P38A engine stalling at idle

[DT-P38]

Ok, how do you know that?

[adm333]

Ummmm my engine was running like a dog on gas and it was because one oxygen sensor was shot

[glenhendry]

Having said that, the throttle positioning sensor can cause that and also a loose or faulty MAf sensor cable can cause this also. There is a way to test this by putting the MAF sensor into limp home mode. My mechanic did this, funnily enough it ran better than with the MAF sensor functioning. I'm not sure of the procedure on doing this, maybe someone else can shed some light on this method.

My handheld diagnostic confirmed for me that the MAF and TPS are both returning valid values in real time.

I dont have gas.
But i thought that the RR will/should start on Petrol warm upto 50c then switch over automatically to gas.
Not sure if you have a non injected gas system?
Could check your air filter ignition leads and your fuel filter under the drivers door.
If you havnt done the fuel filter in a while i would look at this any way (watch the pressure in it if you take it off)

I have a non-injected system. It doesnt change over automatically. I have a fully manual switch. Air filter is new, ignition leads and near new (but I will check) and I cant find the fuel filter under the drivers door. I will have to look harder or in rave.

Glen
Get the oxygen sensors checked. If they aren't right they can really mess with the engine. The gas system uses the readings.
Ask me how I know that ....
Dave

Well, the handheld ODB diagnostic says that 2 of the four are working, but the values dont seem to be obviously correct. Its strange voltages like 0.4v etc. Anyone know what the valid range is for O2 sensors?
The OBD standard allows for four O2 sensors for the NAS cars presumably, but I only have 2 sensors.

[wayneg]

From memory there is no fuel filter on the Thor engine cars, Correct me if I am wrong.

[RR P38]

From memory there is no fuel filter on the Thor engine cars, Correct me if I am wrong.

Fuel filter is under the drivers seat inboard side of the chassis rail.
This is a much neglected filter on a P38.
If you change it be aware it is a pressurised line, you will spill fuel.

[bee utey]

My handheld diagnostic confirmed for me that the MAF and TPS are both returning valid values in real time.


I have a non-injected system. It doesnt change over automatically. I have a fully manual switch. Air filter is new, ignition leads and near new (but I will check) and I cant find the fuel filter under the drivers door. I will have to look harder or in rave.

Does your gas system have an O2 sensor emulator when running on gas? It passes a slow square wave to the petrol ECU to prevent it from self learning incorrect mixtures when idling/driving on gas. It allows richer idling (open loop) on gas to reduce backfiring on take-off, without the petrol ECU ending up too lean to run properly.

ell, the handheld ODB diagnostic says that 2 of the four are working, but the values dont seem to be obviously correct. Its strange voltages like 0.4v etc. Anyone know what the valid range is for O2 sensors?

Approximately 0.1V lean, 0.9V rich. 0.45V is the happy medium but it will vary anywhere from 0.2 to 0.8 while the petrol ECU/gas computer adjusts fuelling in closed loop operation.

[wayneg]

Fuel filter is under the drivers seat inboard side of the chassis rail.
This is a much neglected filter on a P38.
If you change it be aware it is a pressurised line, you will spill fuel.

Are you sure on that? post 99 cars? Cant see that on mine, have you a pic. There is also a thread about this and the conclusion was no filter except in the tank. The 99 on tank sender / pumps are different to earlier cars

[glenhendry]

I agree Wayneg, I can see no fuel filter that is easily replaceable in my 2000 RR.

[PaulP38a]

Correct... 99+ models have the fuel filter in the tank with pump... WTF were LR thinking? Perhaps the end of servicable vehicles to the dawn of disposable vehicles?

I spent ages trying to find the fuel filter on the chassis rail of my '99 until someone here pointed out the error of my ways.

Cheers, Paul.

[glenhendry]

Does your gas system have an O2 sensor emulator when running on gas? It passes a slow square wave to the petrol ECU to prevent it from self learning incorrect mixtures when idling/driving on gas. It allows richer idling (open loop) on gas to reduce backfiring on take-off, without the petrol ECU ending up too lean to run properly.

Approximately 0.1V lean, 0.9V rich. 0.45V is the happy medium but it will vary anywhere from 0.2 to 0.8 while the petrol ECU/gas computer adjusts fuelling in closed loop operation.

Thanks. Good info.

The O2 sensors definitely work, returning valid values, along with the MAF. However, I have discovered something odd. On petrol, the fuelling is happily reporting closed loop and everything is normal. When I switch to LPG, I instantly get a Pending fault on "no value from O2 sensors" which after some determinate time changes to a non-pending, permanent fault. If I then change back to petrol, I will remain on open loop fuelling (ie bad fuelling) until I clear those O2 faults. The annoying thing is, that the O2 sensors work fine, its just that my LPG system must ignore their signals (and/or block them from the Bosch ECU) and then I am stuck in open loop fuelling until I clear the faults from the ECU. If open loop is undetected, after a long time (days/weeks), the mixture gets WAY of of whack and I stink the neighborhood up with rich running black smoke, especially at idle.

So, for posterity, if you have LPG, make sure that you get back to closed loop fuelling after switching back to petrol by examining faults with an OBDII scanner.