Lean Misfires at 2700-2800 rpm

[brattesani]

Originally Posted by DoctorDex

Welcome to the club. Hopefully since enough of us have the problem, one of us will eventually stumble on to a fix.

Badass: Have you made any inroads into the problem at your end?

telling ya, pull your safc and check.

[DoctorDex]

Originally Posted by brattesani

telling ya, pull your safc and check.

I will, I just need the time (and the right weather). I'm hoping to get the chance in 2 weeks time.

[badass3000]

Originally Posted by DoctorDex

Welcome to the club. Hopefully since enough of us have the problem, one of us will eventually stumble on to a fix.

Badass: Have you made any inroads into the problem at your end?

soon, weather is getting real nice down here

[DoctorDex]

With regards to the SAFC-2:

If I was to disconnect Red +12V IG1 power source and then join the pink and orange wires of the SAFC-2, it should effectively "disconnect" the SAFC-2 without having to undo/redo all of the wiring?

The diagram I'm getting this info from is here: http://www.stealth316.com/images/s-a...ng-diagram.gif

[brattesani]

yeah that looks ok.

[UCSB VR4]

Originally Posted by UCSB VR4

This ABSOLUTELY did the trick for me, except I unplugged the O2s without resetting the ECU (just to record any changes before/after resetting the ECU). Now, my car drives like an absolute champ, so I'm not even going to bother resetting the ECU... the idle RPM doesn't fluctuate up/down/up/down, the acceleration is smooth through the entire RPM band, and when letting off the gas in neutral the rpms step down controllably instead of bottoming out at 300-400 RPM (almost stalling the engine).

I don't suppose there are any ramifications to unplugging the O2s beyond freezing your fuel trims (which actually improves performance on my car). The only thing is that my ARM guage is practically useless now, but what a small price to pay, I never used it anyways.

I actually enjoyed driving to work today, the problem is gone.

- Jeff

EDIT: I wanted to also mention that the BRAND NEW ECU I swapped in had no effect on this problem, prior to the fix implemented above.

I wanted to get my ARM I guage working again, so I plugged in the rear O2 sensor and cut the wire going to the ECU. This gave me ARM I functionality but kept the O2 feedback from reaching the ECU.

After driving around for a day or so, I noticed my car was running a little bit richer than it used to... so rich that my ARM I was reading completely rich during cruise... not good. I leaned it out and noticed that the problem came right back, but not to the degree in which it did with the O2s plugged in... so I suppose driving around pig rich solves the lean misfire problem on my car.

I still continue my never-ending quest of modifying/toying with the ARC II settings. I have real problems getting the thing to run lean enough in 4th / 5th gear cruise, and at the same time getting enough fuel during WOT or hard acceleration in lower rpms. I ended up taking the "mid" click down 1 and compensating by setting accel +1 and high +2. This seems to keep it leaner during cruise but just as rich during hard accel. My current settings are here:

Low: 0
High: -2
Mid: -8
Accel: +4

Notes: Cruises lean in 1st-3rd gear, 4th gear cruise is stoich, 5th gear cruise is rich. "okay/good" acceleration/transition from low -> high rpm... lean misfire problem is hardly noticeable. Cannot rev up RPMs in neutral... runs way to lean (not that i ever do this anyways)

Old setting:

Low: 0
High: -4
Mid: -7
Accel: +3

Notes: Cruises rich in 2nd, 3rd, 4th, and 5th gear, "smooth" transition from low -> high RPM (probably due to running so rich). lean misfire problem isn't noticeable at all.

I guess what everyone can take away from this is that unplugging my O2s caused the lean misfire problem to subside 80-90%, but didn't cause it to go away entirely.

[DoctorDex]

Well, disconnecting the SAFC-2 didn't work. If anything it made the problem worse.

Now thats it gotten colder out, I've noticed the rpm range where it misses has moved up in the rpm range a bit. Now it seems to miss most between 2800 - 3000 rpm (whereas before it was 2600-2800).

As an experiment, I left the SAFC-2 hooked up, but with the AF signal disconnected. After doing this, it would bog badly in the 2800 - 3000 rpm range @ part throttle. Upon entering the 2800 - 3000 rpm range, it would start to miss and if i held it at that rpm the missing would quickly get worse and worse and my AFR would eventually hit 18+. It almost seems like once it hits 2800 - 3000 rpm, unmetered air just floods the engine. However, I know this isn't true. I have no boost leaks and I even tried blocking off my PCV system today and it had no effect whatsoever. I suppose I could try blocking off my clutch/brake booster, but I highly doubt this will do anything. This has got to be some sort of fueling issue.

Oh yeah by the way, I pulled the ETACS unit and there are caps inside, but they aren't rubycon caps. Therefore I didn't change them because they looked good.

[DoctorDex]

Made some interesting observations today.

