How to diagnose bad fuel mileage

[Delasangre]

So short story. Bought a 2011 sportwagen TDI. I was averaging 47mpg highway and 37mpg commute. Then I get the delete and tune (MRTuning) and everyone said deleted it would get 55mpg. For about 3-4 weeks my mileage stayed the same then right about that time winter hit and we got the winter blend fuel and my mileage was 35 highway and 31 commute. Really poor. Well now in the spring the winter blend fuel is gone and it's been warming up and my mileage has gotten better but I'm still doing about 35.5 to a tank commuting. To me this is crap. I also feel like I have an intermittent misfire but don't get a CEL. I have done a timing belt and yes I am confident I did it right. I have done all fluids and filters. I have checked I don't have a sticking rear caliper. On a gas car I would just do a tune up and/or an O2 sensor. But on this car I'm not really sure how to diagnose this, do I have a bad injector or some other issue? Where do I start?

 

[Shoveltrev]

id start with the thermostat. the gauge is buffered it will read 190 from 160 to about 212. watch the temp with a scan tool if it cant maintain 87c or 190f on the highway it needs replaced

 

[IndigoBlueWagon]

Why would you not think it's the tune that's killing your fuel economy? I'd talk to them first.

 

[Delasangre]

id start with the thermostat. the gauge is buffered it will read 190 from 160 to about 212. watch the temp with a scan tool if it cant maintain 87c or 190f on the highway it needs replaced

I replaced the thermostat and both temp sensors already

 

[Delasangre]

Why would you not think it's the tune that's killing your fuel economy? I'd talk to them first.

For one I was getting good mileage still for about a month after the tune plenty of time for it to learn or do whatever it was going to do. And I reached out to the tuner and he said my 20 to 25% loss of fuel mileage is normal when you combine winter tires, winter fuel, and cold weather. But I find it hard to believe I'd lose so much. Also I'm not getting it back now in the spring.

 

[IndigoBlueWagon]

That FE loss is nonesense. 5-10% is the max you should see. And it's not even that cold where you are. If the tune was working properly you should still have been seeing high 40s in winter because deleted cars should use less fuel.

It's also possible you have a fuel delivery issue that the misfire is indicating. You should get the car on a scanner and see if you have codes: you may despite not having a CEL.

 

[Delasangre]

That FE loss is nonesense. 5-10% is the max you should see. And it's not even that cold where you are. If the tune was working properly you should still have been seeing high 40s in winter because deleted cars should use less fuel.

It's also possible you have a fuel delivery issue that the misfire is indicating. You should get the car on a scanner and see if you have codes: you may despite not having a CEL.

Thanks! That's what I thought.

I have plugged VCDS in a few times and there's not been any codes. I will plug in again this weekend to make sure.

My neighbor works across from a diesel injector shop. I plan to take the injectors in and have them checked soon to rule them out.

 

[oilhammer]

Does it black smoke on acceleration?

My stock 2010 sedan, with a DSG, is hard pressed to get worse than 40 for me, and that's with 17s and my lead foot. Most of the deleted CJAAs go past 50 MPG pretty easily.

 

[Delasangre]

Does it black smoke on acceleration?

My stock 2010 sedan, with a DSG, is hard pressed to get worse than 40 for me, and that's with 17s and my lead foot. Most of the deleted CJAAs go past 50 MPG pretty easily.

I get smoke when I cold start it and it idles for a minutes. Then I get light smoke when I'm full throttle. But since I'm not behind myself I can't tell if it smokes under light/normal throttle. If it does its not enough to see in my mirrors.

 

[TwoDangerRanger]

I just purchased my first TDI. It’s a 2012 Golf and I’m not getting impressive fuel economy. I drive highway almost exclusively (110 miles a day) and I just averaged 29 mpg’s. It’s been very windy but shouldn’t I be getting more around 35+ with a stock car?

