10 mpg 3.2 - can’t find where the fuel is going

[Discseven]

Originally Posted by Showdown

Karl,

I was out driving today and was keeping an eye on my digital gauge and the MPGs thinking of you.

A few things I noticed:

1.) Any movement in 1st or 2nd gear at any speed and the MPG was low, lower than my total avg. Once I hit 3rd, the MPGs shot way above my avg. Keeping the car in 3rd I could consistently see MPG in the mid 20s around town. In 2nd it was 15ish and 1st was like 7.
Getting the car up to speed consumes a ton of fuel, even for a very light car like mine...

2.) Even with roughly 30 mins of highway driving and MPG in the high 20s, it had minimal effect on the overall average for the 90 minute drive. That is to say that city driving MPG is so low that hwy driving needs to be more time to offset the city...

I also was keeping an eye on my fuel gauge and have determined that like the oil gauge, it's at best, a suggestion.

Julian... Thank you for driving around with this fuel puzzle in your head. Where Dave is coming from is understood in that this was not an apples-to-apples comparison. Still, am open to all input and if possible, converting ideas to some notion that I can apply to what I have in hand. From what you say Julian, perhaps I should try starting out in 2nd rather than 1st. Whether there's more initial load for the air-fuel to deal with but less on the tail end... has to be seen. Will test this beginning at the next fill. Obviously this would not be a solution to the puzzle. Rather, an experiment.

Originally Posted by ant7

Bump
There is a valid reason for asking this!
This may help those who are interested in why I asked the question.
https://www.bellperformance.com/blog/bid/117629/octane-and-mileage-does-low-octane-hurt-your-gas-mileage
Ant.

A snip for the link Ant provided:



Octane aside, and not concerned with when this text says "explosion" occurs (I believe ignition begins BTC with peak expansion occurring shortly after TDC), any "less mpg" comment gets some attention. In this case, snip raises a timing question. Problem with timing being off here is the engine's performance... Not a moment of hesitation through all rpm, and pull is strong to redline through all gears. Given engine performance, O2 readings, and spark plug color, can we rule timing out completely, or is it a consideration in the least?

Ant... no worries about hijacking Is interesting stuff. RON and MON... sidetracking into the educational zone for sure. But all good.



Status on progress...

Looking at wheel wells and brakes for friction is delayed. Weather's not cooperating. Rain. And can't do what needs doing in condo's garage. Association rules don't support raising cars onto stands---anywhere on the property. Rushing it in the condo, possible but not going there. I get away with breaking the rules to a degree... want to keep that program on the menu.

All the input provided from each of you is highly valued. Know that this "project" is not done until the answer---the solution---is found.
.

[KMoore68]

Hey,

I wanted to add my $.02, how are you measuring the distance travelled? Are you using google maps or the odometer? On my 1977, the odometer is slightly delayed, meaning when I start the car, the speedometer is working however it takes several miles for the odometer to start working.

I wanted to make sure you weren't seeing the same type of behaviour on your car.

[Mr. Merk]

Originally Posted by KMoore68

how are you measuring the distance travelled? Are you using google maps or the odometer?

He did a pretty thorough test last week

Originally Posted by Discseven

Odometer test

Was done with Google maps. Stride app was not run.
Route followed had 2 legs.

Combined route total per Google = 12.3 miles.
Trip odo reading = 12.4 miles.

Main odo reads same as trip.
Reference below.

Google Maps, routes 1 & 2.





Start and end mileage for Google route followed.



Current city mpg = 11.55

[KMoore68]

Originally Posted by Mr. Merk

He did a pretty thorough test last week

Thanks, I missed that post!

[REVerend]

Originally Posted by Discseven

…Looking at wheel wells and brakes for friction is delayed. Weather's not cooperating. Rain. And can't do what needs doing in condo's garage. Association rules don't support raising cars onto stands---anywhere on the property. Rushing it in the condo, possible but not going there. I get away with breaking the rules to a degree... want to keep that program on the menu.

