New Tech Article: Swapping BMW Injectors...

Wayne 962 · Wayne 962's Garage

Hi folks. I just finished a new tech article:

http://www.pelicanparts.com/bmw/techarticles/E36-Injector-Replacement/E36-Injector-Replacement.htm

Check it out and let me know what you think...

-Wayne

blkongry · 11-17-2003, 01:13 PM

Nice article... and very well written (I love tech articles that have pics coinciding with the text... best way to do it). I do have a question about stock injectors and the air fuel metering control system. At what point would you want to change the injectors to a higher flow/pressure... I mean, the computer should compensate for what ever air volume you are moving through the engine.... go to a higher altitude, the computer leans it out... go down to the coast, the computer makes the mix richer to compliment the more dense air... if you threw on a small supercharger, wouldn't the computer then compensate for the added air up to the limit of the injector? But what is the upper limit of the injector? Bosch is the OEM injector, but Bosch makes injectors for nearly all car manufacturers... are our injectors custom designed for the I-6 engine, or are they more generic. Either way, what is their limit and how can one tell? Lastly, how do you know if your fuel pump can provide the required pressure necessary to run higher flow/pressure injectors? I'm just trying to get a feel for what can be modified (and to what extent) before other systems need to be upgraded to 'keep up'. Thanks!

Wayne 962 · Wayne 962's Garage

All good questions. I'm not 100% sure on any of them however. What you say about the computer compensating is true, however, the injectors are an open-loop part in a closed loop system. The only feedback the car has on mixture is from the O2 sensor, which is affected by a number of factors.

I believe that the computer knows what the injectors' flow rates are, and compensates and controls them accordingly. Like a water hose with a squeeze nozzle on the end. If you replace the nozzle with one that doesn't flow as much, will the engine know it? I'm not exactly sure it will compensate correctly.

-Wayne

blkongry · 11-17-2003, 02:51 PM

Hmm... that helps. Thanks!! So, I guess the injector will just try to give the computer what ever it demands. Will the computer tell you if your mix deviates from what it is trying to give? meaning show up as an O2 sensor error or something? Hmm, I imagine, since the injector is an open loop, the computer is not that smart nor informative to it's owner. If the mix is not right (meaning at some point the injector can't keep up) I presume the computer will just keep trying and trying to make the right mix, and we'd never know the difference. Thanks again for the article... keep 'em coming!!

Wayne 962 · Wayne 962's Garage

Someone sent me an email sortof disagreeing with me on this. Let me clarify - the injector is an open-loop component - it doesn't have any internal feedback (other than sending resistance back to the computer to tell the computer it's connnected and working). Let's say that one injector fails and is stuck open, spilling fuel into the manifold - your O2 sensor is going to incorrectly diagnose that because it will think that all the injectors are too rich. So it will lean out the mixture. You'll have one injector running rich, and the others running lean...

blkongry · 11-19-2003, 06:53 AM

Yeah, one would be hard pressed to be able to measure injector performance. The only way I can see would be to monitor it (not measure) by having the computer watch the pressure to each injector and make sure it is where it should be when not firing... a low pressure injector means a leaky one. Although, how often does this happen? Are injectors so tried-and-true in this day in age that it is a rare event to have one fail?

Wayne 962 · Wayne 962's Garage

You could have an O2 sensor on each exhaust port, although that wouldn't distinguish mechanical problems (like valves not seating properly)...

-Wayne