MyE28.com

I finally installed a wideband in my 6/83 745i to monitor my afr & had some puzzling findings. With a stock 2.5bar fpr the car cruises lean at 16.3 afr. Cold starts sucked & I had to pedal it to get the car started. Once warm it idled fine. I swapped to a 3bar fpr & cold starts were awesome but idle was way rich (obviously). The car would start & stay at 14.7 for about 20 seconds & then creep to 10.5 afr. I've isntalled rebuilt oem 745 green injectors, new cts, new thermotime sw, new iacv/slider, new fuel pump & filter. I also tried swapping to a different afm & new 2.5 and 3bar fpr's. Additionally I cleaned all ground points & installed all new battery cables. The ecu has a Jarmo chip installed.

Thoughts on what may be causing this lean condition on cruise?

stats with fpr:
2.5bar fpr
idle 12.8
cruise 16.3
boost @5psi 14.7

3bar fpr
idle 10.9
cruise 14.3
boost @5psi 12.5

Nebraska_e28 wrote:

Thoughts on what may be causing this lean condition on cruise?

Unmetered air.

T_C_D wrote:

Nebraska_e28 wrote:

Thoughts on what may be causing this lean condition on cruise?

Unmetered air.

Hmm, that would make sense.

Something else I just noticed. On lunch break I started the car & just sat there with it idling. Warm was afr 13.5, then suddenly it dipped to 11.9 for a few seconds then came back. Would this point to something electrical?

Nebraska_e28 wrote:
Something else I just noticed. On lunch break I started the car & just sat there with it idling. Warm was afr 13.5, then suddenly it dipped to 11.9 for a few seconds then came back. Would this point to something electrical?

Maybe a lazy narrowband o2 senor.

T_C_D wrote:

Nebraska_e28 wrote:
Something else I just noticed. On lunch break I started the car & just sat there with it idling. Warm was afr 13.5, then suddenly it dipped to 11.9 for a few seconds then came back. Would this point to something electrical?

Maybe a lazy narrowband o2 senor.

Hmm. New AEM wideband & o2 as of 3weeks ago.

You're using an O2 sensor on a 745?

I don't think 16 is too lean for cruising. My buddy has a Datsun 510 with a stock turbo red top motor in it and stock ecu. His car runs like a top and cruises at 16 just fine. I've tried it that lean with my m106/mergasquirt at cruise, but acceleration enrichment has to be spot on to avoid stumbles under quick throttle changes.
His is a more modern four valve/cyl motor and factory tune, that may have some bearing.

My 10/84 runner (and the 12/82 Ljet) have these air/fuel figures -

2.5 Bar FPR, stock 0.4 bar boost:
14:1 hot idle, park/neutral around 850rpm. This flutters a bit, but pretty steady
15.5:1 cruise at steady freeway speed of say 70mph, flat road
12.5:1 full power, until about 5400rpm then goes way fat. Have not relogged with modified K27

I still think the issue is harness or CTS related.

More details. At idle if I unplug the wot switch on the throttle body, idle goes from 13.1 to 14.9

Also if I gently crack the throttle on takeoff (like my grandma is in the car), afr instantly goes lean 16.x

demetk wrote:You're using an O2 sensor on a 745?

Yes, in conjunction with a wideband.

What vacuum are you pulling at idle and at cruise? IIRC it should be about 18inhg at warm idle and 10inhg at cruise.

Has the AFM been messed with at all? I think just about all of these cars have been "played" with, maybe your AFM is adjusted wrong.

My car (which has been messed with too much) currently idles at 11.4, cruises at 15.2 and WOT is 12.8 @6psi. I could NEVER get my car to idle at at 14.xA/F and not go super lean while driving, the one time I tried that I was pulling 17.2A/F at cruise. SicStang03's 745i I believe idles at 10.8A/F.

