P38A engine stalling at idle

[glenhendry]

I have a 2000 4.6 P38A with 230kkms. It is dual fuel/LPG. It's blue ;)

It has been running and starting fine on both LPG and fuel for 6 months. In the last few weeks the cold idle has degraded to a point where it now idles dangerously low for a few seconds, surges occasionally and ultimately stalls a couple of times until it is warm unless I give it some throttle. This seems to happen on fuel and LPG. It is obviously noticable because I also start it (using key only, no throttle, every time I drive it and wait patiently for a minute or two before I drive it). ODBII shows no fault codes.

I was thinking I would crack open the idle stepper and clean it out and also the throttle body and air inlet passages. Any other wise suggestions?

I also flick quite often between LPG and fuel. Is this a bad thing? I am nervous of backfires popping my new MAF, so I never climb hills or overtake on LPG, but cruise on highways and around town no problems. Often I start and finish on LPG - maybe this is screwing the fuel maps?

[RR P38]

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?

I had a similar problem with my idle some time ago so i replaced the TPS throttle position sensor.....no improvement.
So i pulled the stepper motor out and it fell apart in my hands/ unrepairable it was.
I got a new one fitted/ they have got a setting that requires them to be adjusted some how.
Any how long story short i had some continuing idle problems/minor.
I took it to a specialist LR w/shop and now she is fine.
The stepper was original 14yrs old and i had 260k km on the clock.

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)

[drewandkate@iinet.net.au]

We recently had a similar problem with our 1995 R.R HSE stalling at idle on the odd and becoming more frequent occasions.Our HSE has been gas injected for 700 hours and im not sure if it is related to the gas, but it has had stalling issues since the gas was fitted and not prior.I asked my mechanic mate what an idle speed motor does and mentioned my idle issues and he suggested we clean the throttle body and idle speed motor, so far no stalling.

[It'sNotWorthComplaining!]

It could also be as simple as a vacuum hose coming off or leak at the rear of the motor, the one that goes from the throttle intake to the rear of engine.

[wanglemoose]

i have the opposite problem with mine since the new engine, it idles at 1800rpm untill it warms up and then attempts to idle between 650 and 1100 (cam lope causing the warm problem).
not very helpful i know but interested to try and nut it out.

[redandy3575]

I had that issue when my fuel pump was failing, and due to the low pressure in the fuel manifold, the engine was starving fuel. 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.:)

[jsp]

check the big orings in the air intake pipe are still in one piece or even there at all.....

[PeterH]

I would tackle the simple stuff first before delving deeper.
My '89 classic had idiling problems pretty much exactly as you described. I cleaned the throttle body with throttle body cleaner and guess what? Instant fix! Hope that's all it is for you.

[DT-P38]

I have recently had similar problem of low idle (on Gas only) after a little hot start backfire. Was there anything came of this as far as the fix for the original problem?

[adm333]

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

[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? :twisted:

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.

[Hoges]

Hi Glen, just a wild thought:

In an earlier post you implied that the problem occurs when the engine is below operating temp : so, given that the vapour point of LPG is lower than for petrol am wondering if you are getting ice buildup at idle due to the generally high relative humidity of SE Qld.

Also, the LPG system is not a sequential fuel injection type?

Got me thinking about problem icing which can occur with the carburetor on piston engined aircraft, e.g. throttle reduced on descent in high rel humidity conditions... solution is to pull on "carby heat" which directs warm air into the induction system thereby reducing the relative humidity of the air supply.

With the Thor setup on LPG, does it stall when the engine is at full operating temp?

Given it's not gas injected, perhaps after running rich there might be some other form of metering device which needs cleaning hence the gradual deterioration in performance?

cheers

[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 certainly does seem to pass this emulated value to the engine ECU, but the OBDII shows a solid 0.45v as soon as I switch to LPG. That doesnt seem square to me :eek:

Is this a correctly emulated value? It doesnt stop the ECU from throwing a code, as 2x O2 sensor faults show up as soon as I switch to LPG and then it reverts to open loop fuelling (when I run next on petrol) until I clear the faults. It seems like the LPG ECU is not clever enough to fool the engine ECU. After many minutes on LPG it also throws 8 injector faults.

FYI: I have the SGA299 self learning feedback system LPG ECU (by AEB?).


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.

I do notice the O2 sensors flipping between these values nicely on petrol, 0.1 on throttle and 0.8-0.9 on coast. On idle it alternates. So I suspect the O2 sensors are fine.

[bee utey]

Find out where the oxygen sensor feed is cut, reconnect the sensor straight to the vehicle ECU and disconnect the emulator output. This way you rely on the gas processor functioning in closed loop and cycling the mixture. Straight 0.45V is useless as an emulation value. It should mimic petrol operation.

Find the injector cut wiring, solder a 68 ohm 1 watt resistor between the 12V in and out wires. This is quite adequate in my experience to allow a small diagnostic current to flow without opening the injectors on sequential injection, including D2's. Don't ask me where exactly these wires are, I don't like the AEB system and won't use it. You should be able to identify the injector cut wires where they cut into the main loom's positive injector feed, brown with orange stripe wires.