Poor MPG

DrWeird

Member
Joined
Dec 3, 2010
Location
KY
TDI
2000 New Beetle TDI
2000 New Beetle TDI 5-Speed. Engine has 197,000 mi on it. Has Greasecar kit on it (not currently used). Story goes from the seller that a husband and wife owned the car, husband purchased and drove it from new to 126,000mi running veggie oil primarily. Then she drove it from 126,000 to 187,000, never running oil. The dealership that sold it to me specializes in Greasecar conversions and such, and they ran a couple of tanks of oil through it, and I've tested it with the oil that was in the tank when I bought it, but haven't gotten my filtration set up to run any more through it. I suspect the lady granny-drove it, which may be related to my problem.

Problem: the car gets about 30-32mpg mixed city/highway over trips of about 10 miles (back and forth to work). On the highway (drove 1000mi to NY recently), got consistently 36-40mpg, maintaining 70mph, only stopping for fuel. Needless to say, this is a bit low from what I expect from a TDI. The car has gotten better than this mpg in the past, maybe 40 city, 50highway, when first purchased, but has declined over the past 10,000mi to the point its at now. Replaced fuel filter, no difference. Regularly run diesel injector cleaner, doesn't seem to make any difference. Doesn't exhibit any symptoms of running poorly, just doesn't get very good gas mileage.

Sort of at a loss here, wondering where I should start in investigating this. Any VAG-com checks I should run, or things to do to try to determine the cause?
 

NickBeek

Veteran Member
Joined
Jan 9, 2011
Location
Upstate, SC
TDI
2013 Passat TDI 2006 Dodge Ram
Not sure on the VW TDI, but Cummins engines that were not worked properly will get build up on the injectors causing poor atomization. This leads to poor economy. From my limited research on veggie, it can amplify the buildup issue.
 

dieselfuel

Veteran Member
Joined
Dec 28, 2008
Location
ohio
TDI
2003 Jetta TDI
Drweird,

First and easiest thing to do is to check your coolant to see if your thermostat is working properly.

Check my sig for a link.

Post here and let me know what temp your coolant is.
 

DrWeird

Member
Joined
Dec 3, 2010
Location
KY
TDI
2000 New Beetle TDI
In terms of the injector buildup, is there a way to check flow via VAG-COM, or would that just be a "remove and clean professionally" sort of thing? I have been running injector cleaner regularly on it, but I suppose WVO buildup might be a bit more stubborn than normal injector crud.

As for the intake, that's what I'm thinking might be contributing. I'll take a peek and see. I think maybe the lady grandma-drove it and might have contributed to the buildup. Also, at 200k miles I don't think its ever been cleaned, so it might be bad buildup anyway.

I'll check the thermo too, since that definitely could be the issue. I didn't really think about it, but being only experienced in gas-car mechanics, I didn't think it would make that big of a difference. I figure since the heat is more critical to the diesels' operation (and their performance is thusly more temperature sensitive), this could make a big difference. I'll check it out.

Thanks for the feedback, I'll let you guys know how it goes, and pictures if the intake is especially nasty :]
 

AndyBees

Top Post Dawg
Joined
May 27, 2003
Location
Southeast Kentucky
TDI
Silver 2003 Jetta TDI, Silver 2000 Jetta TDI (sold), '84 Vanagon with '02 ALH engine
As already suggeted, Intake/EGR should be inspected for crud build-up!
Check Intercooler to see if it is "loaded" with oil!
Check the timing with VAG COM!
Check the Turbo Actuator for full movement! The Veggie junk and lack of exercising the Vanes in the Turbo unit may have resulted in poor operation!
Does it appear to be running in limp mode? That can happen without the CEL popping on.....any number of reasons too!

I seriously doubt that the injectors have "build-up" resulting in poor performance. The nozzles have 5-very tiny holes that the high pressure injection process will keep clean and open ....................However, running the Veggie Crap may have eroded them leading to poor atomization of fuel, thus an unacceptable spray pattern resulting in poor fuel economy!

Trivia: I love to tell this story - The Original FUEL FILTER on my brother's 01 Jetta TDI was not changed until January of 2009.......... just under 171,000 miles on it! He was not experiencing any performance issues! As with Ripley's, Believe it or not!

