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Old 03-04-2009, 05:55 AM   #1
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Diesel ecomony?

Ok, so i have done alot to an 01' ford focus ZX3 and an 87 toyota MR2 but i purchased an 06 jetta TDI a few years ago and am now realizing that diesel is up and coming in the U.S. but still has some drawbacks with fuel economy.
So, i have the TDI, 5 speed manual, all stock. The air intake, calipers, tires, hell everything is pretty good. It's the 06 NOT the 06.5 they made to fill demand which cost several thousand more.
I had some trouble with this car in the past, when i first got it, and had the engine replaced, the clutch and transmission three times because of a HUGE issue with the transmission siezing while driving. But now, it's all fixed.
The past year, since getting the new engine, i have been averaging about 40mpg. about 70% is highway and being in NJ that doesn't account for much real highway driving. Still 40mpg isn't bad but i wanted to explore more with the diesel and see if i can push that a little more.
Any advice or has anyone tried to 'modivate' the tdi's for economy and reduce its diesel consumption? Since the 'new' diesel cars are relatively unknown in america i'm def expecting some mixed results, but let's give it a try and see what happens!
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Old 03-04-2009, 06:02 AM   #2
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I would first do the things that apply to all vehicles like:

-increase tire air pressure to max side wall
-look into aero mods
-look into grill block (for aero and faster warm up)
-get a scangauge 2 (the best little gauge for MPG)

also, maybe look into a block heater. this may give you varying results but it will cut back on warm up times.

if your car already has an MPG meter on it, you may not want to spend the $160 or so on the scangauge but it has made a world of difference in my car.

most of this isn't diesel specific but it is a good place to start. I say start with the basic and easy stuff and work towards the more complicated things. I would first get a scangauge and alter your driving style. several people attribute driving style to a 30% increase. I have always driven slow so mine wasn't that significant.

good luck
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Old 03-04-2009, 06:31 AM   #3
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Be aware than the Scangauge may or may not be accurate on a TDI. There are accuracy issues with some diesels, while others are fine.

While driving, aim to keep the rpm low,low,low. That's where a diesel thrives.
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Old 03-04-2009, 06:57 AM   #4
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Yeah, a scanguage is out of the question for me because it doesn't work with my vehicle :-( The jeep liberty diesel and the 05-06 tdi's don't work (or early mkV tdi), but the 06.5 up and the MK IV TDI's it works fine. I tried it with the one from my 01 Focus, and sadly, it just read 9999 all the time for everything. That was a sad day.
Grill block i have, but it concerns me since the air intake is at the front of the grill and i have the skidplate on. I replaced the front end with a newer black front end from the high speed hatchback. Any reccomendations for that intake?
I thought about the block heater but was concerned about the additional weight. The new TDI's come with an internal block heater coil for the fuel to allow for cold weather startup by keeping the car in AC for a minute before starting. So i was a little concerned about additional weight for a blockheater, but i'm no expert on that.
As for the RPM, i have a question about that. I try to keep my TDI about 2.5-3k RPM. Granted the TDI's have 1k less RPM than their gasoline counterparts. I found anywhere between 2-4k rpm to be it's 'sweetspot' for picking up. When staying below 2 or 3k rpm will i risk making the engine work much harder in order to slowly speed up rather than boosting my rpm's slightly?
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Old 03-04-2009, 07:11 AM   #5
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oooh, would a bump start on a dielsel be a good idea? Especially with suchj high torque i'd be afraid of cracking my clutch housing, or twisting some motor mounts.
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Old 03-04-2009, 07:19 AM   #6
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I've done hundreds of bump-starts in a diesel with no trouble.
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Old 03-04-2009, 07:19 AM   #7
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I don't have any diesel experience, but I'm very interested in finding out what works for you. A 2006 Jetta TDI would be a definite posibility for my future. I currently have a 2008 Rabbit with the 2.5L I5 gasser and 5 speed manual; I really like a lot about it, but a Jetta TDI would be an improvement. The Jetta is like a Rabbit but less embarassing, and the TDI...well, it's a diesel!

Can you tell me more about the transmission seizing problem?

Also, can you tell me about your gear ratios? How many RPM at 70mph in 5th? I wonder if VW gives you the same low geared close ratio tranny that I've got. I bet with decent ratios I'd get another 10mpg.

I imagine the diesel would do better at low RPM than the gasoline engine. Mine is very happy to run WOT almost from idle when I shift at 1200-1500 rpm and enter the next gear at 900-1200. I normally don't get above 1500 rpm until I run out of gears (no 6th to shift into!). When I first got the car, I was shifting more like you, but every time I tried shifting lower, my FE improved significantly. My results run counter to what many people think, which is that the engine needs to run lightly loaded at higher RPM or it will waste fuel by struggling.