 

[Delasangre]

I just purchased my first TDI. It’s a 2012 Golf and I’m not getting impressive fuel economy. I drive highway almost exclusively (110 miles a day) and I just averaged 29 mpg’s. It’s been very windy but shouldn’t I be getting more around 35+ with a stock car?

I was getting 47 highway and 40 on hills when it was stock, then about a month after tuning it dropped hard. The only things I can think are o2 sensor throwing off AFR or a leaking injector. Going to check both soon. The members on the forum here have not been super helpful in diag.

 

[TwoDangerRanger]

I was getting 47 highway and 40 on hills when it was stock, then about a month after tuning it dropped hard. The only things I can think are o2 sensor throwing off AFR or a leaking injector. Going to check both soon. The members on the forum here have not been super helpful in diag.

I’m new to the whole TDI scene but I have some common sense in my tool box. I think in your case the winter mix messed with something in your fuel system. If you’ve gotten your injectors checked then it’s probably something electrical and that’s beyond my limited knowledge, or maybe a partially clogged fuel line from the pump to the injector.

As for me I’ve only had the car for 2 weeks and it’s staying steady at the low 30’s. I had the transmission serviced at the dealer just this first time (bit the bullet on that one $$$) and I’m having the timing belt done at the end of the month just as preventative maintance (local shop, not the dealer this time). I bought the car for fuel economy and 29 is better than my ranger at 19 but still.

 

[Delasangre]

I’m new to the whole TDI scene but I have some common sense in my tool box. I think in your case the winter mix messed with something in your fuel system. If you’ve gotten your injectors checked then it’s probably something electrical and that’s beyond my limited knowledge, or maybe a partially clogged fuel line from the pump to the injector.

As for me I’ve only had the car for 2 weeks and it’s staying steady at the low 30’s. I had the transmission serviced at the dealer just this first time (bit the bullet on that one $$$) and I’m having the timing belt done at the end of the month just as preventative maintance (local shop, not the dealer this time). I bought the car for fuel economy and 29 is better than my ranger at 19 but still.

The winter fuel and cold are long gone and I STILL get 30-32. Max I have seen was 38 highway where before I was getting 47 and SHOULD be doing in the 50's with the tune. Its not winter fuel or snow tires in my case.

I know I feel you, I wouls start with a fuel filter as those tend to get clogged from what I heard. The other big culprit is sticking e-brake cables or calipers so you could check that. Jack up the back and try to spin the wheels without the ebrake on and see if they spin or stick.

 

[TwoDangerRanger]

The winter fuel and cold are long gone and I STILL get 30-32. Max I have seen was 38 highway where before I was getting 47 and SHOULD be doing in the 50's with the tune. Its not winter fuel or snow tires in my case.

I know I feel you, I wouls start with a fuel filter as those tend to get clogged from what I heard. The other big culprit is sticking e-brake cables or calipers so you could check that. Jack up the back and try to spin the wheels without the ebrake on and see if they spin or stick.

Yeah since the warm weather is here I understand the winter mix isn’t being put in anymore. What I’m saying is the fuel might have caused the injectors to spray poorly or there might be some sort of artifact from the winter mix that is in the fuel system. Maybe a fuel filter is a good one to try for your situation.

I did get the fuel filter done at the same time as the transmission service. But that sticky e-brake might be part of the problem. The lever is super….firm…it only lifts a few clicks to engage the e-brake.

 

[Delasangre]

Yeah since the warm weather is here I understand the winter mix isn’t being put in anymore. What I’m saying is the fuel might have caused the injectors to spray poorly or there might be some sort of artifact from the winter mix that is in the fuel system. Maybe a fuel filter is a good one to try for your situation.

I did get the fuel filter done at the same time as the transmission service. But that sticky e-brake might be part of the problem. The lever is super….firm…it only lifts a few clicks to engage the e-brake.

I understand, I just did a fuel filter and it was super clean. I have done ALOT of work to the car so the only things that really can be is a sensor or injector I think. I wish somone had more pointers since this is my first diesel too.

