The PO on my new 1988 325i put in 18LB injectors and swapped in a 173 DME to replace the 153 that the car came with. The engine runs smooth & strong but only gets roughly 18mpg highway. Would re-installing OEM 14LB injectors or putting in a MarkD 17.5/19LB chip likely improve that mpg level substantially?
-
"If the sky were to fall tomorrow, the tall would die first."
-Dr. Paul Forrester
Do I LOOK like I need a psychological evaluation??? -
If you are interested in improving fuel economy, larger injectors and/or a chip is moving in the wrong direction. Reverting to OE injectors will help with mileage, but there are other things that also may need attention to get mileage where it should be (eliminate intake leaks, valve adjustment, distributor rotor/cap, ignition wires, plugs, fresh O2 sensor, i.e., a major tuneup/refresh).The car makes it possible, but the driver makes it happen.
Jim Levie, Huntsville, AL -
PO just did a major tune-up w/ receipts, which is why I suspect the larger injectors. Engine barely failed smog, which would also indicate running rich.Originally posted by jlevie View Post
The parking brake seems a little tight, so I'll loosen that up first."If the sky were to fall tomorrow, the tall would die first."
-Dr. Paul Forrester
Do I LOOK like I need a psychological evaluation???Comment
-
The Germans knew what they were doing. If all you want is mileage, stock is the way to go. Or you could throw an M30 AFM at the car to correct the mixture and pick up some power, too.sigpic
Mike
'91 325i track car. Mostly...Comment
-
it's not going to get bad mileage just because of larger injectors.
First off, the injector design of the stock 14lb injectors is crap. The single pintle design results in extremely poor fuel atomization, plus they are noisy and by 100,000 miles probably gunked up and working at half efficiency.
A newer 4 pintle design sprays in a fine mist, the stock injectors is more like a stream. The 4 pintle design is vastly superior, the best thing would be to find a stock size injector in a newer design, but there aren't any that I'm aware of.
Second, you probably just need to replace your O2 sensor and fix any vacuum leaks. With a functioning O2 sensor, Motronic will gradually dial back the injector pulsewidth until it reaches the lambda values in it's target table. If the O2 sensor is dead or not sending a good signal (they tend to read lean over time, causing the engine to run richer), then it's either running in open loop or is unable to properly dial back to the lambda target.
A Mark D chip may help, but only because it's already been programmed for the larger injectors and Motronic won't have as much work to do to reach the target.
I don't buy the M30 AFM "upgrade" either. That will make it worse because now Motronic can't properly measure load. Any delusions that the M30 AFM sends a signal 1:1 as the original AFM are just that, delusions. It was designed for a different engine with a different tune. The fact that it plugs in and "works" is just coincidence that BMW saved money by using similar connectors and electronics.Last edited by nando; 04-06-2009, 01:51 PM.Comment
Comment