Fuel economy

The last four tanks my fuel economy has really been in the pooper. When I first had my tune done, the only changes I've made to my car is replace the vac lines, and regular service, fuel filter, oil, ect.
Last few fillups I've averaged 33mpg I used to hit 38 with the exact same drive, exact same driving styles.

I just got back from running boost and MAF logs. Not 100% sure what to make of them, but it seems like my MAF actual reading is decaying as RPM's go up. Don't know if that's normal. Log can be found here. Really would appreciate any input on it! I'll be going through the rest of the Fuel Eco 101 thread and check everything such as dragging brakes, and update as I go. Not sure if it's related, but on some occasions when I do a hard pull, sometimes there's no smoke, sometimes its a big ol fart of black haze. I do regularly do an Italian Tuneup as well.

Yea, that's low. I have same issue with my bug (all stock), 38 steady. Read from more knowledgeable folks these cars should hit closer to 50, frustrating. You shouldn't be seeing smoke other than very occasionally. Since you did business with Malone you might ask them to look at your logs. Compression test maybe.

Strange bit is, my fuel economy still remained 38 after the tune. It's only lately its gone downhill. I'll see if I can get ahold of Malone and see what they say too. Still open to more suggestions.

Is this a MAF delete tune? I don't know if it is or that's just the way VCDS is reading it. Also, your actual boost seems to lag request.

However, I don't think either of those are enough to see the FE drop you're experiencing. I'd look to injector balance and fueling.

Boost leak based on the inconsistent power and low boost values. pump timing and injection quantity done as well? both 33 and 38 are really low numbers unless it's aggressive city use.

It's not a MAF delete tune as far as I know. I didn't request it that way. Being as I have dynamic EGR I think it's not a MAF delete.
I had my injectors built and balanced by DFIS, they're about 15 months old now. In group 013, balance is within .19 of each other.


I have a homemade boost pressure tester, cap off at turbo outlet, gauge on boost pipe to EGR valve. I checked it roughly 6 months ago, and it held 35psi for an hour. Lots can change between now and then though.


I'll check my actuator start/stop vacuum tomorrow morning, and check for boost leaks again. Then take another log and upload it.



Also, the last few tanks I've tried to be conservative with my right foot, to see if my FE would come back up, and it's remained the same.


In case my turbo is starting to go south, can I replace it with a VNT17 without a tune without any issues? There may be a week to a month before I can get the tune adjusted in case I have to go that route. Was going to get the VNT17 anyways in the near future anyways.

I get 32-33 mpg on both my tuned ALHs in town and it doesn't seem like I can get any better than that.

I was seeing much better mileage just a few years ago...been working on it ever since i noticed. Now that I have a new turbo, and Rich told me he took the liberty of cleaning my intake while he was in there ("I just had to" lol), I'm hoping to see a marked improvement.
Maybe we all should just copy exactly what IBW's mods are and go from there ...

People tell me my car is a unicorn, and I think there's some truth to that. The other MKIV cars in my orbit don't achieve as good FE as my wagon. That's why I keep driving it.

I cleaned the intakes on both of my cars and had no change in FE that I noticed. The only time the FE went down was when I added the nozzles and tunes. Stage 4 is the way to go IMO.

Any changes made right around the time the MPG dropped?

No, no changes. It's been a slow decrease for a while.


Did some more tests this morning, pressure tested the charge air system from turbo outlet to EGR inlet. Found a teensy little seepage at the elbow where it mates up with the EGR/ASV valve. Otherwise it held 30PSI no problem. Replaced the hose. Upgraded to worm gear clamps vs the spring clamps at both ends of the elbow. Also put a worm gear clamp on the turbo outlet just for safe measure.
Checked turbo actuator with my MityVac. Starts to move at about 4inhg, comes to a full stop between 19-20. No visible binding/looseness in the actuator linkage., guess changing my shifting until warmed up helped clear the binding I thought I had. Holds vacuum.

Not sold fuel economy math, but I’ve gotten roughly 150 miles on a quarter tank. Bought the 2000 wvw mk4 2 door golf Monday w/350+k, was told she consistently got 44mpg.

zaeli said:

Not sold fuel economy math, but I’ve gotten roughly 150 miles on a quarter tank. Bought the 2000 wvw mk4 2 door golf Monday w/350+k, was told she consistently got 44mpg.

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That's kind of meaningless, I mean how do you measure a quarter tank? Just do full tanks, mileage/volume. Do it every tank, you'll know what you get after repeated calculations.

Hopefully you'r getting that 44 or better, decent number for urban driving.

zaeli said:

Not sold fuel economy math, but I’ve gotten roughly 150 miles on a quarter tank. Bought the 2000 wvw mk4 2 door golf Monday w/350+k, was told she consistently got 44mpg.

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Now you did it. I wasn't going to brag, but this is from a trip home from NC last Summer:


That tank was 52.2 MPG.

