long downward mileage trend over 5 years and 120k. What to check?

[mike944]

Hey everybody,
I just typed in a few years of fuel usage data this weekend, and it's now causing me to notice a slow downward trend in fuel mileage over a period of about 5 years, and about 120k miles. (if you "normalize" the step-change between the automatic and 5-speed) You'll notice the trend starts about mid-year 2008

Over this long period my commute time has remained long, so the highway/city mix is fairly constant. There have been different tires, different brakes, different timing belt, different suspension bushings, different almost everything except for engine internals.

I checked my records, and can't find anything that was changed at the start of the downward trend. I'm stumped as to a possible reason, and things to check.



My only thought is injectors. They have about 200k on the nozzles. WOuld i see this big of a drop from worn nozzles?

Anybody have any ideas of other things to check?

 

[VWRacer1]

Did you periodically check timing with a VC? Between 150k and 200k my mileage also slipped several mpg, but I expect that a new TB and advancing my IP timing a bit will bring it back. I'm driving up to Seattle and back this weekend, so will know for sure by next week.

 

[BobnOH]

Excellent data set. It seems odd that the mileage dips in the summer, maybe you own a boat? All things being equal, I would definitely renew those nozzles (200k)

 

[mike944]

Excellent data set. It seems odd that the mileage dips in the summer, maybe you own a boat? All things being equal, I would definitely renew those nozzles (200k)

I suspect my summer mileage drop is due to air conditioner usage. i use it all the time.

you may, or may not notice that i get peak mileage in spring and fall. dropoffs in both the summer and winter.

I will check timing, but no, i don't periodically check it. i last changed my belt Jan of 2011, and definitely reset the timing there. i didn't notice anything unusual at that time, that would make me connect it to the resetting of timing. I will check it though.

 

[mk3]

A Thermostat can do this. The trend is toward cooler operating temperatures. Replace the thermostat with a new dealer-sourced one.

Also - clogged intake.

The timing setting is debatable. by some accounts this is a closed-loop control system and moving the pulley on the pump will just cause the control system to re-adjust and have literally no change. by other accounts the control system only does so much or only controls over a certain range. I haven't figured out the truth of it yet.

maybe nozzles.... but those are expensive so I feel you shouldn't do this unless it is critical. You may end up spending money first on nozzles - then on pop-testing and calibration - then on new fuel lines if you cannot get your new setup to stop leaking - and then what if it was not the root cause.

Also - fuel formulation changed back in the day... was that 2007? the ULSD fuel came into being.

 

[mike944]

A Thermostat can do this. The trend is toward cooler operating temperatures. Replace the thermostat with a new dealer-sourced one.

I will check this. It has been running cooler for several years now. But wouldn't this be a step-change? not a long downward trend?

Also - clogged intake.

I disabled by PCV system many years ago, and i keep an eye on the intake, but i will check it again this weekend.

The timing setting is debatable. by some accounts this is a closed-loop control system and moving the pulley on the pump will just cause the control system to re-adjust and have literally no change. by other accounts the control system only does so much or only controls over a certain range. I haven't figured out the truth of it yet.

maybe nozzles.... but those are expensive so I feel you shouldn't do this unless it is critical. You may end up spending money first on nozzles - then on pop-testing and calibration - then on new fuel lines if you cannot get your new setup to stop leaking - and then what if it was not the root cause.

Also - fuel formulation changed back in the day... was that 2007? the ULSD fuel came into being.

Regarding the fuel formulation, again, wouldn't this be a step-change? not a long downward trend, getting worse and worse?

 

[mk3]

My impression is that the thermostat can drift slowly down. Possibly due to some hard to understand metallurgical phenom. bit of a wag.

It probably isn't the fuel... but it might have been blended in over time - just a pretty short time.

Could the cat have gotten plugged? I blasted mine out a few years back when I patched up the crack in the weld on mine.

Has your car been starting well?

