Fuel Mileage Problems


Capt. Mike

Moderator
Your difficulty with cold starts indicates a malfunction of the cold start system. It's quite possible this system is not shutting off once its purpose is achieved (after a couple minutes) and thus keeping an overly rich mixture. It may also be operating at max or when it is not called for such as mild weather starts or warm engine restarts.

Unforunately, when you started mixing systems, many of the problems become outside the VW experience curve (at least mine) and one must resort to basic principles.

Browse the FI system forum under the Electical group for additional information of cold start systems. All FI questions go there.

The FI system requires a different fuel pump because the system operates on a much higher pressure and flow. In fact, the system is designed to send way too much fuel to the engine using an overflow return for the excess. Thus replacing the mechanical fuel pump is a given, but does require that the electric be properly speced for the system AND that the return system be intact.

In Europe, I'd consider checking for a Bosch FI facility rather than a VW shop; you might have better luck.

Fuel: Can't guess because you've gone to a modified engine. If OE compression, the US spec is 87 M+R/2 which equates to about 91 RON. I think Europe has stayed with RON. As you saw in the fuel post, too high an octane is difficult to ignite and can cause running problems that would have an effect on mileage, though not likely to that extent.
 

darkroast1

New member
I'm seeking trouble shooting advice for poor fuel mileage with my 84 1.9l westy. I'm averaging 14mpg. The van runs good, but requires a little throttle and cranking when cold. The obvious or typical components associated with mileage are in good condition. It has new tires that are inflated to recommended pressure, fresh plugs, wires, cap, rotor, O2 sensor, injectors and air filter. Are there any common maladies associated with the digijet injection system that result in poor fuel mileage that someone would recommend I start with as I trouble shoot the issue? I have a copy of the bently manual and intend to jump in to the engine compartment with mulitmeter in hand but am hoping for some direction on possible known issues to start with. The mileage quoted above is averaged over at least 6 tanks of fuel, under normal driving conditions, lightly loaded with every attempt made at keeping a light touch on the accelerator.
Thanks for your help -
Rich
 

Capt. Mike

Moderator
You answered your own question: "The van runs good, but requires a little throttle and cranking when cold." This indicates you have a problem with your cold-start sequence, which can translate into poor mileage.
 

darkroast1

New member
Hi Mike
Thanks for the response to my post. I'm still getting used to how the site works, but certainly have learned to appreciate it.
I haven't seen specific reference to a cold start sequence. Would you define it as the following components along with the air mass meter and ECU : Throttle vavle/deceleration/idle switch, Temp SensorI & II & Auxilary air regulator?
I have only checked the auxilary air regulator of the parts listed above, but have a gut feeling about the temp sensors. Do you know what kind of information they supply, is it a simple hot/cold (open/closed)signal or does it actual tell the ECU a range of temp?
Thanks again,
Rich
 

Capt. Mike

Moderator
Cold start 'sequence' is just an adjustment by the ECU, based upon input sensors, to set the FI for that particular state -- temps, etc. As the engine warms up, the readings change and the ECU corrects the output data. It measures temperatures at sensor II (coolant) and Sensor I (intake air). Those are variable inputs and change as temperature changes (they are not on/off). But it also measures air flow with the intake air sensor which also adjusts with atmospheric pressure. Once the engine starts, these values quickly change and the ECU adjusts. Readings from other sensors & switches, such as the O² (variable input), throttle valve/decl/idle switch & full throttle enrichment switch are all factored in. The primary adjustor for cold starts on a Digijet is the aux. air regulator, 24.28. What it basically does is fool the ECU by injecting even more air (most cold-start systems either restrict air or add fuel to make a richer mixture) but this added air causes the ECU to compensate by directing even more fuel, apparently in a progressive manner, to richen the combined mixture for easier starting & warm up.

All FI is so interrelated, that it usually requires full set diagnostics to isolate one problem. Specs are on 24.18; Troubleshooting 24.20 and individual component layout and checks/repairs begin on 24.24a.

Although Digijet and Digifant are different type systems and NOT interchangeable, you may find some checking tips in Digifant that make a procedure a little easier to understand. Do NOT supercede a Digijet spec with a Digifant when there is a conflict -- Digifant just has some better pictures & drawings.
 

darkroast1

New member
Hi Mike
Thanks for the detailed FI summary for cold starts, its helpful to step back and see the sum of the parts. I'm going to get under the hatch the next time it gets above 20 degrees out, all though I'm going to recheck the air regulator this AM since it such a quick check -the westy doesn't fit in the garage!
Thanks
Rich
 

nagster

New member
I have an 81 West Vanagon, aircooled of course. It runs very well, has rebuilt engine 15k ago, has new temp head sensor, air/fuel mixture and timing is all set, new fuel filter, new plugs, clean air filter, .... some of this work was recent. Last year I was getting only 400Ks (250Miles) to a 60 litre tank (Canada), this is a 15.8 US gallon tank) therefore getting 15 miles per US Gallon. Is this is poor or is it to be expected. Does this engine have a 5th injector for the idle, prior to the new sensor it thought the engine was cold and reved high in idle.
What is the expected fuel economy (or lack of) for an air cooled Vanagon?
Thanks in advance.
Nagster
 

Capt. Mike

Moderator
US EPA MPG was 17 and fairly typical. I averaged 19.12 on a '79 (same basic engine) and am pulling 19.62 on a '90 Syncro, but a lot of that is driving technic. Your era FI had a cold-start valve which could be leaking or not shutting off completely, feeding extra fuel after it is warmed up. Or course, just a regular injector that is leaking could also cause low mileage.
 

magowanc

New member
Nagster,
What l/100kms are you getting? I usually drive about 400kms between fills, but am getting 13.5 l/100kms, which works out to 17 US mpg, same as the original gas mileage for an 85 Westy.

