observations regarding FQS setting effect + more troubleshooting
 

A couple months ago I decided to experiment with running the FQS on the DME at 3% leaner. It seemed to run "maybe" a little stronger with no noticable change in fuel economy. I figured maybe 3% really wasn't very much change until a couple days ago I decided to run with the O2 sensor unplugged to troubleshoot yet again a glitch I've been noticing going on forever and ever. The glitch was still there (YES!, at least it narrows down the problem to not being caused by the O2 system), but I definitely noticed a change in the overal engine power behavior and a noticable increase in fuel economy. So I think I have found out what I've been wondering for a while, which is that the FQS only changes the base fuel maps and does not change the way the O2 sensor compensates. It still aims for the standard mixture regardless of the FQS setting. If anyone knows otherwise, please correct me.

As far as how it drives with 3% less fuel, it can definitely feel kind of flat at part throttle until it fully warms up, but once it does, it generally feels nice and strong. I especially like the leaner WOT maps. It really pulls hard on them and I don't notice any detonating, etc especially since I always run premium fuel. The only thing is that it also seems to make whatever my problem is that was the reason I was unplugging the O2 sensor to begin with more obvious. With the FQS set normal, or set for 3% less fuel and the O2 sensor plugged in, the engine feels like the mixture is unstable under part throttle. I can feel the power surging up and down slightly, but seems to run better at WOT. With 3% less fuel and no O2 sensor, it is much more obvious, but when it stabilizes, it pulls nice and strong. So I don't know if it only runs well when set at -3% at part throttle because there is this underlying problem richenning it, or that having -3% only causes the potential intermittent leanness to be even leaner causing the surging to be more noticable.

I think I'm just going to break down and order the lindsey silicone vacuum line kit to rule that out for sure if there are any leaks. If that doesn't fix it, could a bad fuel damper or pressure regulator cause an inconsistent mixture? Or maybe there's a bad solder joint in the DME itself. Or maybe the wiper arm in the AFM isn't putting enough pressure on the resistor strip causing an inconsistent readout only under the vibrating conditions of running, since it did check out fine at the DME connector. Or maybe the injector and/or AFM wiring harness is flakey and acts up only while the engine is running. I've pretty much fixed/checked/replaced everything else and I'm convinced it is capable of running smoother.

Does FR Wilk state on his website what the Euro FQS settings are? Was the mixture the only tweak you made? There's provision for timing too. This is interesting to me 'cause I've got a spare DME that I may mess with. My O2 sensor is dead and I've just been running without it, but I sure notice how rich the motor runs.

From the FrWilk page, the only specificly euro DME listed is the version 1.1 and the FQS settings are the same as the late US DME.
http://www.frwilk.com/944dme/fqswitch.htm

I did not change anything except setting the FQS to position 2 for -3% fuel. There are settings to retard the ignition, but (unfortunately) not to advance it. The ROM chip has to be reprogrammed to do that. I'm currently getting ~25-26 mpg with -3% fuel and O2 sensor unplugged compared to ~23 mpg with -3% and the O2 sensor plugged in. Strangely enough, I never noticed a drop in fuel milage with the O2 sensor unplugged and the FQS set to the stock setting.

I think I'll try this. I've got an early car/DME so the adjustment for me would be to -4% rather than -3%. SHouldn't make too much difference. You're right - it's too bad we can't advance the timing. Didn't BHolmes on this board try that by setting his cam ahead one tooth? I seem to recall the result was less than desirable. I ran one tooth back by accident once...

Running one tooth forward or back only changes the cam timing and has no effect on ignition. Retarding the cam shifts the power toward the top end and advancing shifts it down.