[Gmecm] Strange effect while trying to tune my GM-EFIed beast

[Tomas J. Sokorai Sch.]

On Friday 25 August 2006 18:17, David Allen wrote:
>   What was the last set of modifications you mention?

Uhh ... hard to remember... I'm a bit "non-methodic" when doing table & 
consts. changes: sometimes I get into a "change everything" frenzy  :)
The main were a bit more of fuel (VE lower and upper tables) and a bit more of 
advance. I also played a bit with the DFCO parameters, but I only increased 
the RPM threshold, because before that I was getting engine stalls on 
downshifts on long decels (manual tranny).
The strange thing is that before my last modification session, the engine ran 
at any RPM, but very anemic at high RPMs.

>   Have you scoped the "reference" signal from the DIS module to the ECM? Is
> it faltering away over 2000 RPM? What kind of crank trigger are you using? 

I'm using a "frankenstein-sensor" ;) it is a VR sensor from the TDC diagnostic 
setup BMW put on these old k-jetronic engines, but the signal was too low, so 
I modified the sensor a bit stacking three NdBFe magnets on the back to get a 
higher signal level.

> Could it be gaining resistance as it warms up?  If it's a VR sensor
> (magnetic pin with a coil winding) then the signal is usually dirtier at
> high-revs. Anything interfering with it would be worse when hot (more coil
> resistance) and at high-revs (dirtier signal). Have you verified the
> polarity of the crank trigger coil with relation to the DIS module? To

I checked the polarity very early on my adaption phase: with the switched 
polarity, it barely runs at any RPM :)

> isolate tuning versus hardware causes, you could replace the temp sensor
> with a fixed resistor.  This resistor would give a low "temperature"
> reading which corresponds to the actual engine temperature reange where the
> engine performs properly.  If the engine runs right when fully warned up so
> long as the ECM is being told it is not warmed up, then it is a tuning or
> code issue.  If  the problem persists, then it is a hardware issue and not
> a tuning issue. Hope this helps!

Thanks!.
I'm getting the feeling it is a too big advance, but the strange thing is that 
instead of knock or backfire, I get a complete missfire and even an exhaust 
bang! 
I don't think it is the crank sensor because before my last changes it was 
running more or less smooth at any RPM. 
Now I need to narrow down exactly *what* in my modification-fest gave me that 
result.

Well... it was fun trying to drive the car across the whole city with a 2000 
RPM limit ;) 

-- 
Tomas J. Sokorai Sch.