fuel efficiency conundrum

[jettaman8691]

Howdy all. My daily driver, a laborious effort for years, is a 1.6 td A2 Jetta. It poked along at about 46 mpg until I replaced the auto-adjust clutch cable with the adjustable cable, and I picked up a couple of mpg. Sweet. I then installed an intercooler and it zipped right up to 52 mpg. Great!! But.........the bubbles in the fuel line got worse. So, after spending hours and hours chasing down one thing and another (the water separator came out 2 years ago), I removed the in-line backflow valve from the line leading to the fuel filter. To my relief, the bubbles are gone, apparently for good. I am assuming that the valve was plugged, or otherwise non-functional, causing the IP to pull air into the line as it struggled for fuel.

The problem now is that my fuel economy has sunk back to 48-49 mpg. Crapola!

I use Stanadine fuel additive, and have used an ounce of parowax to the tank (which did make the engine run quieter); the only thing that has changed is the removal of the backflow valve.

I have backed off the smoke screw about 3/4 of a turn, which dissipated the blue smoke nicely, but that did not change my mpg.

The next thing I was going to try is to retard the pump just a tiny bit. The IP was rebuilt last year, so it should be fine.

Any suggestions would be greatly accepted. Thanks.

[82vdub]

So you removed an in-line check valve and your mileage changed but are still getting up to 49MPG? Doesn't sound like anything you did effected the mileage and there's so many factors that could result in a 49MPG rating instead of 52MPG. That's such a little percentage difference.

[Una]

That's 6%. Not exactly insignificant. But I would say you probably were only seeing the better mileage due to fuel starvation. Now that you're getting proper fueling, your using more of it, less air in it's place. Check your tire pressure and wheel bearings though. They say tire pressure is good for something like 6%.

[82vdub]

Even though it's 6%, what I was meaning is that a slight difference in fuel cetane rating number or other factors can change MPG ratings slightly. Couple that with a potential driving difference or conditions (wind etc), that can eat up that MPG difference pretty quickly.

[jettaman8691]

I appreciate recognizing that 6% is a significant number. I will re-check my tire pressure.

Fuel starvation was my immediate thought, and has led me to wonder how to turn that thought into a good decision for fuel efficiency. If my car was giving me the power I wanted before removing the valve, how much better would it be if I reduce the fuel to the pump in a more regulated and proper manner?

So that is where I am at right now. How does one lean out the fuel to the pump safely? I am reading Hagar's thread again for hints, but it sure takes a lot of gleaning to get to the meat of the many directions taken in the course of the discussion.

I will willingly share my observations over the years with those willing to reciprocate. My involvement on this forum has been mostly as a lurker and taker, since I have not felt my offerings were of sound use to anyone. I am a handyman, not a trained diesel mechanic, but perhaps now, having spent years tinkering with this vehicle and this motor, some of my experiences would be helpful to those struggling in this insane economy we now must negotiate best as we can.

[TylerDurden]

Tuning for 50mpg and above is as much art as science.

The smoke-screw can be turned out to the point where no overfueling (smoke) occurs, but you will miss the pull. Hagar opts to run the RPMs higher to compensate for the power reduction.

You can also achieve the same by self-discipline on the right foot.

Every little detail after that can make incremental improvements: timing, valve-grind, rings, injector guts, etc.

IMO, 45-50mpg is good enough to live with (minor tweeking aside).

[jettaman8691]

I have to travel an 800 mile round trip twice each month, so 48 mpg isn't good enough when I know I can, and have, gotten 52 mpg. On one half tank I even averaged 55 mpg! So I know it can be done, and must be done as fuel will go to $5/gallon. It has in the past, and will again. Keeping these motors properly tuned and finding ways to do better is financially a must. But thanks for the input.

[TylerDurden]

I'm all for better smilage, but there is that pesky law-of-diminishing-returns.

Is a valve-job worth 2% increase?
Is a ring job worth a 4% increase?
etc...

Depends on what resources you can afford: Time, Money, Effort...

The biggest consumer of all is speed, so the biggest bang-for-the-buck is driving slower. Not so fun on a 400mi trek, but it gets me into the 50mpg zone.

[jettaman8691]

Thanks.
I have found that my best performance comes at around 2750 rpm and under 70 mph. I am looking for certain gears to custom build an 020 tranny that will allow 72 mph at the 2800 rpm range, but it is not easy. They come from older trannys that are getting crushed more and more these days.
The motor was rebuilt 4 years ago, so it is in fine shape. I change the oil regularly and keep a sharp eye on my oil and water use, as well.

[TylerDurden]

That's 6%. Not exactly insignificant. But I would say you probably were only seeing the better mileage due to fuel starvation. Now that you're getting proper fueling, your using more of it, less air in it's place. Check your tire pressure and wheel bearings though. They say tire pressure is good for something like 6%.

Fuel starvation might not be the best description.

Resistance in the supply line might reduce the IP internal pressure, slightly shifting the dynamic advance.

Definitely not any "leaner"... AIUI, IP pressure has no quantitative bearing on pump delivery to injectors, until the reduced pressure in the pump causes cavitation and missing.

[BlueDog]

I'd like to see 50 mpg. What speed do I need to drive to get this kind of smilage? I regularly get 44.4 to 44.6 mpg. One time I got 44.8 mpg. I drive 70 to 73 mph on fairly flat roads. 64 miles per day round trip x 5 is 320 miles per week. Another 80 miles per week is in town stop and go.

[82vdub]

You can do your own tests. Slow down to 55MPH and see what you get. Then run 60 and see what you get.

[Una]

As I understand it, you get less mileage on biodiesel. You listed B100 in your sig. 44mpg on bio is probably 48-49 on diesel.

[surfcam]

I grabbed a low finial drive tranny ASF and did a 5th gear swap from a .75 to a .71. This brought my mileage up to 51 from 45. RPM's went from 2800 to 2250 at 60 mph. I think I lost about 20% of my power and have to shift a little more but it is worse it to me.

[Ira B]

We don't live too far apart and the weather can be a major factor. Temp, humidity, wet/dry roadways can all have well over a 6% impact on fuel consumption. We have had a lot of dynamic conditions this spring to say the least.