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

[David Allen]

  What was the last set of modifications you mention? 
  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?  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 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!
David



----- Original Message ----- 
From: "Tomas J. Sokorai Sch." <tsokorai at minimania.org>
To: <gmecm at diy-efi.org>
Sent: Friday, August 25, 2006 4:58 PM
Subject: [Gmecm] Strange effect while trying to tune my GM-EFIed beast


> I'm in the tuning stage of the conversion from a mechanical FI '82 I6 BMW  to 
> a '727 ECM with $8D code and DIS ignition, and last set of modifications gets 
> me the following strange effect:
> 
> The engine runs cold very well, a little rough, but pulls strongly and at any 
> RPM.
> But when it gets near its working temperature, it starts to have a strange 
> cut-off at exactly 2000 RPM. It feels exactly like a hard revlimiter.
> The harder I put the foot down, the more pronounced is the cut/resume effect.
> 
> See http://www.xperts.cl/temp/revlim_problem.png
> For a screenshot of a datalog I'm slowly trying to get the 2000 RPM and it 
> starts jerking. Then I try to put a bit more throttle, and I get only harder 
> cut/resume cycles.
> 
> At the strong oscillation, I get some loud "bang"s on the exhaust too.
> 
> Any ideas of what is happening?
> 
> -- 
> Tomas J. Sokorai Sch.
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