Every now and again, about once every dozen starts, the engine will catch and fire nicely but then immediately begin to stumble hard. It will recover quickly and rev to ~2000 rpm then stumble, then rev again. Eventually it settles out with no codes. A week ago it went into RP mode with a "random misfire" code for all 8 cylinders; I cleared it and it did not return. Last evening it fell into RP mode with a "too rich, both banks" condition.

Short and Long Term fuel trims look fine as does the Air/Fuel ratio. The air filter is clean and the intake free from leaks.

Clearly something is not happy. There isn't a smoking gun yet but I'm thinking of things that are common to both banks. Mass Air Flow Sensor? Failing fuel pump?

Any ideas from the experts here would be appreciated!

Thanks...

 

I had a similar issue, I cleaned the MAF with proper sensor cleaner and it transformed the car. May be worth a go for the cost of cleaner & 5 minutes of your time.

 

Agreed, I cleaned the MAFS this morning. I'll take it out tomorrow and see. Thanks