Have you tried pushing the car with your hand while it’s on a flat surface? It’s pretty easy and you would notice resistance. I push my car around a bit just maneuvering it in the garage and a single click of the emergency brake will make it noticeably difficult to push, though the engine would easily overcome if you tried to drive it like that. A simple push test would tell you if there was some gross friction going on.

[scarceller]

Calculating city MPG is not that easy, driving habits, length of trips and stop and go frequency is different for every driver. But even 12MPG is not so good.

Highway driving is far more predictable to compare, typically the 3.2 can see 24-28MPG depending on how well it's running.

Has anyone else ever tracked city MPG? I'm just curious.

But the OP is stating that prior to rebuild he got far better MPG in the city, as I recall around 16MPG but after rebuild it's currently at 12MPG.

[REVerend]

Originally Posted by scarceller

…Has anyone else ever tracked city MPG? I'm just curious…..

I check it every time I fill up, my car is a 3.0 so not exactly the same but close enough. It’s pretty consistently in the 17 mpg range every time I fill up. On road trips where I’m primarily on the interstate it seems to be 26 mpg. For a while I was writing my miles on receipt and shoving them in the glove box every time I got gas but I quit that after a year or so.

[proporsche]

the funny thing is in my ownership of my 911 for 29 years , i have never ever really worried about the fuel consumption_ if someone would ask i`d say,i do not know nor care.I just go fill it up(for long trips only) half a tank a go for drive...

Ivan

[917_Langheck]

Lots of WAGs being floated here, but most have already been addressed or are not relevant - octane doesn't affect mileage to the degree believed, but added undisclosed percentages of ethanol does.

Someone please send Discseven a known good, working DME to swap with his existing so the ECU/fuel driver circuit can be ruled out as a cause of the unusually large disparity between city and highway mileage.

[rwest]

It is interesting to see just city driving. I’m not sure if I have ever done a tank with just driving around a town stop and go without some freeway driving.

I get to third gear very quickly in town and even in a 40 mph zone will probably select fourth gear.

When my car was stock CIS and doing normal around town and using freeways too, I would see about 19 mpg and on long trips could get 27 mpg.

Maybe try driving around in a normal fashion using freeways and running errands in town on one tank and see what your combined mpg is?

[REVerend]

Originally Posted by proporsche

the funny thing is in my ownership of my 911 for 29 years , i have never ever really worried about the fuel consumption_ if someone would ask i`d say,i do not know nor care.I just go fill it up(for long trips only) half a tank a go for drive...

Ivan

I think there is a difference in being worried about miles per gallon and wanting to know if something is running correctly. In one of my other cars a noticeable change in fuel consumption was one of the most readily identifiable indicators that something had gone awry in a carburetor. On long road trips it’s also useful… do I press on another 100 miles to a station where I know I can get 93 octane without ethanol or do I need to stop sooner.

If my car suddenly had an increase in fuel consumption I would sure want to recognize it and resolve whatever issue might be causing it before it led to additional problems.

[proporsche]

Originally Posted by REVerend

I think there is a difference in being worried about miles per gallon and wanting to know if something is running correctly. In one of my other cars a noticeable change in fuel consumption was one of the most readily identifiable indicators that something had gone awry in a carburetor. On long road trips it’s also useful… do I press on another 100 miles to a station where I know I can get 93 octane without ethanol or do I need to stop sooner.

If my car suddenly had an increase in fuel consumption I would sure want to recognize it and resolve whatever issue might be causing it before it led to additional problems.

good point..but-- there is a lot of modifications between the normal mileage and now problematic usage,right?
In my case, same engine different continent;-)..consumption still the same..
Ivan
and yes the gas has to go somewhere...in Karl case

[Showdown]

The gas isn't going anywhere except into the combustion chamber where it's burned effectively resulting in desirable and predictable lambda/AFR.