Sooooo you gonna share what mods you have on your wagon? I may try to copy it

Stock block and head (although I may have an SDI cam, not sure), PD150 airbox, snorkel, MAF, and turbo inlet, PD150 intake manifold, Tyrolsport intercooler, PD lift pump, 11mm pump, PP357s, Rocketchip Stage 3+, Mahle VNT-15, 2.5" downpipe and exhaust, resonator and muffler, no CAT, SBC Stage 2 daily clutch.

I think that's it. One problem with owning a car for 17 years is you forget what you've done to it. Some of the mods are 12 years old. My average on Fuelly for the last 168K miles is 46.3 MPG. The speed you see above is my usual trip cruising speed. Commuting I go with traffic, which around here is often running at 80+. Unless it's stopped.

Nice! Now then, what brands of fuel and oil? Oh, and EGR state?
Thank you, BTW!

IndigoBlueWagon said:

Now you did it. I wasn't going to brag, but this is from a trip home from NC last Summer:

That tank was 52.2 MPG.

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OK
So if you go 405.3 miles using 1/4 tank then you can go 1621.2 miles on a tank?

I rolling my pants legs up because it's getting a bit deep.

No. that's the thing: the fuel gauge is horribly inaccurate on these cars. IBW will pass just over 800 miles when the warning light comes on. The fuel gauge you're looking at is trying and failing to tell the driver he has half a tank remaining. Really wish they were more accurate. Thus Bob's recommendation to do full tanks for calculating FE. Closest practical estimate possible.
EDIT: And...52.2 MPG in an assumedly vent-less tank = around 835.2 miles/tank.

I haven't done the ventectomy yet, have been debating about it. Mostly worried about tank expansion from it being full and wrecking a tank. You've been running the ventectomy for a while, any regrets?

No regrets (years ventless), and I always ask the attendant to fill it to the top. They 99% of the time oblige ( Past 9 years same station every time). Yes, everyone, we Oregonians are not allowed to pump our own fuel...we have highly paid professionals for that. I actually don't mind, and even tip them...not uncommon here so I hear.
But Nero, being the fretful, helicopter owner of this '03, I did a lot of reading here, and convinced myself the vent is good for nada on a diesel tank. No danger to the public even.

I took that photo in part as a joke. As STDOUBT says, these gauges aren't the most accurate. At that point in the drive I hadn't stopped and shut the car down since filling, and when I restarted the car (shortly after the photo) the tank read lower.

Here's the rest of the tank:


Still not too shabby. Vent has been removed on my car forever, probably for 12 years. Never had any problems. I don't have vents in any of my TDIs where they're removable.

Since I have a PD150 intake manifold no EGR cooler. EGR is in place, anti-shudder valve works, but the tune has no EGR duty cycle in it. And no, I don't have any issues with the car warming up in the winter. I usually use Total Quartz 9000, and I have my own fuel tank at my house with good diesel and an Opti-Lube dose in the tank, but nothing special. And on trips, of course I fill wherever. Not that picky.

Nero Morg said:

I haven't done the ventectomy yet, have been debating about it. Mostly worried about tank expansion from it being full and wrecking a tank. You've been running the ventectomy for a while, any regrets?

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I have been doing myself but use the nozzle to open the vent.
My MPG varies a bit but I don't worry about it.

I'll have to do the mod. We're spoiled over here and have fuel attendants here. Not illigal to pump your own diesel but they sure look at you funny.

Yes, do the mod. It allows the car to have more range and gives you consistent fillups, so you can calculate FE more accurately. I have done it on two of my ALHs. No problems with it.

I fill until I see clear fuel in the filler neck. Done it for years. No problems. I usually add 15.5 gallons if I fill right when the reserve light comes on. Most I've ever added is 17.1.

Something that I think may have been overlooked. Have you checked your tire pressures lately. As I see my MPG's go down in the winter, I check my tires. and usually I have 1 or more low. I run 40 psi on all 4 corners and have you had an alignment lately?

I check the tire pressure at least once every two weeks, keep a gauge in the door pocket. I have yet to get an alignment though. It doesn't feel off when driving on the freeway, but that still doesn't mean it's at least 1 degree off.

Here's my mileage since last year January.
Hope it pastes OK

Model Date Gallons Per Gallon Total Mileage Additive Notes Vender MPG
Jetta TDI 03 1/31/2018 6.861 $3.06 $20.99 232.8 $1.30 Idle 30 minutes for accident Quick Check Ramsey 33.93
Jetta TDI 03 3/6/2018 12.817 $3.06 $39.21 446.4 $2.60 Cleaner & Anti-Jel Quick Check Monroe 34.83
Jetta TDI 03 6/16/2018 12.012 $3.12 $37.47 507.7 $1.30 Quick Check Monroe 42.27
Jetta TDI 03 9/22/2018 13.795 $3.00 $41.37 528.1 $1.30 1st Time Vented Tank 260211 Quick Check Monroe 38.28
Jetta TDI 03 11/9/2018 15.233 $3.12 $47.51 521.8 $1.30 2nd time venting 260733 Quick Check Monroe 34.25