83 air cooled gas mileage decrease

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83AC
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83 air cooled gas mileage decrease

Post by 83AC » Tue Oct 24, 2017 7:08 pm

Okay friends - our 83 air cooled with engine rebuilt about 15,000 miles ago recently had a change in gas mileage from an average of 18.5 mpg to about 16. The change happened after about an 80 degree day in the Washington DC area where we sat in traffic that was craaaawling for a long time with no way out of it. Our oil temp gauge was reading up to 260ish, but I didn’t stress it much because it has tended to read high almost always, sometimes by 40 or so degrees. Anyway, the fill up prior was averaging 18.5 and since then on the same stretches of straight highway driving 55 we now only get 16.5 at best. I’m wondering where I should start looking first. No change in gas type. Was thinking I should do a valve adjustment. Oil is new and not different from what was in before (Castrol 20W50). Thinking temp II sender, but beyond that I’m not positive what to check. Thought maybe timing but that doesn’t make sense. Temps in the engine have seemed to remain stable as well. Not hearing alarming noises from the engine. Love some ideas.
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Re: 83 air cooled gas mileage decrease

Post by Randy in Maine » Wed Oct 25, 2017 7:44 am

If it were me, I would look back at the engine first to make sure that no hoses have fallen off anything. I would probably then check the dwell (if you are running points) and the timing to confirm that the vacuum advance is still indeed working correctly.

Then I would check the tire pressure to confirm they are what they should be.

Nobody gets great mileage if they are stuck in traffic. Might take it for a country ride to see if it performs better.
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Re: 83 air cooled gas mileage decrease

Post by SlowLane » Wed Oct 25, 2017 8:10 am

Even though you are putting in the "same type" of gas, keep in mind that fuel composition varies over the year, with gasoline producers tweaking the chemistry to accommodate changes in temperature, humidity, etc. It could be that your most recent tankfuls are of the winter formulation. I have no idea whether this would fully explain such an increase in consumption, but just trying to point out that there are external influences at work, over which you have no control. Does the engine performance seem any different?

But yeah, check your tire pressures.

Oh, wait. Yours is a California model, isn't it? If so, try checking/replacing the oxygen sensor. Also if so, check for plugged-up cat.
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Amskeptic
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Re: 83 air cooled gas mileage decrease

Post by Amskeptic » Thu Oct 26, 2017 8:32 am

83AC wrote:
Tue Oct 24, 2017 7:08 pm
Okay friends - our 83 air cooled with engine rebuilt about 15,000 miles ago recently had a change in gas mileage from an average of 18.5 mpg to about 16. The change happened after about an 80 degree day in the Washington DC area where we sat in traffic that was craaaawling for a long time with no way out of it.Love some ideas.
Inescapable is the richening that both temp sensors provide when cold. Your gas mileage is likely to be less as the outside temps drop. The intake air sensor reads the day's ambient temperatures at all times. If it is cool outside, then you are getting more fuel.

Yes, check timing if you have points. It does retard over time as the gap closes.
Colin
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Chloe - 1970 bus . . . . . . . . . . . 206,787 miles
Naranja - 1977 Westfalia . . . . . 80,070 miles
Pluck - 1973 Squareback . . . . . 55,510 miles
Alexus - 1991 Lexus LS400 . . . 72,113 miles

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Re: 83 air cooled gas mileage decrease

Post by TrollFromDownBelow » Mon Nov 27, 2017 7:41 pm

Where I live in the Midwest, we get a 'winter blend' about this time of year, which coupled with the lower temps and ergo, more rich running time; I always get lower mpg from Nov to about march/april. Check the things as mentioned, but if everything checks out, don't sweat it until next spring.
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Re: 83 air cooled gas mileage decrease

Post by 83AC » Wed Dec 20, 2017 2:05 pm

Amskeptic wrote:
Thu Oct 26, 2017 8:32 am
Inescapable is the richening that both temp sensors provide when cold. Your gas mileage is likely to be less as the outside temps drop. The intake air sensor reads the day's ambient temperatures at all times. If it is cool outside, then you are getting more fuel.

