Really Poor Mileage M50 Swap

[Kerrath]

I've got a 1992 M50 in my 86 325e and so far it's gotten 16-17 mpg at the top. I found out after a stomp test that I had a bad MAF and the OBD equipment at a dealership reported a bad O2. Both show as being clear now, yet, I think my mileage is still really poor. I haven't gotten a full tank out since replacing the O2 sensor, but preliminarily, it's the same as always: Bad.

The engine is stock. Original injectors, correct maf, cams, etc. I have seafoamed it and it burned out a looooot of crap according to the smoke cloud compared to my friend's is. My next step, once I figure out my weird rattle is to take the injectors out and give them a thorough cleaning, but other than that, does anyone have any ideas? My ECU is grounded.

Please help out.

[lifeiskaos]

first thing i'd do would be to check the spark plugs. maybe you were running super rich before the new o2 sensor, and fowled them?

[MR 325]

Is your speed sensor hooked up?

[Kerrath]

Is your speed sensor hooked up?

Yeah, all pertinent sensors are hooked up, including speed sensor.

[E34Dean]

In my e34 I get 19mpg with a 5 speed, its a 94 so the m50 w/vanos. you should be getting better mileage than me. no rough idle? fuel pressure ok? as stated above spark plugs wet and or fouled? also, try some seafoam after you replace the plugs. Clean tha MAF with MAF cleaner, and maybe even dip the injectors in some seafoam to clean em out as well.. all I can think of =]

[th3blue]

what ratio LSD do you have?

[lifeiskaos]

he has a 2.93 open with a getrag 260

[Kerrath]

he has a 2.93 open with a getrag 260

Still gets the same mileage even when like 80% freeway driving. Thanks for the thought on the spark plugs. I'll check them this evening when I'm where the tools are.

In my e34 I get 19mpg with a 5 speed, its a 94 so the m50 w/vanos. you should be getting better mileage than me. no rough idle? fuel pressure ok? as stated above spark plugs wet and or fouled? also, try some seafoam after you replace the plugs. Clean tha MAF with MAF cleaner, and maybe even dip the injectors in some seafoam to clean em out as well.. all I can think of =]

The idle was rough until I replaced a broken vacuum line and the o2 sensor. Now it idles pretty steady ~500 rpm. I don't have a pressure gauge, is there another way to check fuel pressure? I'll clean the injectors and MAF after I check the plugs. Thanks.

[E34Dean]

Still gets the same mileage even when like 80% freeway driving. Thanks for the thought on the spark plugs. I'll check them this evening when I'm where the tools are.



The idle was rough until I replaced a broken vacuum line and the o2 sensor. Now it idles pretty steady ~500 rpm. I don't have a pressure gauge, is there another way to check fuel pressure? I'll clean the injectors and MAF after I check the plugs. Thanks.

Hm, not really but Im doubting fuel pressure the problem. if you end up buying new plugs go NGK or Bosch ONLY I run NGK in my car. Id suspect spark plugs to be honest. Because your not having any ridiculous symptoms, just a loss in gas milage.

[Kerrath]

Pulled and cleaned the spark plugs a couple days back. They looked fine. No carbon or moisture. Just a nice brown sheen. Brushed and cleaned them anyway and after about 30 miles, I see no change in the way it runs or more particularly, the efficiency. Looks like I'll be getting 17MPG again. I drove it about an hour after cleaning it and I FELT like it picked up speed faster, and I wasn't exactly looking for that symptom, but I think it was just placebo.

Could something as simple as a bad coolant temp sender be causing such a severe drop in efficiency, or should I be expecting something way more catastrophic like evenly bad compression on all cylinders? :(

[Farbin Kaiber]

What is this rattle you speak of? Could it be piston slap? Have you done a compression test?

[Kerrath]

What is this rattle you speak of? Could it be piston slap? Have you done a compression test?

No, not piston slap. It sounds like it comes from the rear, and only happens after like 15 minutes of driving. Piston slap would be the opposite of both. So I think it's entirely unrelated to engine.

I haven't done a compression test, don't have the equipment available readily. I imagine if the compression were bad enough to so significantly harm mileage, it'd significantly harm power as well and it seems to run pretty strong.

[Kerrath]

On lifeiskaos' suggestion today, I listened to the injectors with a screwdriver, and none of them sound like they are working improperly to either of us. Sparkplugs and injectors ruled out, I think the next thing likely to fail would be the CTS, however, I don't know that it can cause such a large impact as this. Any word on that?

[Jason89i]

being in CA, i bet your car has a cat. with no hfm and no o2, im sure it was running rich for a long time, it might be clogged. jason

[Kerrath]

being in CA, i bet your car has a cat. with no hfm and no o2, im sure it was running rich for a long time, it might be clogged. jason

Hmmm, it's a possibility I suppose. It's an aftermarket "high-flow" cat. I discovered yesterday that it is responsible for making my exhaust sound really rattly and bad as something inside is loose. If I hit it, I can feel it land after a moment, then settle to standstill. My thought was that one of the 'honeycombs' had broken loose and was just resting at the bottom of the cat, but would that contribute significantly to the problem, or would it be more likely to increase flow by removing one of the gates? Thanks for your idea.