"ECM after a tach signal from coil?"
Does this 6F truck have an ECM? If so, that would be news to me.
I have been under the impression that TWC and an ECM were introduced with the 7F models, and I may be wrong.
Was this truck built as a dual fuel truck?
We used to have some 6FGCU25 trucks in our fleet and none had TWC and an ECM, They all had the simple Aisan vacuum fuel lock regulator which had only 1 main fuel solenoid valve on it.
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