Yes, check timing if you have points. It does retard over time as the gap closes.
Colin
So in this case we started getting the lower mileage after sitting in traffic on an 80 degree day (not cooler temps) though temps since then have certainly been on the cooler side (70s and below). We don't have points. On our trip we drove through much cooler temps than anything we have driven in recently and ended up with really great MPG - all the way up to 20, but definitely between 18 and 19 consistently for months, thousands of miles driven like that. We are living in VA now, and we do get cold temps at times in the winter, but had not been driving in severe cold. My point is, we know that it got fine MPG prior to the traffic incident, which was on the east coast in late September, beginning of Oct. Would fuel composition have possibly changed around this time? Is it more likely to suspect the temp sensors? I want to dive further in and want to have a logical plan.

I did check tire pressures - all good.
Have not yet checked timing.
Currently do have to resolve an electrical issue (battery was dead after sitting a few weeks. Battery is about 2 years or so old. Did sit several months last winter when snowed in without charging in very cold temps - started up no problem later winter early spring. Dealt with weird starting issues throughout the trip. Recently sat for a few weeks and then was dead when I went to start. Started with a jump. Haven't had chance to drive since then due to leaking from brake fluid reservoir. . . had used gasket maker last time this happened in the winter when high and low temps caused this issue. So now I need to look back into that issue either with new gaskets or with teflon tape on the reservoir nipple and another brake bleed. . . ugh... Again, this leak seems to happen when temps fluctuate between very cold at night and warmer in the day - thoughts on that?)

Okay maybe I just need to spend some time with the van, period - bought a house (project) and life is hectic.

Thanks,
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Re: 83 air cooled gas mileage decrease

Post by Amskeptic » Sat Dec 23, 2017 10:17 am

83AC wrote:
Wed Dec 20, 2017 2:05 pm

we started getting the lower mileage after sitting in traffic on an 80 degree day

We are living in VA now, and we do get cold temps at times in the winter, but had not been driving in severe cold.

My point is, we know that it got fine MPG prior to the traffic incident, which was on the east coast in late September, beginning of Oct. Would fuel composition have possibly changed around this time? Is it more likely to suspect the temp sensors? I want to dive further in and want to have a logical plan.
To be logical here, we need to get rid of spurious suppositions of cause and effect, we need data that gives us an insight into how your car specifically feels about its fuel metering duties. Therefore, forget the "traffic incident" and focus on:

*fuel pressure at idle (28 psi) and with temporary vacuum hose off pressure regulator (36)

*actual 68* ambient resistance readings of both temperature sensors (2500 ohms each)

*fuel pressure bleed down after engine shut-off (if immediate slow discharge of pressure, suspect cold start or any of the four main injectors leaking)

*compression/vacuum test (requires a valve adjustment prior)

Colin
BobD - 1978 Bus . . . . . . . . . . .111,130 miles
Chloe - 1970 bus . . . . . . . . . . . 206,787 miles
Naranja - 1977 Westfalia . . . . . 80,070 miles
Pluck - 1973 Squareback . . . . . 55,510 miles
Alexus - 1991 Lexus LS400 . . . 72,113 miles

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Re: 83 air cooled gas mileage decrease

Post by TrollFromDownBelow » Mon Dec 25, 2017 4:36 am

After you've checked everything Colin has recommended and if you are still seeing poor gas mileage, as mentioned in my post above, it could very will be that your area uses a 'winter blend' gas. The winter blend gas is supposed to help reduce emissions in the colder months when cars are running richer. I have found that my vehicles get anywhere from 10-15% poorer gas mileage when running winter blend gas; I don't care if it is 80 degrees outside or not, I will get poorer gas mileage with winter blend gas. Which has always made me wonder if the environment is still benefiting if I'm burning 10% more gas b/c of the blend? :scratch:

In any case, would be worth doing a quick google search to see if your area switches to a winter blend in the winter.
1976 VW Bus aka tripod
FI ...8 completely solid lifters.... now it's both kinda noisy and leaky, but she sure runs good!
hambone wrote: There are those out there with no other aim but to bunch panties. It's like arguing with a pretzel.
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