Fuel issues + low MPG + 91

[Badabing86]

So lately my car has been running on 91 octane because if I don't it pings a bit on acceleration and sometimes it will on cruising highway speeds. Took a trip out to Cali and my average MPG highway was 15-18 mpg per the gauge on the car. Not sure if the pinging and MPG are a common issue. So just trying to see where I should start looking to troubleshoot.

[MrBurgundy]

You could have a leaky injector...

18 mpg going how fast? what diff are you running, eta seta or I?

[mr2peak]

Burning lots of oil?

[Badabing86]

65 I would be hitting 15, 18 would be at 75 mph. Yesterday I put half a tank of 91, drove to work and home 10 miles each way freeway at 65 and turn the car on today and sitting just above empty. 88 325i

I have a small leak at the valve cover that is a slow leak but holds oil well.

I was thinking it was a fuel Injector, or pump going bad or a filter. Is a injector rebuild something I can do on my own?

[Seawolf]

Check your Fuel pressure regulator vacuum line (the thin vacuum line that runs from the FPR to the inlet manifold) if your FPR is iffy fuel will get into that line and overfuel the engine.

How's the engine run generally ? Good idle ? Pulls well ?

[squidmaster]

What are your mods? Could be a number of things, from valve contact to bad injectors. An easy test is to take an IR temp gun to each of the primaries as it's running. A large temp difference will point you toward the cylinder that is having issues, if it's just one cylinder and not an issue down the line (fuel pump/filter/timing belt slipped, etc)

[Badabing86]

Check your Fuel pressure regulator vacuum line (the thin vacuum line that runs from the FPR to the inlet manifold) if your FPR is iffy fuel will get into that line and overfuel the engine.

How's the engine run generally ? Good idle ? Pulls well ?

The car has always ran good, idles at like 700 constantly. Does drop any rpms, and pulls nicely .

[Badabing86]

What are your mods? Could be a number of things, from valve contact to bad injectors. An easy test is to take an IR temp gun to each of the primaries as it's running. A large temp difference will point you toward the cylinder that is having issues, if it's just one cylinder and not an issue down the line (fuel pump/filter/timing belt slipped, etc)

No mods done to it yet, been getting it back to running order from a buddy that I bought it from. But car runs strong for the most part just wondering why it needs 91 now to run without a ping or just gas is getting so bad. Ok I appreciate the help on that, Im going to give that a try and see what my findings are. Hopefully its nothing too big

[squidmaster]

Having the stock exhaust manifold makes that test a little harder since it's cast iron AND really short/poorly designed. You could also pull your plugs and check to see if any are discolored or fouled

[Badabing86]

Not sure why this is happening but went to put gas that wasn't 91 octane and decided 89 would be good but soon as I did I got an immediate CEL