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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old January 26th, 2012, 15:40   #1
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Default 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 because it may be just one small part of the puzzle, but an important part nonetheless!

Tires: this goes beyond simply air pressure, although that is an important part. Tread design is another, as well as the proper size and load rating. When in doubt, look at your car's tire information placard. It will have the proper size, load, and speed rating. Going from the proper 91 load to an 89 (an all too common mistake) can take a 1 MPG hit all by itself over the same model tire. Low Rolling Resistance (LRR) tires are becoming more popular, and are usually marketed as such. Still, keeping the pressure up, and the rating correct, is very important.

Alignment: if the alignment of the car is off, the tires can 'scrub' as they roll down the road. This causes excess friction and drag, which causes the engine to have to work harder. Checking the alignment with proper equipment is the only way to be sure everything is in specification. Worn bushings and steering components can cause the alignment to be out.

Trim pieces and shields: the car was designed to be both slick on the top AND the bottom. Missing shields under the car can not only make the engine take longer to reach operating temperature, they can also cause extra wind resistance when travelling down the road, and the faster you go, the worse it becomes. Missing grill inserts, missing lower splash shields, missing/damaged fender liners, can all contribute to this problem.

Thermostat: if the engine cannot reach proper operating temperature as quickly as possible and maintain it, it will cause more fuel to be used. Don't assume the gauge (or light in the New Beetle and latest Jetta sedan) is enough to tell you the engine coolant is warm enough. The only sure way to know is with a scan tool and watching the coolant temp data. Thermostats don't last forever, and one that cannot do its job properly needs to be replaced. If it gets really bad, the engine controller will flag a fault, however it has to be pretty bad before that happens. Also note that on many TDIs, the coolant temp sensors are notorious for going bad, so you may need to verify both sides (the ECU side and the Instrument side) is working on cars equipped with the 4-pin tandem CTS (specifically, late ALH, BEW, and BHW cars).

Timing: Both the VE TDI and the PD TDI are sensitive to timing. Just a few degrees can make a substantial difference. If the timing belt was installed wrong, your timing will be off. Period. While there is some sliding scale here, as some timing settings favor power over economy, keep this in mind when you set the timing after the belt is installed.

Air cleaner/air flow sensor (MAF): these items need to be in proper condition, and work in conjunction with the Exhaust Gas Recirculation (EGR) for proper engine function. A little bit off, or dirty, or poor quality/fitment can cause some adverse performance.

Lambda (oxygen) sensor (PD only): these can degrade and cause a slow response from the ECU in a similar (but not identical) fashion that a gasoline fueled car can, and can cause some loss of fuel mileage on PDs.

Intake/EGR clogging: while not the issue it once was, due to ULSD fuel, it is still something that should be checked for, especially on VE cars. This can cause a loss of power overall, and call for excessive fuel use since the air the engine needs to run cannot get in properly.

Brakes: many Volkswagens, especially A4 platform cars, have chronic stuck parking brake cables. This causes the rear calipers to drag, and your torquey diesel won't know it... it 'll just eat more fuel to move the car down the road. Also, brake pad perches can rust up, and cause the pads to sieze in their spots, dragging the brakes.

turbocharger: the "T" in "TDI" can't work properly if there is a vacuum/pressure leak, binding VNT/wastegate actuator, etc. While a fault will be flagged if it gets really bad, low boost can slowly creep up and rob you of both power and fuel economy gradually without you even knowing. Again, a scan tool checking requested and actual boost can help check the condition of your turbocharger system.

Driving style: we hear this all too often here, but it bears repeating here. Driving the car too easy for too long can really mess things up in the MPG department. Keeping the pipes clean, by occasional spirited driving, can help to keep the EGR clean, the intake clean, the intercooler clean, the breather system functioning properly, and the turbocharger working correctly, in addition to keep the catalyst and (where equipped) Diesel Particle Filter (DPF) clean and in good working order. A good test on VE and PD cars is to do a floorboarding rolling start (once the engine is warm) and see if it belches out a cloud of black smoke. A healthy, happy TDI, even a mildly modded one, should NOT do this. If it does, and it goes away after doing this a few times, you know you were driving it too easy for too long. Some folks call this an 'Italian tune up'. Whatever you want to call it, it works.