That does sound like you have an issue. It should lift about halfway. Mine are odd because I have one that felt normal as in slight drag, the other side didn't slow down at all so maybe that caliper isn't working at all. I am doing rear calipers, parking brake cables, and rotors/pads this weekend.

 

[turbodieseldyke]

I was getting 47 highway and 40 on hills when it was stock, then about a month after tuning it dropped hard... The members on the forum here have not been super helpful in diag.

Did the members tune your car to get lower mileage? How helpful has your tuner been? Super or Not Super?

 

[Delasangre]

Did the members tune your car to get lower mileage? How helpful has your tuner been? Super or Not Super?

The bad fuel mileage happened/started at least a month after applying the tune. Something happened after. Could be the tune I guess but I feel like its something else. My tuner blew me off as winter fuel and tires, thats normal. Ya sure, normal to loose 25% of economy.

 

[Nuje]

Is it possible that during a cold snap during the winter, you had some dreaded ice/water build-up in the intercooler and bent a couple rods (would also explain the apparent misfire / fuel knock) - bent rod = low compression = poor FE.

Just hypothesizing.

 

[IndigoBlueWagon]

Have it flashed back to stock and see if the FE improves. Then you'll know.

 

[Delasangre]

Is it possible that during a cold snap during the winter, you had some dreaded ice/water build-up in the intercooler and bent a couple rods (would also explain the apparent misfire / fuel knock) - bent rod = low compression = poor FE.

Just hypothesizing.

I mean it's a possibility. Still pulls really hard and only shudders in 4th at low 1600-1800 rpms or so. But yes I've been looking into getting a compression tester kit to rule any issues there out.

 

[Delasangre]

Have it flashed back to stock and see if the FE improves. Then you'll know.

I'm not sure how it would run without the emissions system in there.

 

[turbodieseldyke]

You can check compression with vagcom, group 141 iirc

 

[IndigoBlueWagon]

I'm not sure how it would run without the emissions system in there.

Forgot it was deleted. Then find another tuner. This one doesn't seem to want to be helpful.

 

[Delasangre]

Forgot it was deleted. Then find another tuner. This one doesn't seem to want to be helpful.

I reached back out. I was very clear its not a winter fuel or snow tire issue. He sent a revised tune to try out. I will get it flashed this weekend and fingers crossed. I still doubt its a tuning issue, but we'll see.

 

[Delasangre]

You can check compression with vagcom, group 141 iirc

Thank you for the suggestion. It was 141 I think anyways. It showed 4 boxes with between 42-45 BAR across the board. Seems like a good number to me.

 

[TwoDangerRanger]

I think I found my issue. It’s a EGR / DPF situation.
On that note, I have no problem deleting them, no emission checks where I live. Where did you get your delete kit? I’m not looking for insane performance just a reliable commuter. I’ve found the Malone 0.5 tune that’ll let me delete those but not require a DSG tune. I’m on a budget is the only reason I’m not doing a higher stage tune.

 

[Delasangre]

I think I found my issue. It’s a EGR / DPF situation.
On that note, I have no problem deleting them, no emission checks where I live. Where did you get your delete kit? I’m not looking for insane performance just a reliable commuter. I’ve found the Malone 0.5 tune that’ll let me delete those but not require a DSG tune. I’m on a budget is the only reason I’m not doing a higher stage tune.

It usually is. That's why so many delete it. Malone is good, I've heard mixed feelings on Stevenson, but bang for the buck if you want fully deleted with the exhaust bits you can't beat the kit from EOC which is a Flowdirekt downpipe and MRTuning tune.

However the customer service sucks so if I was to do it again I'd buy just the exhaust from EOC and get the tune direct from MRTuning. You can check out Josh's Jetta's on YouTube since he does a nice comparison of a few popular tunes and breaks down total cost and stuff.