Running problems with Ford vehicles fitted with LPG can be tricky, especially for those who don’t do a lot of work on LPG vehicles.
This diagnostic article attempts to help workshops that may not be familiar with issues in AU, BA, BF and FG Ford Falcons.
The problem vehicle is a Ford Falcon fitted with factory LPG using the Vialle converter venturi-based systems.
These diagnostics also apply to older Ford factory Tickford LPG systems where the balance line is in a different position to the vehicles featured in this story.
- Most symptoms can be detected within seconds using a OBD scanner/software.
- Stalling at idle
- Stalling on an overrun going down a hill prior to coming to a stop
- Stalling during low speed cornering.
First, do a basic vehicle inspection, drain the LPG converter and clean the idle jet.
If these check out OK, with the vehicle running, connect the scan tool and bring up the parameters PIDs, bring up the on board diagnostics parameter IDs (codes used to request data from a vehicle) and do the following:
- Check for engine codes
- Check that the 0 sensor is cycling with the engine on and at idle
- Check the LPG stepper motor count (normally 150 to 175 at idle and similar at 1,500 RPM).
The 0 should remain cycling while the 2 engine is at idle.
The oxygen sensor should have 0 to 1 oscillating voltage if it is working OK. The stepper motor count should be about 150 to 175 at idle if all conditions are OK.
If the engine is running lean, the 0 sensor 2 will be showing lean around 0.1 V and the stepper motor count will be lower than 150 steps.
Now check the PCV (positive crankcase ventilation) valve that can often be damaged by an engine backfire.
Check for air leaks within the intake manifold system, from the air box through to the manifold gasket. If the fault is still apparent, test drive the vehicle with the scan tool hooked up and record the stepper motor count.
If the mixtures go lean as indicated by the 02 sensor being below 0.1 V and the stepper motor count is ramping lower to (say) 140 when the engine stalls, it may be a split or a hole in the balance line.
To check the balance line remove it from the air filter end and, using a smoke machine, check for leaks but use only very low pressure.
Alternatively just seal one end of the flexible line and blow in the other. There should be no pressure loss.
Due to the design being like a convoluted tube, complete removal and a thorough visual check for splits must be done.
If no splits are found, the sensitivity diaphragm in the converter may be split. This pressure balance system must be completely sealed.
An easy way to help diagnose this is to test drive the vehicle in a safe area, accelerate up to 40 to 60 kmlh and shift the gear to neutral while coasting.
If the engine stalls it is most likely to be a balance line problem.
The balance line must be completely sealed as the incoming ram air is required to draw the correct amount of LPG through the idle circuit.
Note: Stepper motor count- the higher the number, the richer the mixture. The lower the number the leaner the mixture.
Car diagnostics software like TOAD can help diagnose common engine issues.