2000 4.6 running bad on petrol, good on LPG

[glenhendry]

The other day I noticed RR was a bit smelly. It reminded me of the early days when I got it when the O2 sensors were out of action. I noticed via OBDII scanner that the MAF was reading zero, so I threw another one in. It shows realistic values but somehow its now much worse. It starts and idles, but as soon as I touch the throttle it instantly flounders badly, it never stalls, ever, but you cant even lightly touch the throttle. Attached are the EMS captures from Faultmate. O2 sensors and MAF seem to be providing valid values.

I can switch to LPG once its the engine is warm enough, and it instantly switches to beautiful and strong running across the rev range. If I switch back to petrol and I get dog running again. Doesnt that prove the intake system, the CPS, etc and many other things? What exactly is in play (and failing) on petrol and out of play on LPG? Fuel pump, injectors, fuel filter?

I put premium petrol in to make sure it wasnt low on petrol and it didnt help. I do have one additional very interesting fact, if I clear the O2 fault codes on the EMS, it runs on petrol much better for a while, so I know the O2 sensors are involved. I also notice on petrol that the short term and long term trims are all high (~20) so I think the EMS is trying to add fuel. This is supported by the fact that the o2 sensors seem to be sensing lean, as the value sits very close to 0v hovering around 0.05v.

Have had troubles in the past inserting pics, so also they are linked here (https://picasaweb.google.com/105878078807082914833/DropBox?authuser=0&authkey=Gv1sRgCJKGgoDg9pOdDA&feat=directlink).

[dragcharger]

Have the same problem, only worse as cannot use petrol at all. Seems to be starving for petrol.

[glenhendry]

I now believe that its the MAF. I believe LPG ignores the MAF sensor too. I did put in a MAF that shows changing values in the OBDII scanner, but I dont think the values are correct. I am buying a new MAF from Island 4x4 and will advise. Anybody in need small items from Island 4x4 can go in with me for postage reduction, advise by COB today. I will send express post from Brisbane when it arrives.

[glenhendry]

I now believe that its the MAF. I believe LPG ignores the MAF sensor too. I did put in a MAF that shows changing values in the OBDII scanner, but I dont think the values are correct. I am buying a new MAF from Island 4x4 and will advise.

The MAF worked! It is fixed. I am idling and driving fine on petrol and LPG again now. I am amazed though, that a MAF can fail, but still look like it is generating good numbers on the diagnostics.

Note that when idling, a MAF should be returning ~9-12 mg/sec airflow. The old one was down to about 5. If you get severe misfiring upon acceleration - check MAF for low values.

I have also confirmed that my LPG, uses TPS and O2 sensors, but not MAF.

[clubagreenie]

Yes, the MAF will cause very random fault codes and symptoms. My symptoms were gearbox, O2 sensor and lean codes. Under load it would change from 3rd to 4th but have no drive and then limp mode until back on level ground (unless driven hard then load would fault it again). If it doesn't see any air but the O2 sensors do it gets very nervous as it doesn't know whether to lean it out (lack of air from MAF) or richen it (too much air (lean codes))from O2 sensors. If you think the V8's economy is bad you can try my old MAF.

Your LPG doesn't have a MAP (manifold absolute pressure) sensor screwed in somewhere does it?