"Making the engine work harder" is not necessarily a problem, in and of itself. Gas engines generally produce work most efficiently when running at full load -- putting out all the power it can at its current RPM (google for 'BSFC' to learn more). Part of it is pumping losses at the throttle, which is a component that your diesel does not have (right?). I don't know if the same strategy applies.
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Old 03-04-2009, 07:26 AM   #8
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I was not aware of the issue with diesels and the scangauge. there is also the kiwi. I know verry little about it other than it is expensive. it has many more features than the scanguage. it may be more compatible with the diesels than the scangauge. it may be something to look into.

one thing about your grill block. I purposefully take in warmer air from under the hood (actually from the exhaust manifold) to get my intake temperatures higher. not sure if this will benefit a diesel but it may be worth a try.
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Old 03-04-2009, 08:31 AM   #9
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Thanks Palemelnesian, i will practice bump starting soon, hopefully that'll help my stop light coasting mpg.

Oooo, i like the rabbits. I wanted an 06 TDI Golf but i just couldn't find any. Good for me, though, they have the MKIV motor.
I love my TDI, i was getting an average of 33mpg on the first engine and i think it was an issue with the engine. I had the car a month and the clutch stuck to the floor in rushhour in the fast lane on the Garden State Parkway, those familiar with the road know how bad that it. The dealership said the tranny siezed and killed my clutch, both were replaced after fighting with them and telling them it wasn't how i drive. Second time the clutch started whinning, like a jet aircraft taking off. Took it in and the tranny again... it also destroyed my clutch and bother were replaced. I was driving in February of 06 in Vermont in the middle of a blizzard and a car ran me off the road. I rolled the car on its side but eveything checked out a-ok. Purged the brakes and was back on the road with just a little body damage. 8 months later i was driving down the road and was speeding up from a stop light my tranny siezed again, this time because my foot was on the gas when the tranny siezed the engine kept trying to turn the gears and ended up ripping 3 out of 4 motor mounts thanks to the torque. Dealer said it was because of the accident of the autobody that did the repairs (yeah right). Whatever, i got it fixed and a new engine because the ripped motor mount cracked the block. After that my MPG was great! I delieve part of the problem was that the car i bought was one of the last 06's made before the 06.5, it was sitting on the lot for a long time, and had 2k miles on it so obviously someone drive it before me and seeign as i used to work at a dealer i know how abused new cars can be.
As for my gear rations...
1st gear i redline at about 25 mph, i shift around 3k rpm about 12-15 mph depending on how cold the engine is.
2nd red lines at about 45 to 50 mph, i shift at 40mph or 3.5k rpm when i feel like getting frisky, or 30 to 35 mph at 2.5k rpm.
3rd is tricky, i usually keep that gear until 60mph at around 3k rpm, or i'll shift at 50mph at 2k rpm.
4th i stick in 90% of my highway driving. At 60mph i'm running just under 2.5k rpm. at 80mph i'm runing at 3.5k rpm.
5th gear is my baby. I understand why people in automatic 5spd card don't realize the have a 5th gear. At 70mph in 5th i'm running about 2.3k rpm, maybe a little higher. There's not a huge difference between 4th and 5th but you feel it in the handling. The car seems more relaxed in 5th but in the snow it's not worth it because handling suffers.
With the tdi you MUST do over 1500 rpm alot of times or else you will not speed up at all, and there is no resistance in the clutch so shifting at ultra low rpms can cause a stall. Trust me, it's not just me on this one, i know i'm not the BEST driver out there but i'm pretty good, and people much better than me stall it out too in low rpm shifting.
You're right, the same strategy does not apply with a new generation TDI. Def with an old generation tdi which is an gas engine converted to run on diesel fuel. But the new ones are diesel specific, designed from the ground up to be a mini powerplant. As for the whole BSFC calculations.... lets just say i suck at math alot.
I'll get a more accurate RPM rate tonight on my way home, highway and city.
The biggest thing i have done to increase the fuel economy is teh grill block. I filled in the grill holes with 1/2" foam pipe insulation and stuffed it in the grill holes, i also put a piece of heat resistant plastic BEHIND the grill covered in aluminum tape to refect the heat back at the engine (same stuff used for repairing muffler holes but much cheaper). I want to cover it the plastic with a reflictive sun visor cut to fit. That made it great this time of year, but i'll have to do something else once summer rolls around because it's going to be a hot one.
So far, the economy for TDI that i have noticed has been driving slow. At 80mph i get 38-42 mph on highway, driving at 65 i get 45. not a huge difference but it's something, the problem with driving 65 around here is tehre are so many aggressive drivers or the police will stop you for going too slow (happened to me once, they though i was drunk because i was going the speed limit).
I'm also going to try bump starting the car and see if that helps too with traveling downhill or to stoplights.
My motto with the TDI is, if you don't use the turbo you're doing good. but i'm no expert. I'll try your RPM stuff this week, too, i'm willing to give anything a shot, lol.
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Old 03-04-2009, 09:05 AM   #10
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sorry, i just realized i should have posted this is the diesel section. Sorry, about that, guys and gals!
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