Bad Fuel Mileage Troubleshooting

[h00drat]

Alright, trying to trouble shoot decrease in gas mileage. Yes, I've searched and read a ton of forum posts. Just want to sort out the logic and figure out the best route of resolution.

The back story (sorry if this gets long).

Motor - 1987 2.1 litre wolfy tin-top

Van ran fine all of last summer and got 17mph (ish).

This year I planned a good bit of tuning up before the season. Just wanted to be prepared. So I ordered the tune up kit from GoWesty along with new fuel lines.

Before I even started in on any of this, I noticed my fuel mileage begin to decrease all to the point that I got 7mpg on the tank (all around town driving).

I started with the o2 sensor and hit the road the next day. 200 miles of highway and I got 15mpg's. Better, but not great.

The next weekend I did the tune up kit. Plugs, wires, distributed, air filter, oil change. I realize this shouldn't have affected my mileage, just including it here so that you know the whole story.

New tank of gas and I'm at like 40 miles and around half a tank, maybe a little above. So back to research.

I checked the connections on my temp 2 sensor and they look okay. Can replace if advised.

From here, I still need to do the fuel lines, but again, I doubt this will make a difference.

Wondering where to go from here? Things I have seen based on my research...

- Vacuum
- Fuel injector seals (I have new ones with the fuel line kit, so these will be done)
- Timing

One other noteworthy thing...I appear to be losing coolant, slowly, but I'm not sure where it's going. I'm not seeing puddles under the van, and my oil was not milky, making me thing it's not a head gasket.

Thoughts??
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<<Tyler>>
1987.5 Wolfsburg Weekender / CHC / 2.0 ABA
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[Ahwahnee]

My first thought when I hear about single digit mileage is that the coax shield on the fat green wire to the O2 sensor is shorting to the center conductor. That will cause the ECU to run the engine very rich and your mileage will drop precipitously.

Let me search a bit rather re-enter the tedious details of how to check & fix this (if it is indeed the problem).

Mystery loss of coolant is usually either air in the system self-bleeding out or coolant going out the tail pipe. How does your exhaust smell? Like pancakes?

[h00drat]

Exhaust smells weird. Not sure about pancakes...but I'll check again.

Another anomaly...I was expecting my plugs to be black from running super rich when I changed them. They weren't at all. Granted I ran 2 tanks of gas through it AFTER changing the o2 sensor before changing the plugs, but I still would have expected them to be a bit black. Don't get me wrong, they didn't look great, but definitely not as black as I would expect from a motor getting 7mpg's.

This o2 sensor shorting wire is news to me, but sounds worthy of examination.
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<<Tyler>>
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[Ahwahnee]

[h00drat]

Right on. Thanks for sharing! I will inspect that wire and see what I find.
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<<Tyler>>
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[levi]

Since it does make a noticeable difference, automatic or 4 speed ?

Unsure of the timeline.
Was this big drop in mpg following the replacement of o2 sensor?
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[h00drat]

Automatic trans.

Initial big drop in mileage was before new o2 sensor.

2 tanks of gas AFTER o2 sensor were in the 15mpg + range.

Current tank of gas (3rd after o2) appears to be back sub-10, but I would have to fill it up to confirm. Gauge is showing half tank.
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<<Tyler>>
1987.5 Wolfsburg Weekender / CHC / 2.0 ABA
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[levi]

Do you have a meter to check that o2 sensor?

As mentioned above it is very often the cause of a huge drop in mpg.

There's a detailed process on the first page of this link by tencentlife showing how to check it.
Too bad tencent doesn't post much these days on this forum, his posts were awesome.

http://www.thesamba.com/vw/forum/viewtopic.php?t=330442&highlight=o2+sensor
_________________
One of these days I'm gonna settle down,
but till I do I won't be hangin round.
Going down that long lonesome highway,
gonna see life my way

https://youtu.be/cSrL0BXsO40

[Paulbeard]

Ignore the tank/gauge. Mine shows half when it should be up at 3/4. Consistent but unreliable. If it were me, I would fill up, do 50-100 miles and then fill up, do the math, repeat.
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Formerly -> Steward of a 73 Super Beetle (Beater) and 67 Beetle 1300 (Little Max) both names by POs

[Terry Kay]

Make sure the TPS is working.
Adjust the TPS.
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T.K.