The premise of the question was that the gas was disappearing somehow- either it was going out the muffler as unburnt fuel or leaking form a hose, etc... all of those have been ruled out.

Drag and resistance has been largely ruled out (and I suspect Karl will pursue this further and find negligible resistance if any).

I asked about timing but had no clarification- Does the stock timing for a 3.2 play well with the timing for 964 cams?

Suggestions to swap out the DME are great but ultimately there's no evidence of unburnt fuel. It's all being burned... Therefore the fuel is not missing- we know where it went.

City driving kills MPG, maybe these cams give a lot more up top and the price is down low they consume a lot of fuel...

I sincerely hope Karl finds a smoking gun and this thread has a lot of fantastic diagnostic tests for posterity, but for my money, I don't think there's a culprit other than the cams.

[mysocal911]

Originally Posted by Showdown

The gas isn't going anywhere except into the combustion chamber where it's burned effectively resulting in desirable and predictable lambda/AFR.

The premise of the question was that the gas was disappearing somehow- either it was going out the muffler as unburnt fuel or leaking form a hose, etc... all of those have been ruled out.

Drag and resistance has been largely ruled out (and I suspect Karl will pursue this further and find negligible resistance if any).

I asked about timing but had no clarification- Does the stock timing for a 3.2 play well with the timing for 964 cams?

Suggestions to swap out the DME are great but ultimately there's no evidence of unburnt fuel. It's all being burned... Therefore the fuel is not missing- we know where it went.

City driving kills MPG, maybe these cams give a lot more up top and the price is down low they consume a lot of fuel...

I sincerely hope Karl finds a smoking gun and this thread has a lot of fantastic diagnostic tests for posterity, but for my money, I don't think there's a culprit other than the cams.

Yes, I agree, but most have ignored/discounted it. New power typically requires additional energy input with tradeoffs, e.g. the low-end vs high-end.

[ant7]

Cam Timing and ignition timing play a big roll in power, and therefore fuel consumption, obviously the amount of time the injectors are held open will also eat into MPG, all this will be determined by the ECU, when a car is standard, from the factory, the ECU is pre-programed to take into account the type of fuel used, compression ratio, and the timing curve needed for performance and economy, across a broad range of driving situations etc, if you change anything in the chain be it cams, or cam timing, or compression ratio or even different types of fuel, the ECU must either learn these changes and adapt, or it must be re-programmed to best take advantage of the changes.
Ant.

[Shaun @ Tru6]

I haven't read the whole thread but I suspect an air leak, has a smoke test been done? I have a 2010 Cayenne GTS that went from highway 17 mpg (driving nicely) to 12-13 after I changed the starter motor and introduced an air leak buttoning everything back up.

[Mr. Merk]

Originally Posted by Shaun @ Tru6

I haven't read the whole thread but I suspect an air leak, has a smoke test been done? I have a 2010 Cayenne GTS that went from highway 17 mpg (driving nicely) to 12-13 after I changed the starter motor and introduced an air leak buttoning everything back up.

His AFR is perfect and his highway MPG is excellent. Just chasing city MPG at this point. It's been an interesting journey for sure. I applaud his thorough documentation and thoughtful testing of the entire system. I think we've all learned something from this thread.

[MikeD930]

Since the engine has been rebuilt I suspect that the rebuilt engine is 'tight' so give it time to fully break in. Then you may see better mileage.

[Shaun @ Tru6]

Originally Posted by Mr. Merk

His AFR is perfect and his highway MPG is excellent. Just chasing city MPG at this point. It's been an interesting journey for sure. I applaud his thorough documentation and thoughtful testing of the entire system. I think we've all learned something from this thread.

I learned to read through a thread before commenting.

[ant7]

Originally Posted by MikeD930

Since the engine has been rebuilt I suspect that the rebuilt engine is 'tight' so give it time to fully break in. Then you may see better mileage.