2008 gmc sierra 1500 engine 4.3l v6 - Not starting - Question.

[Mike717]

Hello all, 

New to the forums here so I'll try to be as detailed as I can. My pops has an 08 gmc sierra 1500 4.3l v6 and Im trying to find the issue here of the engine turning over but wont start. It seems as if its deprived of fuel being a fuel issue potentially.

We went to auto parts store and got the pressure valve test kit for fuel and connected it to the tap in my screenshot with key on its 60PSI and while turning over 60PSI doesn't seem to be dropping so I think the pump is fine????

Check this screenshot out -
https://imgur.com/a/6rXTwdR

On the top of the throttle body where I pointed out the cap is where we tested from but in that boxed area we suspect this part may be at fault. Does anyone know what it is?
Also on the throttle body itself is a sensor on the left side and another just up top of that also pointed out. We've checked, cleaned and reseated the connections there no luck.

Its also notable that when the issue first started as he was driving a few days ago check engine came on and says "Reduced engine power" he did stall out several times before he crawled home multiple times.

It wont even start anymore, we did manage to get OD tool to check for engine light its code reporting is:
P0641 "5V Reference 1 Circuit" - only 1 code no others.

We think the little wire just up top of the throttle body where the test point was on the line is measuring something under that black cover perhaps its having the issue. Any feedback greatly appreciated.

Edited May 5, 2024 by Mike717
Added more relevant info

[asilverblazer]

If memory serves... that circuit will power an assortment of sensors. 

 

gmupfitter has wiring diagrams that I would look at to trace that circuit looking for faults. 

 

Without looking to see what all is on that circuit, I suspect one of them has cut fuel delivery. 

[newdude]

16 hours ago, Mike717 said:

Hello all, 

New to the forums here so I'll try to be as detailed as I can. My pops has an 08 gmc sierra 1500 4.3l v6 and Im trying to find the issue here of the engine turning over but wont start. It seems as if its deprived of fuel being a fuel issue potentially.

We went to auto parts store and got the pressure valve test kit for fuel and connected it to the tap in my screenshot with key on its 60PSI and while turning over 60PSI doesn't seem to be dropping so I think the pump is fine????

Its also notable that when the issue first started as he was driving a few days ago check engine came on and says "Reduced engine power" he did stall out several times before he crawled home multiple times.

It wont even start anymore, we did manage to get OD tool to check for engine light its code reporting is:
P0641 "5V Reference 1 Circuit" - only 1 code no others.
 

 

 

Its the 5V reference that is likely pulling something down and not letting you start is my guess.

 

Items on that 5V reference 1 circuit:

 

Circuit/System Description

The engine control module (ECM) has 2 internal 5-volt reference busses called 5-volt reference 1 and 5-volt reference 2. Each reference bus provides 5-volt reference circuits for more than one sensor. Therefore, a fault condition on one 5-volt reference circuit will affect the other 5-volt reference circuits connected to that reference bus. The ECM monitors the voltage on the 5-volt reference busses.

 

The 5-volt reference 1 bus provides 5 volts to the following sensors:

The manifold absolute pressure (MAP) sensor

The fuel tank pressure (FTP) sensor/front fuel pump sender assy

The air conditioning (A/C) refrigerant pressure sensor

The accelerator pedal position (APP) sensor 2

The engine oil pressure (EOP) sensor

The camshaft position (CMP) sensor

 

So the 5V reference will need to be checked at each component to see where it's being lost.  Most likely culprits (but still could be any above) would be the MAP, cam position, engine oil pressure, accelerator pedal and the low ref for the tank pressure/fuel pump.  Could be a bad component or just a bad wire or corroded terminal or wire.  

 

The diag below can be quick to jump to the ECM but don't just condemn that right off the bat.  Seen good techs get burned by the way GM writes their diag charts.    

 

Circuit/System Testing

Important: Additional DTCs may set when disconnecting components.

Important: A short to voltage on the signal circuit of certain components may cause this DTC to set.

1. Ignition OFF, disconnect the harness connector of all the sensors connected to the 5-volt reference bus for the applicable DTC. Refer to Circuit/System Description.
2. Ignition ON, test for 4.8–5.2 volts between one of the 5-volt reference circuits and ground.
   • If less than the specified range, test all of the 5-volt reference circuits for a short to ground. If all circuits test normal, replace the ECM.
   • If greater than the specified range, test for a short to voltage on all of the 5-volt reference circuits, or the signal circuit for each component associated with the appropriate 5-volt reference bus for a short to voltage. If all circuits test normal, replace the ECM.
3. Connect each component associated with the appropriate 5-volt reference bus, one at a time, while monitoring the voltage. The voltage should not change more than 1 volt.
   • If the voltage display changes more than the specified range when a component is connected, replace the component.

Edited May 6, 2024 by newdude