Realize that 5 liters of fuel makes a big difference in the calculations. Lets say you filled up at 400kms exactly, and the needle was in the red (top of red is 10 l left). 55 l/400 kms = 13.7 l/100km = 17 mpg
 

Capt. Mike

Moderator
To repeat a note posted elsewhere on the site -- mileage by the tank is an almost useless statistic. The design of the tank and filling practices make huge differences in the amount of any particular fill. Combined with a rather poor accuracy (on Vanagons) fuel gauge, and it's not unusually to see swings from 12-24 mpg on an individual fill.

Since you've found this site, we presume you have can do a simple spreadsheet. Run an extensive records for a dozen or more fills and see if it doesn't average out.

The 4WD Vanagons have an almost horizontal fill pipe that will shut off the fill early if it gets an air bubble or if the vehicles isn't level or preferably down a touch at the left front.

The 2WD Vanagons have a tank with literally two upper halves -- look at a picture of it in the shop manual. It requires perfect venting through a rather complex system to fill consistently. It too benefits from a lower left side during filling. Stations deliberately slop away from the fill islands -- pick the pump & side that gives you that little downhill position wanted.

Even long trips, where I keep my Westy mileage records, my '90 varies from 17.71 to 21.38 mpg; the '79, with a vastly superior tank fill design, still varied from 18.18 through 20.70.

Those numbers are long trip mileages which already average extensive swings in individual tank fills.
 

westfaliarage

New member
Hey all,

Just to throw in some more mileage numbers. I have been keeping track for the last 4 fills with mostly highway driving @ 3200 rpm and have been 23-25 MPG (later with tail wind). Mainly have been gotten 23 MPG with only low to moderate camping gear weight.

Road goes on forever the party never ends!!!!

David

PS no fish stories
 
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fullgro

New member
as anybody tried the tornado fuel saver ki 55
you suppose to save on fuel and gain a little power also just look at the tornadofuelsaver.com
 

Capt. Mike

Moderator
Consider it with all the reservations of the infamous 100 mpg carburators and the magnetic fuel saver already discussed this topic Oct 10, 2000.

Think about it! Your air & your fuel are mixed AFTER the device and computer (on FI cars) controlled so it can't have any control over the fuel-air ratio that reaches the combustion chamber. Even in a carburated car, the carb would restablize any air flow change. If it sounds too good to be true -- it is! Better results from "snakeoil.com".
 
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treejay

New member
Hey all. I have a question about my fuel mileage. Firstly, I'm a novice mechanic and a am trying to learn all the different concepts to what makes my vanagon (1983 water cooled) run. Anyhow, i used to get roughly 18mpg but recently it has been around 12-13. i had a rebuilt engine put into it. afterwards, the mechanic mentioned that the vacuum retard of the distributor needed replacing. could this defective piece drop my gas mileage to it's current level? I suspect problems in the FI system which I'm trying to figure out how to diagnose myself. My best conclusion is that I'll need a volt/ohm meter which I don't have. Any advice is greatly appreciated. Thank you.

ej
 

Capt. Mike

Moderator
I presume you have now checked out all of the above recommendations and they are not the cause(Guideline #3).

Distributor advance and the advance/retard mechanisms have a strong bearing on not just mileage, but also performance and engine life. Distributors are discussed in the FUEL SYSTEM, IGNITION . . . forum.

Trouble shooting a FI requires a volt-ohmmeter and are discussed in the "Fuel Injection Troubleshooting . . ." topic in this forum. A useable volt-ohmmeter can be bought for less than $10. Analog is satisfactory, but digitals readouts are preferred for this purpose. See TOOLS forum, "Electrical diagnostic tools. . ." topic.

A table of troubleshooting is in the Bentley (You have a 'water-cooled Digijet' FI system, pages 24.20-24.21.) as well as other tests in the respective component sections such as sensor testing. It is not difficult but read and understand the entire FI section of the Bentley first as it contains a couple of warnings to avoid shorting out the ECU.
 

jack1985ww

New member
Why a I getting 10mpg?

Help! I just bought a 1985 VW full camper yesterday. I drove it 325 miles back to my house and it took three fill ups of over 12 gallons! I grew up driving a 1971 VW camper so I do know how to drive these things and I was not pushing it at all. It seems to run well, supposedly the engine and transmission were rebuilt 16k miles ago and it just passed the CA Smog Check yesterday. There were a few hills and passes but I would guess that at least 275-250 miles out of the 325 miles were completely flat. So, does anyone have any idea why I am getting less than 10 miles per gallon? (I burned at least 36 gallons to drive 325 miles = 9.027 mpg)

Any suggestions would be greatly appreciated.
 

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