Common causes of M42 fuel economy issues?

[Chappie]

Alright guys, looking for some help as I'm an m42 newbie. Picked up a 91 318is a couple of months ago and have noticed my fuel economy is probably worse than my old m20, but there are no immediate signs of where I should be looking, other than smelling gas over 3/4th filled tank. I haven't noticed any spotting that makes me think leak, my air filter is clean, temps seem in order, tried swapping spark plugs as the PO included them with the sale of the car, and it didn't change anything. The gasoline smell is most prominent on passenger side, near fuel fill up door. Car also has Mark D 93 octane chip and only gets 93 octane from Shell and Sunoco (no cheapo gas).

Where should I go from here? I want to get under it either this Sunday or next Thursday, whenever I get some free time.

[oldmann]

Ck under the wiring cover on the fire wall, should be a single wire plug visible, should not be connected, Connected is for cars without cat.
O2 sensor may be bad.
Fuel pressure regulator may be bad, unplug vacuum line, rpm should rise, look for fuel in vac. line, if theres fuel fpr is bad.
Injectors may be leaking, or someone may have put bigger in place of stock.
And as always vacuum leaks can kill mpg.
How bad is you mileage? 22 to 25 mpg around town was normal for me. 30 on the interstate.


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[Chappie]

Common causes of M42 fuel economy issues?

My speedo doesn't work at the moment so I can't be completely sure, but since I use it mostly just during work commutes, I'd estimate I'm getting 130-150 miles on a full tank. Half of it is in town driving and half highway.

[roguetoaster]

First, you have some sort of fuel or fuel vapor leak (likely, pull the fuel pump cover to look), so that's probably killing some. Check it out ASAP, don't let your E30 burn up, and replace all of the soft fuel lines. Just because you don't see the spotting doesn't mean you don't have a leak.

130-150 on a tank means that on an average 12.5-13 gallon fill up you are getting 10-12 MPG, which isn't realistic. For reference, the M42 powered Chump Car got ~14 MPG on track, granted with ~700 pounds less weight. Even my 4.45 rear end only knocked me down to 25 MPG, and I drive like an idiot as much as possible.

I'd start with a check of your plugs to see if rich/lean conditions exist. Then I'd test the crank & cam sensors, DME temp sensor, and TPS to see if they are within spec. I'd also verify that you have a genuine Mark D chip, and while you're there have a look at the DME board for obvious failures. Perhaps do a compression test as well. Beyond that I'd consider looking at the AFM, swapping O2 sensors, cleaning injectors, and other parts replacing measures that'd I'd encourage you to avoid if possible/unnecessary unless you plan to keep the car long term.

It's also worth examining your driving style. The stock gearing means that the car is happiest (MPG wise) cruising at 60-65, but that's not too realistic in modern driving.

[TobyB]

If you smell fuel around the filler, pull the right rear wheel and the cover that's behind it.
There's a 'Y' pipe made of steel back there, and it loves to rust.

Otherwise, reread what Rogue said, paying attention to the condition of your plug wires-
oil in the plug wells can make a mess of 'em.

t