So, .......... I doubt seriously that changing the fuel filter will ever increase fuel economy!
 
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DrWeird

Member
Joined
Dec 3, 2010
Location
KY
TDI
2000 New Beetle TDI
Yeah, my impression on the fuel filter maybe making a difference was that it might be leaning out the mix because of a lack of flow through it. I also had no idea when it was changed by the last owner.

In terms of determining if its in limp mode, how would this be done? I know its making boost, since I can hear it on accell, and it's never ever thrown a CEL. It appears to be pretty peppy in terms of performance (no drop since I've had it anyway). Is there a secret or definite way to determine if its in limp mode?
 
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AndyBees

Top Post Dawg
Joined
May 27, 2003
Location
Southeast Kentucky
TDI
Silver 2003 Jetta TDI, Silver 2000 Jetta TDI (sold), '84 Vanagon with '02 ALH engine
Limp mode due to under boost from the Turbo generally occurs between 2800 to 3100 rpms ....... the vehicle will tend to just fall on its face. Of course, when it occurs depends on how hard you're pushing it. Limp mode from over boost may (never experienced) be immediately to avoid damage to the engine.

If the Mass Air Flow (MAF) sensor is acting up, Limp Mode will occur around 2700 RPM. But, typically the MAF going kaput doesn't significantly reduce fuel economy assuming everything else is performing properly!
 

DrWeird

Member
Joined
Dec 3, 2010
Location
KY
TDI
2000 New Beetle TDI
Using VAG-COM, I tracked the coolant temp today. Looks like the thermostat is the issue. With the ambient temp being about 5C today, the coolant temp raised steadily up to about 60C. At this point, over the course of city and highway driving with redlights, etc over about 20mi in 30mi, the temperature ranged from 62.2C to 66.6C (LOW!). As an experiment, while I went in to the auto parts place for a few minutes I left it idling in the care of a friend. The temp crawled up to 70C while idling, but dropped back down immediately once we got going again.

Looks like the thermostat is stuck open IMO. I'll replace it soon, and that should make a big difference.

On another note, what temp thermostat do you guys recommend for a normal VW with 200k mi, the outside chance of more WVO in the future, and primarily city driving? Is 180F the normal choice for thermostat? When I look for thermostats there is everything from 160F to 205F around...wondering what to get.

It seems 87C is normal for the TDI? I found this after some time.
 
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dieselfuel

Veteran Member
Joined
Dec 28, 2008
Location
ohio
TDI
2003 Jetta TDI
Buy a Genuine VW thermostat. About $60 U.S. I know, it's a lot of cash for a thermostat.

But, I went the Genuine route because some on this forum have stated that they bought aftermarket t-stats and they were no better than the old faulty t-stat they replaced.

I bought Genuine VW because I didn't want the hassle of possibly doing the job over again.
 

UFO

Veteran Member
Joined
Jul 31, 2007
Location
A mile high
TDI
2001 Beetle
Buy a Genuine VW thermostat. About $60 U.S. I know, it's a lot of cash for a thermostat.

But, I went the Genuine route because some on this forum have stated that they bought aftermarket t-stats and they were no better than the old faulty t-stat they replaced.

I bought Genuine VW because I didn't want the hassle of possibly doing the job over again.
That's what I did, and I don't regret it. Not yet anyway, the first one only lasted 70-80k miles.
 

DrWeird

Member
Joined
Dec 3, 2010
Location
KY
TDI
2000 New Beetle TDI
Thanks for the info guys, I will replace it, and let you know once I get a new tank's MPG values.
 

lmtman

Well-known member
Joined
Oct 10, 2010
Location
Hydro, OK
TDI
02 Jetta TDI 5sp
Not tellin you what to do, but i spent 8 bucks on a t-stat at the autoparts store and it works great.
 

Rusty Tub

Active member
Joined
Nov 19, 2010
Location
Oregon
TDI
2001 Jetta TDI 275K
I will be trying a wahler brand T stat, I use them in all the early water cooled vws. So I will try it in my tdi and if it works for me over the long haul then and only then will I use it routinely. I know for a fact that the meyle brand sucks and I will not use their t stats.
 

dieselfuel

Veteran Member
Joined
Dec 28, 2008
Location
ohio
TDI
2003 Jetta TDI
Imtman,

How many miles do you have on that $8 t-stat? How long has it been installed?
 

