need help diagnosing "limp" mode
1993 750iL (prod. 12/1992) increasingly went into limp mode over about a 15 minute span, starting out intermittent and ending up full time limp (low power, runs rough, etc.)
0) Reading the codes with the Peake shows only O2 sensor (for both banks).
1) Disconnecting the right MAF had no effect, meaning it still starts and runs rough. Disconnecting the left MAF and she won't start.
2) Putting a timing light on the right coil wire shows intermittent pulses at idle and no pulses at all above about 1K rpm. Checking the left coil wire shows strong consistent pulses.
3) Both distributor caps, rotors, wires all check good. Both coils have correct resistance on primary and secondary sides.
4) Both crankshaft sensors have correct resistance (540 ohms).
Assuming one bank of the V12 (right side?) is not running, I tried swapping some parts to isolate the issue.
- swapped MAFs and #1 above still holds true.
- swapped throttle bodies and #1 above still holds true.
My gut tells me this is a sensor somewhere, but what to check next?
Re: need help diagnosing limp mode
I also swapped the ECUs and that made no difference.
Could a bad Oxygen sensor cause limp home mode? I thought they only came into play after the engine warms up.
Sorry, this is NOT limp mode
I know this because if I put the shifter in '3', it starts in 1st gear, then shifts later to 2nd as it accelerates, and finally up to 3rd gear. My understanding is that limp mode starts and stays in 3rd gear.
I swapped the DME connectors and the problem doesn't change.
I'll check the voltages on the MAF connectors next.
Any suggestions would be strictly followed and greatly appreciated!
Update: no fuel pressure at left bank
Forget the mention in the original post of intermittent spark on the right bank. I double-checked and it is fine. That was my bad timing light connection to the right bank's coil wire.
I put the timing light on each of the 12 plug wires and there is voltage for spark, so I'm ruling out the ignition system for now.
I pulled off the left bank's fuel line where it meets the fuel rail and started the engine. Not a drop of fuel came out. It must be the fuel pump, relay, filter, etc., right?
time to check the fuel pump, relay, fuse, etc.
Thanks to Sean and Johan's site, I'll be probing further for the source of the lack of fuel pressure on the left bank. Stay tuned...
no, bad oxyden sensor cannot cause this
Shogun E32 Tech Tips: http://twrite.org/shogunnew/topmenu.html
Re: no, bad oxyden sensor cannot cause this
After doing some more research (and even the Bentley manual mentions this) I learned that the O2 sensors are used by the DME for fuel/air mixture only. Anyway, their signals aren't used until the engine warms up.
I also found a post on Roadfly (and Sean and Johan's site confirms it) that states that a faulty fuel pump will NOT set a diagnostic code (through the CCM). This is what really tripped me up, and is definitely a design flaw.
I hope my posts help someone else in the future.
good idea with the Y-connection
I believe one pump will be enough to pump enough fuel as long as you do not drive full throttle.
Original the capacity is laid out per pump for 875 ccm/30 seconds at 3 bar, so theoretical that would be >100 liters per hour!Per pump of course.
You can also try to install these cheaper pumps from TrePerformace, they are claiming to pump our 255 l/h. I have asked someone to take the dimensions.
Of course most of it goes back with the return line.
Read this here
http://ww w.bim merb oa rd.com/f orums/pos ts/696070/Shogun E32 Tech Tips: http://twrite.org/shogunnew/topmenu.html
that would be nice if you can get the data
or after you changed the fuel pumps, compare both in size, the orginal Bosch 69493 with that one.
The Tre-performance pump might be enough with regards to capacity, although that is probably under optimum conditions, maybe without counterpressure. The original Bosch pump has to produce the capacity I mentioned under counterpressure of 3 bar.
Another point to consider is: study the wiring diagrams, what happens if one pump is only installed? Any feedback to the EML or the MOTRONIC?
Can't find it on the MOTRONIC pinout, maybe o.k.
http://www.europeantransmission.com/Bulletin/DTC.BMW/lateversion_zf_pinout88-89.pdfShogun E32 Tech Tips: http://twrite.org/shogunnew/topmenu.html
1 pump works just fine, thank you
Bought a nice Y-connector for $2.99 at Kragen Auto. Connected the double ends to the fuel rails and connected the single end to the right side fuel line (the one connected to the remaining good fuel pump); all using some short sections of new fuel hose.
Capped off the left side fuel line using an extra vacuum cap from when I replaced the ones on the throttle bodies; fit perfect.
After warming it up I found some open road and did some hard acceleration and she behaved just fine, no lurching or sluggishness.
I won't leave it this way for long. The plan is to get a couple TrePerformance fuel pumps and retrofit them in the plastic holder in the fuel tank. I'll report on the when it's done.