 

[mike944]

well, if the temperature has been drifting down, the gauge isn't showing it. it's been running a bit on the cold side for years now. but no drifting. In fact, for the last few fillups, it's been running back to normal. I will change it though.

ok, i'll check the cat this weekend too. I'm overdue for a turbo vane cleaning. starting to get overboost codes. while i have the exhaust off, i'll check it with a shop vac (as recommended by a friend)

Starts just fine. i messed with the IQ a few months ago due to an idle vibration, (went from 1.2ish to 4.5) but it had noticably less power, and got noticably worse mileage, so after a few fillups, i put it back. I know that's an unusual result, most people get just the opposite, but i don't understand it.

 

[AndyBees]

Exel is awesome, especially when you put all the data in perspective with a graph.

I've experienced the same trend with my '00 Jetta. The downward trend started at about 200k miles. The downward decline dipped even more when I retired.

1. Wear on the Injectors probably affected the first down turn.
2. After I retired my driving was mostly stop and go vs long commute to work and on the job road trips.

Oddly, my son now drives the car and the MPGs are trending back up somewhat. So, at over 325k miles on the clock, I suspect a new set of nozzles would be a big help!

Side note: I've never had starting issues with my car. Fuel filter change once per year regardless of miles on it.

 

[nathan_b]

temp gauge on dash reads "190f from about 150f-215f.

 

[mike944]

temp gauge on dash reads "190f from about 150f-215f.

Yeah, thanks for the reminder. I guess i forgot that fact that the temperature gauge is nothing more than an idiot light with a needle. it's not a real temperature gauge.

is there any way to "fix" this with vag-com?

 

[Genesis]

No. Unfortunately.

My money is on the injectors and MAYBE the pump's lift side (the vane pump); if case pressure is unable to be maintained quite to spec the pump will be a bit noisier than it should and the quality of injection will suffer, which will impact mileage a bit.

You're in the zone where either or both of those can be the cause.

 

[shoebear]

I bought a 2005 NB TDI in January and have been fixing things one at a time. The thermostat was bad, and I was seeing about 150-160* temps according to ScanGauge. When I replaced it, my temps are now in the 204-209* range, and it gave me about a 2mpg bump. Thermostats can wear out gradually over time as the wax pellet leaks and/or becomes contaminated. For much, much more than you EVER wanted to know on this subject, click HERE.

Nozzles may help also, but I would verify the need before doing them. For example, if you idle your car for 5 minutes, then take off, do you get white smoke (from fuel dribbled, rather than sprayed, into your cylinders; accumulating, then expelled as white smoke when you accelerate). When I replaced the stock nozzles on my 1998 Jetta TDI with DLC1019's and chipped it, I gained about 3-4 mpg. Of course, I don't know how much of that was due to 1) replacing worn nozzles, 2) moving to larger nozzles, which allow quicker injection and a more efficient burn, or 3) the chip.

 

[sirpuddingfoot]

Going to suggest some very basic stuff just so we've got all the bases covered:

You have changed the fuel and air filters, right (hopefully a couple times)? Cleaned/deleted snow screen?
Intercooler caked with crap because you're missing a fender liner on the right side?

 

[IndigoBlueWagon]

Like posts above have observed, routine maintenance items can affect FE. If you don't want to throw parts at the car, I'd suggest you run logs of fueling, boost, and MAF output. See if they're all reaching request. If not, clean the intake, replace the vacuum hoses, and check the actuator to make sure it's moving through its full range and not notchy in operation. After that I'd look at the N75 and MAF. Keep in mind that a clogged intake or underboost will also show low MAF readings even if the MAF is OK. Setting timing is also a good idea, and you might want to run a can of diesel purge through it before replacing nozzles: cheaper and you may get a good result.

Some items in these cars do a slow fade and the decline in performance is not immediately noticeable, but your FE numbers tell a story.

 

[mike944]

ok, here's the update from this weekend. A bunch of stuff checked, no smoking guns found yet.

