924Board.org

[jg4454]

My car is well tuned, new plugs, wires, cap, rotor, syn oil, coolant, no overheating in over a year.

But in mostly highway mileage i am only averaging 15 mpg. I don't floor it all the time.

i use 87 octane, mostly Chevron or Shell.

Any help would be appreciated, for obvious ($!) reasons.
_________________
"Speed is a matter of dollars. How fast do you want to go?" - Somebody said this in Road and Track once.

[MunkPuppy]

[924guy]

im with monk here, and id bet the 87 is hurting your mileage big time.. and your performance... feed your 931 better gas, it deserves it...
_________________
Eric
78 924
82 931 SE "smokey"
99' VehiCross
Y2K Honda Insight
http://www.cardomain.com/id/924Guy
Performance by Pasha

[Paul]

87 octane in the US is fine for a stock US 931, it's what's the owner manual tells us to use.

Don't be fooled by the different rating scales between the US and other countries.

Also ROW 931s had higher boost levels and required higher octane.

Higher octane gas has no more "power" than regular gas, it just allows higher compression ratios, etc. Putting high octane gas in a low octane motor can burn valves and reduce power.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

[dpw928]

Could be a bad O2 sensor. Has it been replaced in the last 30K miles?

Dennis
_________________
81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

[924guy]

ive never heard of anyone burning valves on "normal " high octane pump gas, thats interesting stuff.
my 931 must think its a row car , because it runs like crap on anything under "super" grade..
_________________
Eric
78 924
82 931 SE "smokey"
99' VehiCross
Y2K Honda Insight
http://www.cardomain.com/id/924Guy
Performance by Pasha

[jg4454]

I've tried premium US gas, and it didn't make a difference. BTW, Octane is one of a dozen additives to gasoline, and only matters if the manufacturer designs the engine (timing, CR) to use it. The US 931 calls for 87 octane.

The o2 sensor has been replaced, and the fuel filters, both of them. Also a new external fuel pump, and I have no leaking lines. (No leaks at all, for that matter.) The highest I ever got was 19 mpg on an interstate trip.

I'll try higher octane again, it won't burn valves. I'll also report on my tire pressures when I check.
_________________
"Speed is a matter of dollars. How fast do you want to go?" - Somebody said this in Road and Track once.

[numbbers]

Well, on my '80, I can get 28+ MPG on the road, if I keep my foot out of it. But, around town, I only get 12 to 13. That is because I am running high boost, and my foot is always to the floor. If your car is running well, and passing emissions, then it must be driving habits.
_________________
1980 924 Turbo

[Paul]

Are you sure your odometer is working correctly and not slipping?

Do you have any signs that the car is running rich?

The reason high octane gas can burn valves in a low octane motor set at stock ignition timing, is the fact that the gas burns slower and is still burning as it exits through the exhaust valve. This causes the exhaust valve to run hotter.

My car always passed emissions with 87 octane, never with 93.

[jg4454]

It does seem to be running rich, some smoke, a little backfire between first and second gear, and a gas smell at idle. I adjusted the idle screw and I idle at 900.

Odometer is working fine, I use the trip odometer and it is always ticking.

My tire pressures were low, so I filled them to the pressure in the manual, 29 psi front, and 36 psi rear. Made for a better handling car.

My driving habits could account for much of it. I like to accelerate.
_________________
"Speed is a matter of dollars. How fast do you want to go?" - Somebody said this in Road and Track once.

[john h]

My 924GT replica used to do about 30+ miles per gallon (note the NZ gallon is the same as teh imperial gallon 4.54 liters).

I check your air filter, also check your brakes aren't binding. My old Merc used to give bad milage and took ages to change into top (auto box). When I checked the brakes the rear pistons weren't retracting and after applying the brake sthey would still have slight presur eonteh disc. Once new seals were installed and the pistons and culinders cleaned I noticed one hell of an improvement in gas milage and performance.

On the 931 park in on level ground apply the brakes hard then release them and get out and try to push the car it should move reasonable easy - if not jack it up corner by corner and see which wheel is dragging.
_________________
Remember a Porsche is not just for Christmas,

if you take it to pieces slowly it can provide anguish all year long!

[Paul]

Sounds as if a mixture adjustment might save you some fuel. It's best done with an exhaust gas sniffer.

[jg4454]

It's not the brakes.

Probably the air/fuel mix. I'm scared of my distibutor, because it's old, and the rubber piece that's sandwiched in there is probably worn out. According to a post on here, there's no way to correctly adjust the distributor unless you have a new rubber part, and it's NA from Porsche, so you have to make your own. A new distributor from Perfomance costs $1000+.

As I've said on my other posts, the 931 is a very well built car, but the rubber parts wear out after a quarter of a century, and cost a lot of money to replace. And the 931 engine has a lot of rubber. My wastegate is also giving me problems, another big fix. Too much boost is probably eating gas.

I'm still a sh%&load of money away from getting this car mechanically sound, and I haven't even touched the interior.
_________________
"Speed is a matter of dollars. How fast do you want to go?" - Somebody said this in Road and Track once.