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andyh1956 · 01-29-2018, 10:43 PM

If your truck still has EGR if it sticks open the manifold vacuum will fall & when the engine goes to idle the MAP will see out of parameter condition & set a code.
The spark timing code is set because the low MAP makes the ECM think you're going up a big hill at wide throttle & it tries to adjust the spark for it. But the engine is idling & so now the rest of the parameters it needs- Engine RPM, TPS, VSS don't match the low MAP & so the ECM just gives up & sets the SES light.
We went through this with our '88 K5 a long time ago.
Get a Vacuum Gauge & hook it to manifold Vacuum & mount it where you can see it. Watch as you're driving normally & you can have a value to compare to when the engine goes haywire.
Oh yeah if you aint already done it, get all those grounds on the front of the intake onto fresh wire & fasten them to the stud the neg batt cable fastens them to.

01-29-2018, 10:43 PM	#4
andyh1956 Registered User Join Date: Mar 2010 Location: thomastown mississippi Posts: 1,261	Re: More TBI weirdness If your truck still has EGR if it sticks open the manifold vacuum will fall & when the engine goes to idle the MAP will see out of parameter condition & set a code. The spark timing code is set because the low MAP makes the ECM think you're going up a big hill at wide throttle & it tries to adjust the spark for it. But the engine is idling & so now the rest of the parameters it needs- Engine RPM, TPS, VSS don't match the low MAP & so the ECM just gives up & sets the SES light. We went through this with our '88 K5 a long time ago. Get a Vacuum Gauge & hook it to manifold Vacuum & mount it where you can see it. Watch as you're driving normally & you can have a value to compare to when the engine goes haywire. Oh yeah if you aint already done it, get all those grounds on the front of the intake onto fresh wire & fasten them to the stud the neg batt cable fastens them to.