Gas Mileage in HALF!

[aTron12]

A few weeks ago I had my entire rear end redone.. all bushings, camber kit, suspension etc. The day I got it back, I burned through my tank of gas super fast. After a few tanks I have discovered that I am getting about 140 Miles per tank. I verified that my odometer and trip miles gauge are working normally. I have a new ICV, Fuel Injectors (rebuilt), fuel filter. I smell absolutely no gas anywhere. I drove like a grandma on the last tank and there was no improvement.

This issue seems to have absolutely nothing do with the last repair but it is very strange that it happened immediately after picking the car up. My mechanic was not under the hood. All that aside, any ideas on what could be causing this or where to start looking? I want to get my head wrapped around this one before I start talking with my mechanic.


Thanks,

Adam

[nomansland92]

When was the last time your o2 sensor was changed? Even though a o2 shouldn't make that big of a difference but it may help

[jlevie]

It may sound like a silly question, but after "burning through" a tank how much fuel does it take to fill the tank? And what car (model & year) is this?

[aTron12]

Has a year old 02 sensor. 1988 325is (plastic bumper). Takes about 15 gallons.

[MTechnikII]

I had the same problem, after I did all maintenance myself according to Bently manual the gasmilage dropped to 10 11 mpg, i changed what ever was changable, no difference at all, i was ready to dump the car, couldn't afford 4$ a gal at 10 mpg.
I even adjusted the valve clearance per bently,
I figured it out somehow that the valve clearance is the cause of bad MPG and good enough it was the cause of it,
adjust your valves and go a little bit larger than the max recommended, don't remember how much is the recommend clearance, google it.
don't worry about a little valve noise , this engines don't like tight and quiet valves.
Good luck
Henry

[aTron12]

Thanks Henry. I'l check into that. Adjusted valves a few months ago. I'll consider that during my investigation.

I had the same problem, after I did all maintenance myself according to Bently manual the gasmilage dropped to 10 11 mpg, i changed what ever was changable, no difference at all, i was ready to dump the car, couldn't afford 4$ a gal at 10 mpg.
I even adjusted the valve clearance per bently,
I figured it out somehow that the valve clearance is the cause of bad MPG and good enough it was the cause of it,
adjust your valves and go a little bit larger than the max recommended, don't remember how much is the recommend clearance, google it.
don't worry about a little valve noise , this engines don't like tight and quiet valves.
Good luck
Henry

[whodwho]

Pull the vacuum hose from the FPR and make sure there is no fuel coming out or in the hose.

[paperplane94]

This leads to another question about adjusting valves. Where and to what decimal should you adjust the valves?
Is it different when you do it rocker/cam lobe as opposed to valve stem/eccentric?


I did mine eccentric/stem and my MPG sucks. Adjusted to .254mm in the firing order.

[CarsSuck]

I know this might sound silly, but have you checked the rear brakes and e brake to make sure they're releasing properly? After you've driven for a few miles without heavy braking, feel for heat. Start at the hubs and work in so you dont burn yourself.

I only bring this up because you just had the rear end apart.

[aTron12]

Just got a call from the shop.. Looks like there is a bad rocker end plug which is causing massive vacuum. I don't really understand this but he's replacing it and re-adjusting the valves. We'll see how it goes.

[MTechnikII]

go atleat 10% more than what you have as max in bently , 2 desimals should be ok, there is no diffrence between the gaps if they are adjusted at either end, lob end or accentric, but depending on your tools and experience one is easier than the other, the main thing is to make sure the lobe an adjusting valve is facing completly down , and when you tighten the nut make sure the accentric stays still with the feeler gauge under it , the feelr should be just snug not grabbing and not loose, it's dificult to explain but once you do it a few times you'll get the feel.
Good luck and dont be afraid to do it over a few times till you get it right.
Henry

[whodwho]

There is a ratio built into the rocker so there will be a difference between the two ends

[aTron12]

So the mileage has not changed at all. I guess we got a vacuum issue taken care of but it didn't resolve the gas problem. Could the computer be an issue? It is obviously running too rich. Fuel pressure is dandy, O2 sensor values are within perfect range. Valves are perfect. Not sure where to go from here..

[bmwstephen]

Adam,

I know how you feel and it seriously could be all in your head. First off, lets see if your fuel tank reading or gauge might be off before we conclude that your mpg really sucks.

On your current tank of gas, drive the car until it hits the yellow light on the fuel gauge (1.5 gallons left over). Then fill your tank up, it should take about 15 gallons of gas to get full. After that, reset the odomoter pin so you start out fresh at 0000 and see how many miles it takes you to reach the same spot on your fuel gauge.

For my car, I narrowed it down using this test to the fuel pump and fuel level sender on the drive side. Replaced both and got accurate mileage.

[MTechnikII]

you are right I've never thought of that , but I have done either way
Henry