a VW dealer, and I don't know of any TDI expert independent mechanics in Orange County. Maybe someone here does.

 

[coolusername]

A full delete is a recipe for disaster in California. Even a tune will get you in trouble these days.

You need to find someone who can accurately diagnose this car. Unfortunately that's probably not a VW dealer, and I don't know of any TDI expert independent mechanics in Orange County. Maybe someone here does.

I've had a look at TDIClub's "trusted mechanics" page, but like half of the listed shops have closed down. The remaining half I have no idea if they're still good shops, as the information is old. They could've gone downhill over time for all I know.

 

[coolusername]

Sounds like a bad fuel metering valve

Any way I can definitively check that this is the case before firing the parts cannon? I have VCDS and am willing to use it.

 

[sauron18]

Didn't saw you live in CA well in that case you can't delete. Only way is to take the car to a well known shop and check the car out.

 

[Shoveltrev]

that duramax fuel valve update provides more fuel earlier and acrossed the board. thaat will make your problem worse . your dpf is partially faceplugged . its regening too often and requiring MORE fuel to pull the egts up. it needs a dpf . they have several modes of failure and this is one .

 

[coolusername]

that duramax fuel valve update provides more fuel earlier and acrossed the board. thaat will make your problem worse . your dpf is partially faceplugged . its regening too often and requiring MORE fuel to pull the egts up. it needs a dpf . they have several modes of failure and this is one .

I still have Dieselgate warranty until May. Problem is, I don't have any sort of check engine light or fault code in VCDS or anything. I don't know how I can get them to honor the warranty without a code. Maybe I could show some values are out of spec?

 

[Shoveltrev]

did you find soot in the tailpipe ?

 

[powermandan]

Just out of interest. How does California find out that you have modded you car?

I.e removed the core of the DPF and have it tuned out in ECU?


Not from usa

 

[coolusername]

Just out of interest. How does California find out that you have modded you car?

I.e removed the core of the DPF and have it tuned out in ECU?


Not from usa

California's emissions tests include a tool that hooks up to your OBD-2 port. That tool reads and compares the "hash" of your ECU contents to a database, and they can tell if you have a custom tune of any kind. They also read the federal OBD-2 emissions readiness monitors, of course. Additionally, they floor your car and check for visible smoke, and have a thorough visual inspection of your emissions components, so they'll see if you have, say, a downpipe. You have to go through this set of tests every two years if you want to renew your registration. Surprisingly, though, out on the open road there's basically zero enforcement. Cops don't really pull people over, you can do what you want if you find a way to cheat the smog, and aren't running a smoke tune or something stupid. Day to day there's no smog control points of any kind like there are in European inner cities.

 

[powermandan]

Aside from your milage problem.

I would probably get the ECU cloned. And a gutted DPF and swap in and out every 2 years.

What happens if you disable the ODB port?

 

[coolusername]

Aside from your milage problem.

I would probably get the ECU cloned. And a gutted DPF and swap in and out every 2 years.

What happens if you disable the ODB port?

The way I'd hypothetically do it, is I may or may not know someone that can pass me my smog regardless. So I'd just delete and do that. Worst case scenario I'd probably do what you're suggesting, parts in and out every two years. First I'm going to try the dealer, one last time before my emissions warranty runs out.

 

[powermandan]

If they are anything like dealers in the UK. Then without a CEL your unlikely to get anywhere.

You can imagine it. Hundreds of customers walking through the door. Blaming their MPG on exhaust