Design III injectors -- Do I really need 3.5 bar FPR

[mholbrook]

Just installed the Design III Mustang injectors. On my previous 89 535im, I installed Design II and there was a noticeable flat spot off idle. Corrected with 3.5 bar FPR.

The newer 92 535im seems to be ok with the Design III but I do notice the idle slightly lower. I have not driven it much since I just made the change.

Anyone else have the DIII injectors and what is your experience?

[Alexlind123]

I installed design 3 19# injectors not too long ago. I didnt think much about it until now, but it does idle maybe 75rpms lower, it runs better and quieter though.

[mholbrook]

My feeling is that it is smoother and runs a bit stronger but there does not seem to be that flat spot that I noticed immediately with the Design II injectors. I think these must flow at a higher rate than the D5B injectors.

[Andrem5]

Hi, Mike:

Did you reset the DME after installing the new injectors?

Somehow the lower RPM result doesn't sound quite right. I don't doubt your accuracy at all but are you sure about this? Idle control is a different animal than part-throttle fuel flow, based on crank sensor/rpm not O2 sensor. As I understand it the idle switch basically disables close loop operation at idle and WOT (75%). I can imagine that your idle is now much richer but that is not necessarily a good thing or a desirable result. Wouldn't richer idle lead to highrer rpm? What would lower the rpm?

The DME does have an air/fuel/temp table for idle but the key target value is rpm, not AFR as it is for closed loop. If your old tired injectors were a tiny bit lower flow than nominal the idle control may have adapted to adjust for a leaner condition by restricting bypass air input. The new injectors now could be flowing more fuel but they might bump the idle a bit, not lower it. I do not know the time constant for adaptation of the idle table but I am about to experiment with that as the final stage of tuning my Motronic 1.3 M20, as you know. I want to increase the nominal target idle rpm for that motor, not change the mixture. (I finally found the memory offset for that value.)

Again, what would cause the idle rpm to drop? Perhaps the idle will return to nominal with adaptation. Reset the DME and give it 200 miles of mixed driving.

This is a different problem but the first time I experimented with the Design III injectors on my old M20 (increase from 14.5 to 19 #/hr) the car idled and ran like crap until I reset the DME and trimmed back AFR with the piggy back. On another car, when we replaced the FPR we ham-handedly crushed the fuel rail o-ring and made the car lean despite bigger injectors.

You already know my suggestion about not using the 3.5 bar FPR. WOT would be way out of calibration if you install it.

If you could borrow Ron's or my WBO2 you would have the answer. Ha! Road trip.

[Alexlind123]

Update: I noted my slightly lower idle speed shortly after starting the car after the computer had been reset.

[Felixdacat]

hey guys, I was considering doing the same with my 89 535i. Does it really make that much of a difference? Would I have too add a fuel pressure booster, or is it just a swap out? I have seen that cobra 24# injectors would work also.

[mholbrook]

Update. After about 50 miles of driving around, the idle has gone back to normal and there is no hesitation noticeable. The car runs smoother and has a brighter response to throttle. Just filled up so I'll be keeping some gas mileage data for a while to see if there is an effect.

All in all, a pretty smooth upgrade. DIII 19# are probably just fine with the stock DME. I don't know who has tried the higher 24# and other injectors but you might be running too much fuel with those.

A side note, when doing the install, be sure and have 12 new BMW 0-rings for the top and bottom of the injectors.