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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 March 5th, 2011, 07:50   #1
DrWeird
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Unhappy Poor MPG

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?
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Old March 5th, 2011, 08:28   #2
truman
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The intake may be clogged. Remove egr valve a take a peek.
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Old March 5th, 2011, 09:25   #3
NickBeek
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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.
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Old March 5th, 2011, 20:06   #4
dieselfuel
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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.
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Old March 5th, 2011, 21:15   #5
DrWeird
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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 :]
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Old March 6th, 2011, 05:21   #6
AndyBees
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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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Last edited by AndyBees; March 6th, 2011 at 05:23. Reason: Add info
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Old March 6th, 2011, 07:46   #7
DrWeird
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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?

Last edited by DrWeird; March 6th, 2011 at 07:56.
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Old March 6th, 2011, 10:19   #8
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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!
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461 fill-ups, in Excel spreadsheet, 348k miles...
'84 Vanagon ALH TDI engine (33 mpg) Malone Stage 1 tune, Polaris Diesel ATV, Allegro Bay RV 5.9 cummins, VWs 41 years, VW diesels 34 years, ALH TDIs 12 years
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Old March 6th, 2011, 16:03   #9
DrWeird
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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.

Last edited by DrWeird; March 7th, 2011 at 09:49.
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Old March 7th, 2011, 13:14   #10
dieselfuel
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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.
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Old March 7th, 2011, 14:41   #11
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Quote:
Originally Posted by dieselfuel View Post
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.
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Old March 7th, 2011, 15:32   #12
DrWeird
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Thanks for the info guys, I will replace it, and let you know once I get a new tank's MPG values.
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Old March 7th, 2011, 17:52   #13
lmtman
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Not tellin you what to do, but i spent 8 bucks on a t-stat at the autoparts store and it works great.
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Old March 7th, 2011, 20:09   #14
Rusty Tub
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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.
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Old March 8th, 2011, 15:40   #15
dieselfuel
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Imtman,

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