Intermittent Rough Idle / Gmc Safari 2000

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Good day all. I have been getting an intermittent rough idle on my cargo van.

 

Vehicule info : 2000 GMC Safari Cargo – 4.3 L / 106 000km (66 000miles)

 

On an intermittent basis, maybe each month or so, the engine will “almost” die once warm ( in closed loop).

 

At that point, it is obvious the engine is not burning fuel properly – there is a very bad smell and the engine hesitate seems flooded.

 

At times, shutting down the van, and starting up 5 minutes later would make the problem go away.

 

Most of the time p0300 would come up:

DTC for which Freeze Frame was Stored P0300

Fuel System 1 Status Closed Loop

Fuel System 2 Status Closed Loop

Calculated LOAD Value 20.00 %

Engine Coolant Temp 188.60 °F

Short Term Fuel Trim - Bank 1 50.00 %

Long Term Fuel Trim - Bank 1 25.00 %

Short Term Fuel Trim - Bank 2 -4.69 %

Long Term Fuel Trim - Bank 2 -14.06 %

Intake Manifold Absolute Pressure 20.08 inHg

Engine RPM 1653 rpm

Vehicle Speed Sensor 46.60 mph

Air Flow Rate Mass Air Flow Sensor 4.52 lb/min

Absolute Throttle Position 25.10 %

 

The last time it happened, it was solid, the check engine showed :

P0300 – Random Misfire

P0171 - System Too Lean (Bank 1)

P0135 - O2 sensor heater circuit malfunction (Bank 1 sensor 1)

 

Once the spark plug removed, the bank 1 (driver side) spark plugs, where completely black.

Of course once replaced the intermittent problem was gone.

 

My theory is that something is causing bank 1 to show the ECM that the cylinder are running LEAN therefore telling the computer to pour more gas in it (make the air/fuel ratio richer).

 

The voltage of the o2 sensor would drop dead to 0v when all of this is happening.

 

I have replaced both O2 sensors (BANK1 upstream) and the downstream O2 sensor.

 

Since then, it didn't do it yet (the rough idle), but it feels like it going to do it again.

I find the idle, is not that great, but it does not hesitate when stepped on.

 

I have replaced coil and spark plug wiring, spark plugs, caps, rotor, coil @ 100 000km (60 000miles approx), all GM/Delco parts

 

What could cause only the BANK1 cylinders to run rich and cause black spark plug – other than the o2 sensors ?

 

Thanks in advance.

Ben