Excess weight: we can call this a minor one, but let's face it, dragging 50 pounds of tools, a stack of bricks, your entire CD collection, your golf clubs, the collected works of Charles Dickens, and 70 bottles of water around in your car is not really necessary.

Hopefully some of the above listed items can help you track down a possible loss of fuel economy!

Last edited by oilhammer; September 11th, 2018 at 06:02.
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Old January 26th, 2012, 15:56   #2
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Another helpful contribution to the TDIclub. Thanks Brian!

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Old January 26th, 2012, 16:08   #3
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Old January 26th, 2012, 16:52   #4
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What scan tool would you guys advise getting to monitor temp, boost, etc
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Old January 27th, 2012, 01:20   #5
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ScanGaugeII is quite widely used.

Audi A2 TDi 1.4*, 6M/0.588, mild aero mods, 103hp remap (* 3 pot version of 1.9PD)
Y2000 Passat TDi 115 PD RETIRED, M5, Best 75MPGuk, usually 60-70. 900m clubx7. 1000 mile clubx3.
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Old January 27th, 2012, 01:38   #6
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All good tips. Thanks oilhammer. I also use the ScanGauge II for coolant temp.
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Old January 27th, 2012, 09:45   #7
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Ultragauge has more functions and is cheaper than scangauge .
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Old January 27th, 2012, 11:47   #8
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Great write up!

Here's a link for the gauges:

I haven't tried the ultragauge yet but looks interesting.
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Old January 27th, 2012, 12:28   #9
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Lower GPM (gallons per month). Carpool!
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Old January 27th, 2012, 17:44   #10
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Any advise on the 130 BHP TDI Engine in a Golf Estate
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Old January 28th, 2012, 03:29   #11
Join Date: May 2009
Location: Yuba City, California
TDI(s): 2004 Jetta

Slowing down a bit has helped me. I was at 38mpg going 72mph. Slowed to 68 or 69 and I'm above 40mpg now. I just started setting the cruise to 65mph. We'll see what that does. I would really like to average 45mpg. I'm putting most of the other suggestions into practice already.

Thanks for the thread and ideas.

BTW, at 25,000 miles a year and $4.00 a gallon fuel, 1 mpg increase is about $55/year savings. Hmmm, $200 every 100,000 miles is almost the parts kit for my timing belt change!

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Old January 29th, 2012, 19:25   #12
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Wow thanks Vekke, just saw your post after I ordered a scan gauge.......
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Old January 30th, 2012, 22:40   #13
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I keep all the excess trunk netting and "stuff" out of my car. Also, no one ever rides in my back seat. I'm looking to take the back seat head rests out.
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Old February 6th, 2012, 13:28   #14
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Default confused about driving car too easy

does "driving the car too easy' mean driving in 5th gear in city driving? I just got 407 miles per tank this last time, and noticed thats the most ive gotten lately (I did just change the timing belt)....

One diesel mechanic told me its good to drive the TDI's above 2000 rpm always.....driving it under, wont give you good mpg.......

is that true?
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Old February 7th, 2012, 04:27   #15
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Originally Posted by PDX_TDI View Post
One diesel mechanic told me its good to drive the TDI's above 2000 rpm always.....driving it under, wont give you good mpg.......

is that true?
'always'? Nope. However it IS good to drive it over 2000 reasonably frequently (and occasionally over 3500) - you need to get the turbo spinning to clear off the coke. What the TDI seems to like best is a bit of variety. It's OK to baby the engine as long as you also thrash it sometimes too. But if you only baby it, or only thrash it, you will end up with problems & reduced performance.

Audi A2 TDi 1.4*, 6M/0.588, mild aero mods, 103hp remap (* 3 pot version of 1.9PD)
Y2000 Passat TDi 115 PD RETIRED, M5, Best 75MPGuk, usually 60-70. 900m clubx7. 1000 mile clubx3.
VW Fuel saving tips

Last edited by MikeMars; February 7th, 2012 at 04:32.
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