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LLY wont idle / runs rough

141 Views 1 Reply 2 Participants Last post by  Ron Nielson
2005 LLY around 350k miles..
Within the last year or so it would randomly (every few thousand miles) the engine would fuel knock hard (like rail pressure was sky rocketing) and the low oil pressure light would come on and oil pres gauge would read zero. Sometimes truck would stall sometimes not. It would only happen for a second or two then be fine back to normal for another few thousands miles. No check engine light ever.. would start back up and on my way.

Now truck won't idle without throttle. oil pressure reads zero. No check engine light... I suspect it's a 5v reference shorting to ground .. probably a p0642 (I think?)
Does the fuel rail pressure sensor and oil pressure sensor share a 5v reference? Any other sensors on that 5v reference? Is the fpr on that 5v ref? My other forum searching rabbit holes have suggested cam / crank sensors and fuel level sensor / air intake heater possibly on same 5v reference? Does anyone have a concrete wiring diagram on which sensor share a 5v reference between oil pres sensor and fuel rail / fpr

According gm upfitters ecm gives oil pres sensor a 5v reference from a grey wire at ecm j1-34. Fuel rail sensor has a 5v reference from j2-48 tan wire.

Drives fine down the highway...

Any idea where to start?


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The ECM provides a 5V reference to several different sensors but all come from the same internal source within the ECM, think of an electrical bus with several outlets. If you tested all outlets at the ECM, the voltage should be the same
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