[vegpedlr]

Ditto on the O2 sensor and coax cable. My WBX did the same thing, and when I went to replace the O2 sensor saw the green cable exposed. Once everything was sorted mileage went back to normal.
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[h00drat]

Valuable input. Thanks guys.

Going to check the TPS, then move on to that coax cable. The fact that it appeared to get decent mileage on the highway but not in town makes me think that it may be an idle issue, pointing to the TPS.

I need to get better about using the bentley manual. That thing makes my head spin a bit.
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<<Tyler>>
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[T3 Pilot]

Easier to follow and very logical.....

http://www.mk2g60.yolasite.com/resources/Digifant/...rt%201.pdf
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[h00drat]

Update...

Per Terry Kay's suggestion, I did research on the TPS and started there. It was NOT clicking when coming off of idle. If I understand correctly, this means that at idle the fuel mixture would have been far too rich. I adjust the TPS so that it WAS clicking off of idle. Going to see how this changes things on another tank of gas.

One thing to note, which I should have probably mentioned in my initial post is this...

My van typically upon start up idles around 1100 for a few minutes and then settles in at 1000. But I rev the engine and drop off the gas quickly, either in neutral or revers usually, the rpm's drop to about 800 and engine starts to sputter. I can save it with some throttle, but otherwise the engine will stall. This isn't a huge issue as it only happens in reverse and neutral, and when cold, but I thought it might be worth pointing out.

Thanks for all the input thus far.

I have another 150+ mile freeway trip scheduled this week, so I'll fill up before that trip and see how it does on the highway again.
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<<Tyler>>
1987.5 Wolfsburg Weekender / CHC / 2.0 ABA
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[h00drat]

Update after TPS adjustment.

Based on my last half tank of gas, it's looking like around 12mpg's city/highway. Even with my tire size (27.5") and a roof box, I feel like I should be getting at least 15 combined. So I might keep looking into this.

Idle when warm is fine, but cold idle is a little rough.

might take a look at timing and then that coax cable as suggested at the beginning of this thread.

Glad I started with TPS as it was definitely off. Was not clicking at idle.
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<<Tyler>>
1987.5 Wolfsburg Weekender / CHC / 2.0 ABA
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[bluebus86]

with a working tps, youll save brakes as youll have more engine braking, fuel shut off when coasting off the gas pedel.

rich operation is caused by a failed water temperature sender if it fails in the cold state.
Check the resistance (ohms) of this sender at both a low and high temperature and compair to the graphs on the Bentely Officail VW Manual. Only read ohms after sensor is disconnected from van wire harness.
If it fails the test, replace and enjoy better mileage.

good luck
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[amwaj]

As I understand it, the o2 sensor signal is disregarded by the ECU when the engine is cold, the fuel is enriched, and the aux air valve is open to accommodate the enriched fuel..thus providing a higher idle until a specific temperature is reached...when this temp is reached, the aux air valve has closed, the idle fuel is leaned as the o2 sensor is now "online"..
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[levi]

Since anything below 17 isn't fair, and anything below 14 hideous,
I offer the following.

File what comes below in the category of long-shot ideas;

Is this mileage GPS?
I ask in case you don't realize how far off the ODO can be with these, or if you put larger diameter tires on without factoring that in.

Dragging brakes; lift each wheel off the ground and give it a spin. If it immediately stops that's not too good. Front brake lines can fail internally and keep pressure on the brake piston.

Soft tire pressure creates extra friction and will cost you mpgs.
I hate to even mention that word "tire" as it may draw a tire ocd member with posts of charts and graphs on 4x6 cards, but it's worth mentioning.
I ran into a fellow vanagon driver a couple years ago in a parking lot who mentioned horrible mpg numbers, and when I put my gauge on her tires it was in the 20's ! She said she liked the soft ride.
_________________
One of these days I'm gonna settle down,
but till I do I won't be hangin round.
Going down that long lonesome highway,
gonna see life my way

https://youtu.be/cSrL0BXsO40

[h00drat]

[bluebus86]