BF Falcon V8 Boss 260, Missing at idle, bad fuel trims P0171


Ford Falcon BF XR8 5.4L 4 valves per cylinder 2008


The vehicle drove into the workshop running the vehicle was running ok but not smooth, better when cold, than when warm, hesitation when on take off.

Diagnosis: 2 hours

To start of the diagnosis, The Scan tool was used to look at the fault codes and the live data to see if the problem was at a particular RPM speed, it was found that at idle when the o2 sensors warmed up the fuel trims on the Bank2 would rise all the way up to +15%, not really enough to log a fault but there is clearly an issue, the RPM’s where brought up the 3000Rpm’s this made the fuel; trims come into the +4%

The fault codes where checked: 

  1. P0171 – system too lean bank2 (above max threshold)

Due to the low speed bank 2 lean mixture it was a good idea to start with smoke testing the intake manifold, there was leaks from the PCV lines and Evap Lines at the quick clamp fittings, as a temporary measure they had been tapped up and rechecked the fuel trims at idle had then lowered from +15% to +11% max, Which is good but still not enough to cause the issue and did not solve the imperfect idle.

the spark plugs and ignition coils where checked all spark plug gaps correct and 6 new coils where found, all coils outputted 30KV with out earth leakage. GOOD.


The next test to conduct was an In-Cylinder compression test, the test was performed, from bank 1 to bank 2.
this showed that the cam shaft timing chains where worn and that cam timing was advanced which meant the Bank 2 that was lean was pushing exhaust back into the Cylinder.


The repairs started by removing the plenum from inlet manifold replacing the quick fitting swivels with new and installing new inline quick fitting,

The rocker covers where removed,
fan belts, belt tensioner , radiator fans and coolant drained, upper radiator hoses and steel tubes removed and front timing cover removed.

Once the Crankshaft is TDC Cylinder 1 From here it does not move.

Timing Chain Slack

Looks like bank 2 timing chain is advanced a whole tooth, this is due to chains wearing out.

Exhaust cam shaft timing mark correct

bank 1 Intake camshaft mark correct

Ford Falcon BF V8 4 valve Boss 260 Timing Chain Marks


A full timing chain kit was installed including new sprockets and guides including new timing chain tensioners.
the timing covers and rocker covers where washed and reinstalled, all other parts refitted in reverse order, coolant refilled.

The engine was then started, The engine ran smoothly, the scan tool was then setup and fault codes cleared. the Oxygen sensors where then mapped out when warm to check if the engine was running in closed loop, the vehicle was then test driven.



A quick way to check to timing of the engine is to carry out a cylinder compression test running. this is a great way to find the fault and prove the fault is gone after repairs have taken place.

The vehicle had never ran this well the whole time the client has owned the vehicle.


Link to an in depth over view of repairs carried out and Installation guide.