AndyBees

Top Post Dawg
Joined
May 27, 2003
Location
Southeast Kentucky
TDI
Silver 2003 Jetta TDI, Silver 2000 Jetta TDI (sold), '84 Vanagon with '02 ALH engine
In January of '10, when I did the TB job on my car, I replaced the T-stat with a $14.00 one.........cannot remember the brand name (it come from one of the infamous vendors we all shop at here in the club)! Seems to work quite well. The OEM T-stat had been failing since late summer of '09. Into that fall, it would never get the temp gauge straight up! The ScanGauge was showing mostly in the high 160s at best.

Now, a year later, the temp hangs around 188 to 194 depending on the circumstances!.........let's see! $60 / by $14.00 = hmmm!
 
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Rusty Tub

Active member
Joined
Nov 19, 2010
Location
Oregon
TDI
2001 Jetta TDI 275K
Well I just installed a wahler t stat, right out of the box I have to admit the quality was right n par with what I have come to expect from this brand it is marked made in germany. Like I said before I use this t stat in ll other vws ith no issues. I will continue to update with how this t stat works.
 

TDS

Well-known member
Joined
Aug 2, 2010
Location
N.H.
TDI
2003 Jetta GLS & 2003 Dodge 2500 4x4 Cummins
Just read this thread as my mileage per gallon had started decreasing on my Jetta w/ 171,000 miles. And lo and behold, my coolant temp is only ranging btwn 148-153 F after driving 30 minutes at 65 mph. I'll change the T-stat and see if that's the issue.
Thanks DieselFuel for the quick link!
Tom
 

DrWeird

Member
Joined
Dec 3, 2010
Location
KY
TDI
2000 New Beetle TDI
Replaced the thermostat, runs now at ~90C with ambient temp of ~25C yesterday. Interestingly the EGR duty cycle was something around 85% before, now its muuch lower, maybe 5% or so when warmed up idling, not sure what this means, but probably means more mpg. The thermostat that was in there I think was the original one to the car. It was a VW one, and was 87C (stock). I guess the springs are just weak in it, since it was closed when I got it out. Anyway, gonna fill up today, and track the mpg.
 

dieselfuel

Veteran Member
Joined
Dec 28, 2008
Location
ohio
TDI
2003 Jetta TDI
If you're getting 90C in the tank, that's great! That is 194F.

What brand thermostat did you use?
 

TDS

Well-known member
Joined
Aug 2, 2010
Location
N.H.
TDI
2003 Jetta GLS & 2003 Dodge 2500 4x4 Cummins
Update--Installed a Wahler T-stat, my coolant temp went from the 148-153 F range to the low 180s and I picked up two MPG (went from avg of 40 to 42) based on four fill ups. Although this may seem low to some I'm usually driving at 70-75 on the heavily populated :rolleyes: northern sections of I-91 and I-93.
Tom
 

dieselfuel

Veteran Member
Joined
Dec 28, 2008
Location
ohio
TDI
2003 Jetta TDI
It's low for a manual trans, but you may need new injector nozzles and / or an intake cleaning. Or, possibly your timing adjusted.

I believe I need new nozzles, though I'm going to try a can of Diesel Purge, first. I've never cleaned my injectors. If I get no improvement, I'll for sure get new (bigger!) nozzles this summer.

Even with my original nozzles, I'm able to get anywhere from 46-52 mpg on the Interstate driving 75 mph. (City/Hwy, I only get 42 mpg)
 

smck

Veteran Member
Joined
Mar 27, 2008
Location
south east indiana
TDI
2000 golf man, 2003 golf man
Seems like more than just a bad t-stat to me. Replaced mine last year which was keeping me in the 150-160F range and did pick up a few mpg but I was still able to maintain 60+ with the bad one. You may want to check your timing, MAF and injectors. Replacing bad MAF in mine made a huge difference (no error code will be set for this). Doesn't hurt to clean your intake either but don't expect much difference in mileage unless it's almost completely closed off.
 

UFO

Veteran Member
Joined
Jul 31, 2007
Location
A mile high
TDI
2001 Beetle
In a Beetle at 70mph, I would not expect any better than low 40s. It sounds like a lot of little things and you should be back up there. How about your tires and pressure? Brakes in good shape?
 
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