Intake: not too bad, mostly open, but i cleaned it anyway.
Replaced turbo actuator, old one was indeed notchy.
cleaned variable vanes (to solve overboost codes, not necessarily to fix a mileage issue)
replaced a bunch of vacuum hoses
exhaust was breathing free. (It breathes even "free-er" now
Engine is indeed running cold, and i ordered an OEM thermostat from IDparts.com (OUCH$$$)
couldn't do a proper timing check because it was running cold, but the graph (at 65-70C) showed dead in the middle.
fuel filter changed
ordered a new air filter

other stuff:
Snow screen was removed before i got the car (220k ago)
didn't check intercooler yet
MAF actual vs request was definitely down last week before all the work. didn't re-check after turbo work.

To be done:
replace EGR/anti shudder valve, because i broke the nipple off the anti-shudder actuator, and the EGR has been leaking for years. (on order)
put some miles on it to check fuel usage. It definitely seems a bit "peppier"


I will report back in a few tanks of fuel (a few weeks) if anything made a difference. If not, i'll order new nozzles.

 

[shoebear]

Engine is indeed running cold, and i ordered an OEM thermostat from IDparts.com (OUCH$$$)

The Stant 45379 fits and is probably available locally for about $10 (try Advance Auto if you have those there). It will run a bit hotter than stock, which will help your mileage. I have this t-stat in both my TDI's, and it works well.

 

[mike944]

The Stant 45379 fits and is probably available locally for about $10 (try Advance Auto if you have those there). It will run a bit hotter than stock, which will help your mileage. I have this t-stat in both my TDI's, and it works well.

Which i had known that a few days ago..... Somebody farther up in the thread suggested that i only get the OEM thermostat.

 

[shoebear]

Which i had known that a few days ago..... Somebody farther up in the thread suggested that i only get the OEM thermostat.

Well...better late than never. The Stant does have its detractors because it appears to be built more cheaply, and the temp doesn't seem as consistent from one to another. For example, my Jetta runs about 195*, but my NB runs about 205* with Stants in both. However, I'm not sure whether that's the thermostat, or the difference from AHU to BEW engines. The BEW has three glow plugs right by the temp sensor to heat the coolant. Is that why the NB runs hotter? Not sure, but the Stant has worked well for me.

 

[dremd]

My $ is on the thermostat.
Let us know how the next tanks look.

 

[mike944]

Ok everybody, as i promised, i have the latest mileage report after the turbo vane cleaning and thermostat change. I wanted to run a few tanks of fuel (8 to be exact) to make sure i normalize any tank-to-tank variation.

I have also been running heavy doses of Powerservice DIESEL KLEEN for all 8 tanks (probably 2-4x the normal dosage) to try to clean nozzles.

As you can see, the change (which i pointed out in the image) is almost in-the-noise. Hardly noticable. Not any more than my normal spring/fall spike between summer AC use and winter fuel

So, thermostat wasn't it. Any other things? should i just bite the bullet, and get some new nozzles?

 

[dremd]

How does timing and IQ look after having a fully functioning thermostat?

 

[1.9glstdi]

Have you checked the cam lobes?

 

[mike944]

Timing is advanced, about halfway between the blue and green lines.

IQ is 2.2. I know that's kind of low. i already tried increasing into the normal range it a few months ago, and it made the mileage worse, so i keep it low on purpose.

Haven't had a chance to check the cams yet. I checked them at the last timing belt change (about 70k ago), and they looked perfect. no noticable damage at all. Besides, i thought only the PD engines had cam wear issues. I will check them in a few days.

 

[mike944]

Ok, last update on this issue, since i'm selling my car. (and getting a grey 2004 with 1/2 as many miles)

Since the last update, i've changed the head gasket (due to coolant leakage into the oil) put in a complete timing belt kit (since i was only 20k away) while i was in there. Checked / set timing (halfway between green & blue lines) Cam was perfectly fine.

HG change was only 3 tanks ago, and since my graph covers 245k, 2 tanks on the right-hand end is barely noticable on the graph. But no noticable change in numbers either.

So, still no solution.

I'm selling it to a friend, so perhaps he'll try new nozzles. That's the only thing left that i can possibly think of.