Tuning around a mechanical problem.

[mistermike]

Is a waste of time.

After a record amount of downtime, I finally got my car running with the new fuel system. I have had some idle transition issues which evolved into a severe roughness that set in abruptly at about 60 kPa and higher. On and off like a light switch above and below that vacuum, any RPM, any throttle position. After a while, I noticed that my STFT on only one side was jumping to over 20% (LTFT is disabled) whenever this roughness kicked in. Hmmmm. Loss of spark? Injector cutting out? O2 sensor gone mad?

Well the other day, I decided to put it in open loop to do some VE logging and ran the gamut of RPM ranges as much as road conditions would allow. After a 5000 RPM 3rd gear romp, the problem was no longer confined to above 60 kPa. It sounded like elves were pounding on my motor with a mallet, and I got heaps of knock retard every time I breathed on the throttle.

After a tow truck ride and some wrenching, it turns out that my #6 Scorpion exhaust rocker was broken. Moreover, almost all of the other rockers have stress cracks around the pushrod cups. Evidently, this crack was widening or doing some kind of dance above a certain vacuum before I pushed it over the edge with my right foot.

Apparently, not every running issue is a tuning issue LOL.

[joecar]

Mr.Mike, I agree... mechanical/hydraulic/electrical soundness before tuning... good thing you found the problem before any major damage occurred.

[mistermike]

Update.

Sometimes more than one problem exists simultaneously. After replacing the rockers, the running problem remained, only now it was occurring at 5 inches vacuum rather than 10. The only other change was tossing in a different set of plugs at a different gap. This tended to indicate that the problem was spark related. I had been running an expensive set of aftermarket coils to handle the "severe load" imposed by a supercharger. In this case, severe load turns out to be 10 inches of manifold vacuum, because going back to the stock coils cured the problem. Evidently, bigger and redder isn't betterer.

[5.7ute]

Did you use those crappy MSD coils? I havent heard anything good about them apart for the bling factor.

[mistermike]

Yep. Bling!!!