I had previously examined logs to see if there was any change in the air flow (AF) readings and it wasn't appreciable on the logs, but tonight was a different story. Rather than looking at logs & graphs, I watched the AF readings in real time on the scanmaster. The AF readings were taken from a GM MAF, running in blow through, which means the MAF is situated after the turbos, ~ 1 foot from the throttle body. Here is what I noticed:


While cruising in 5th gear @ < 2500 rpm - AF readings oscillated at most +/- 10Hz and they oscillated/changed very slowly, if at all, and there was no missing. Same thing while cruising in 5th @ > 3200 rpm --> smooth progressive change in AF readings.

Then I cruised at in the 2600 - 2900 rpm and the AF readings became very erratic, oscillating usually +/- 30Hz and up to +/- 100Hz. The large oscillations seemed to coincide with the missing.

At first I thought maybe it was EMI interfering with the MAF, but then I noticed that my blitz DSBC electronic boost gauge was seeing large fluctuations, coinciding with the large pressure spikes. I also noticed similar spikes on my manual boost gauge.

I had noticed similar pressure fluctuations last winter, but at that time I thought it was because the large pressure spikes were coming from the exhaust through my EGR. I have since eliminated the EGR completely, so the EGR definitely is not the cause of the spikes.

So, what is causing the pressure spikes? I'm not really sure. The only theory I can come up with is that the spikes are coming out of the intake plenum and there must be some sort of resonant frequency that occurs in that rpm range where the pressure waves are hitting each other and causing the sensation of missing.

What I can't explain is why do some of us have this problem and some of us not?

[ProwlerGT]

Couple of maybe new ideas, but first..... where is your mods list? Can you add that to your sig? I think that would help others help you.

1. Since the problem seems to change RPMs (raises) due to colder air then how about we focus on the following: BAROMETRIC PRESSURE SENSOR, Engine coolant temperature sensor, and intake air temperature sensor - You need a test harness at the ECU connection point. See about obtaining a broken ECU and harness with at least 1-2 feet of wire on it from a junked 3000. I've never done this, but this is how I would. Remove the connector from the broken ECU and attach it to the test harness with the wiring (take your time and make it a good setup). Find the pins/wires that are responsible for the aforementioned sensors and remove the sheath on the wiring just enough to clamp some alligator clamp jumper wires on - hook that to a multimeter. Once done plug it to your car's harness and ECU connection point and observe the voltage during the trouble RPM range for each sensor 1 at a time.. not all at once (1 at a time to reduce risk of arcing during testing; electrical tape over the spliced wire areas when not in use on the test harness). Know ahead of time what voltages stand for what temperature or pressure and jot down what you see. If the voltages are ACCURATE then this leads me back to the ECU for electrical.
Do you have any ground wiring issues? Can you test your ground and see if its good? Did you ever get the chance to swap ECU into your friends car?

2. Is there a way to trick your ECU into thinking its running at 200 F when its cold? Curious to see what the ECU would do if it already thought it was warmed up. Maybe this isn't a safe test for the engine though. Thoughts?

3. Variable Induction Control Motor and Induction Control Valve Position Sensor - Have you tested these? Very dirty valves??????

4. Turbos - I don't know if you've upgraded beyond stock Turbos or not, but perhaps it happens on some people's cars (mind you I still think its the ECU) and not others cause its a physical/mechanical wear issue. Is it possible to run the engine without Turbos connected or would that not work at all? Maybe 1 or both turbos have seen their life.

5. Remove your exhaust - all resonators, cats, precats, or mufflers - See if you still have the problem. This is just a WILD shot in the dark.

6. What is your alternator voltage during the misfire?

[ProwlerGT]

Thinking out of the box as I try to do I went and did a Google for this problem on other cars. Sometimes I tend to think that only OUR cars experience ODDBALL issues cause of their age and rep amongst others, but the fact of the matter is they are still machines based on basic principles of other machines that do the same thing.

Here is some NEW stuff I found elsewhere on a BMW forum and a Camaro forum:

Valve float
Valve dance
Flat cam
Valve train clearance problem

http://en.wikipedia.org/wiki/Valve_float

http://www.youtube.com/watch?v=_REQ1PUM0rY

- valve float video (very interesting)

Quoted: "just plulled intake off . looks like a valve train clearance problem - was rocker arm to retainer bind . was using long slot stamped steel rockers
& 1.440 springs ( bad choice ) . time to invest in a real set of rockers arms
bearings gaskets cam & etc ."

Quoted: "There are two areas that cause this on the dyno when running in new engines. Ignition, 95% of the time, and flat cam, 5% of the time."

The above information that fixed the problem was for the Camaro. I posted a thread in the Beamer forum and apparently they are just as lost as we all seem to be. I gave them some hope though and told'em that we've been chasing this problem with our cars for YEARS.