m50nv extremely poor gas mileage

[DimitarBond]

I've got an m50nv in my 91 318is, but having some major issues regarding idling, and gas mileage.

First off, at idle the car will do the surging thing that these motors tend to do alot (from what I have read just now online).

I have checked for vacum leaks and all looks good, so after reading online it looks like I should check the idle control valve. I am going to take a look at it tomorrow, and clean it.

The major issue I am having right now is fuel mileage. I am not sure if this idling issue can be causing it, but the car is definitely running rich at times.

Today I replaced the injectors with some rebuilt ones. Two of the ones in the motor were missing the yellow caps on them (not sure what they are called). I figured this could be a problem but after replacing them, and driving the car about 35-40 miles today, I burned through a quarter tank.

Now, with a light e30, and 2.79 rear end combined with my light foot and 70% highway driving there is something wrong with that for sure.

I am not sure if these problems are somehow linked?

At this rate I am putting $35-$40 in the car every 3 days which is just crazy.

Anyone here have these types of issues or any insight?

[DesertBMW]

Now, with a light e30, and 2.79 rear end combined with my light foot and 70% highway driving there is something wrong with that for sure.

Anyone here have these types of issues or any insight?

Engine could be in open loop because of bad oxygen sensor or engine temperature sensor may read cold. These sensor can easily be tested if you got the right manuals and tools.

Other than that, if you are using Getrag 260 then 2.79 rear may be too tall and engine is 'lugging'

[nando]

2.79? Unless it's a diesel, your engine is running way outside of it's efficiency range most of the time. do you pretty much have to floor it from every light?

sounds like about 16mpg? is that right?

[DimitarBond]

2.79? Unless it's a diesel, your engine is running way outside of it's efficiency range most of the time. do you pretty much have to floor it from every light?

sounds like about 16mpg? is that right?

16mpg sounds about right, my odometer is broken but I know my round trip work commute is 30 miles, and I'll do that 4-5 times plus a little misc driving before i have to fill up.

I'm not 100% sure off the top of my head, but I think the car is doing about 1600-1700 rpms in 5th gear doing 55.

The car accelerates fine, I haven't had a problem with that.

Are you saying the engine should be spinning faster when cruising to improve my efficiency?

[nando]

yeah, at 1700rpm the engine's efficiency is not good. it wasn't really designed to operate for long periods of time at such a low speed, especially not on the highway. Stock cruising RPM for an M50 is probably closer to 2500-3000rpm. That's what it was tuned for.

even the transmission won't typically like such low speeds, which can cause higher drag losses. I think BMW says not to drive at speeds 1500rpm below, and you're pretty close to that. I think you'd want to be at least over 2k rpm cruising, where the engine is making more torque and is much more efficient.

of course I wouldn't say it is your only problem but it's certainly not helping.

[Cyrix2k]

I have a very similar setup and my mileage is way better... I'm running a '91 318i, Getrag 240 (1:1 5th gear), and a 2.79 LSD. Sounds like you have a vacuum leak or a bad MAF to me. Running at 1500 rpms should not be a problem - it's outside of the powerband, but it shouldn't hurt anything. Any improvement in volumetric efficiency by running at higher revs are just going to be negated by increased drag inside the engine, trans, drive shaft, and final drive.

[DimitarBond]

Just took a look, and remembered there is some vacuum line that is missing from the equation. The spot on the middle of the intake on top is just plugged with a bolt. I'm not really sure what is supposed to be here and where it goes?

[Cyrix2k]

on top? Do you have a picture? There are two vacuum ports on the intake. One is a small port on the bottom near the firewall which should be connected to the FPR on the fuel rail (right up against the firewall on top of the engine) and the other is a large port on the side of the intake facing the driver's side fender. This large port is connected to the power brake booster, assuming you have power brakes. If the FPR is not connected, the car will run rich and fuel economy will suffer.

[DimitarBond]

I took this attached picture this morning. You can see the bolt plugging it.

[nando]

I don't know about that boot but your hot air intake isn't helping things. :(

[Cyrix2k]

On the intake boot? That's not a vacuum line. One line runs under the intake for the ICV and the other line runs to the valve cover. It looks like you have both of those so I don't know what's up with the screw/bolt. However, your ICV line appears to be patched with tape? Replace the line because it will leak metered (post-MAF) air and make the engine run like crap.

[DesertBMW]

the air from alternator messing up with MAF reading. You need to have a proper intake setup,

[nando]

That will make it run leaner, not richer.