I have good reason to believe the DME in my old 745i was going bad. It got to where it started rough then magically clear up and idle smooth. Years after I sold it ended up at the dealership with no spark, the estimate was to source a new DME.

I can't believe you guys hang on to these 30+ year old DMEs!

There is no reason a 745i, or any vehicle, should idle hot at 11:1 or less.

Factory spec right out of the service manual is 0.5-2% CO, around 13.7-14.2:1 with a CO to WB conversion chart. Let me see if I can attach the pictures.

Edit -

T_C_D wrote:

Nebraska_e28 wrote:

Thoughts on what may be causing this lean condition on cruise?

Unmetered air.

This would indicate a vacuum leak, wouldn't there also be a boost leak? In which case I'd have issues building boost.

Nebraska_e28 wrote:

T_C_D wrote:

Nebraska_e28 wrote:

Thoughts on what may be causing this lean condition on cruise?

Unmetered air.

This would indicate a vacuum leak, wouldn't there also be a boost leak? In which case I'd have issues building boost.

A small vacuum leak at cruise makes a bigger difference in the AFR then a small boost leak at WOT.

I replaced all charge pipe couplings with new units (except for hump/bellow hose on intake pipe), adjusted valves, swapped barometric sensors & it still cruises very lean 16.5afr with the stock 2.5bar fpr. I also inspected the charge pipes, intercooler, & intake mani gasket for leaks but found nada.

Is there anything else I could be missing? The head looks too clean to be original & I suspect it was rebuilt. Perhaps it has b35 intake valves? Maybe that combined with my freeflow exhaust is requiring the need of more fuel? It just seems the 2.5 bar should be more than fine during normal operation.

Does it run fine at cruise? Are you chasing a problem that doesn't exist?

Can you lean out the idle condition with the AFM bypass screw and go with the 3bar fpr? Not sure if the 745i has that adjustment...

T_C_D wrote:Does it run fine at cruise? Are you chasing a problem that doesn't exist?

13.5 idle, & 16.5-17.0afr when cruising with zero boost. Light boost ~5psi afr is about 14.8-15.5

tschultz wrote:Can you lean out the idle condition with the AFM bypass screw and go with the 3bar fpr? Not sure if the 745i has that adjustment...

Possibly, I could try. I tried swapping afm, but that made no difference unless I adjusted the door spring tensioner. If I set Idle to 14.7, it made cruise even more lean.

Nebraska_e28 wrote:

T_C_D wrote:Does it run fine at cruise? Are you chasing a problem that doesn't exist?

13.5 idle, & 16.5-17.0afr when cruising with zero boost. Light boost ~5psi afr is about 14.8-15.5

But does it feel/run fine?

T_C_D wrote:

Nebraska_e28 wrote:

T_C_D wrote:Does it run fine at cruise? Are you chasing a problem that doesn't exist?

13.5 idle, & 16.5-17.0afr when cruising with zero boost. Light boost ~5psi afr is about 14.8-15.5

But does it feel/run fine?

It idles fine but during cruise I can feel something isnt right. There's more hesitation that there should be & the boost is way delayed unless I floor it.

tschultz wrote: Not sure if the 745i has that adjustment...

It does but it makes very minor changes. I don't think I was able to get past a .8 AFR change with mine.

Honestly, this is the EXACT problem I have with my car and its driving me nuts. My idle being at 11.2 is the only way I can get normal cruise and WOT numbers (14.8-15.2 cruise and 12.8 WOT). I just put a new 2.5 FPR on my car and without touching anything the AFR dropped from 11.2 to 10.8 at idle and I'm seeing into the 11's at WOT. So I'm going to try to retune my AFM and see what it does now.


All the more reason I can't wait to ditch this POS M1.0 system and go full MS3. The fact that GM was using hotwire MAF's in the mid 80's with O2 sensors and real idle air control valves but BMW stuck with this AFM crap into the mid 90's is just a joke. I don't like that I can tune my global air fuel with a screw driver and an allen key.