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Graham132white 12-05-2012 01:32 AM

Help! Very technical problem!
Hi guys..

Ok, so I've got a 4.4 V8 E53 2001...LPG converted 80k

Im very mechanically minded but this problem is very hard to get my head round,, basically the car will not run right on petrol, it won't go over 3k revs and splutters and misfired badly although it will idle perfectly. If I unplug the MAF it runs like a race car v8 so I'm discounting bad coils etc, plugs are brand new.
I've got no fault codes either!!:mad:

It runs perfect on LPG while driving and has perfect response on the throttle but idle on LPG is fine until you touch the throttle a little which makes it bounce up and down to about 2k revs down to 0 and sometimes cuts out.
The BRC LPG installer says he has fuelled the LPG correctly to run right, which it does.
On petrol the short term trims go up high only when you rev it and the longs are hovering about +12.

All this would tell me there is a vac leak or bad maf but that wouldn't explain it's smooth running on LPG, or would it? I've sprayed all over the manifold looking for leaks and gaskets were done not long ago.

I just ain't got a clue rom here.... Any help would be greatly appreciated!

Zulu95 12-05-2012 06:09 AM

"If I unplug the MAF it runs like a race car v8"

Sounds like a faulty MAF.

Graham132white 12-05-2012 06:16 AM

Well I thought that but it runs perfect on lpg??? Surely it would be bad on both fuels..

I just didn't want to she'll out for a new MAF to find out it wasn't bad.

Cheers for the reply btw.

Zulu95 12-05-2012 08:40 AM

One of the common "layman" tests for the MAF is to unplug and see if the vehicle drives better. Have you had codes read? You could try cleaning the MAF and see if that helps. How does the LPG system measure the air/fuel ratio and how much is needed for any given driving requirement? There are very few on the board with LPG experience so it may be a bit of a mystery to the usual experts.

Graham132white 12-05-2012 09:48 AM

Unfortunatly no codes at all. I've had the p0171 lean codes both banks but that's just telling me the fuel trims are lean due to vac leak or bad maf. Thanks for your help matey, I think I'm going to have to just buy a new MAF and see if it sorts things.

Setting aside the LPG, I picked up the wife from work on petrol. She kept an eye on the live data while I was driving. The short terms were at 25 on idle but if I put my foot down they dropped to zero while under heavy acceleration but as soon as i let off they went back to 25. Long terms apx 12-15 and even up to 20. Still wont go over 3k revs, just stumbles and burbles if that makes sense.

petriej 12-05-2012 10:34 AM

Before you buy another MAF, try to find someone with a working one and swap it to test.

I'm curious how the LPG conversion worked, and what exactly they did to make it. I guess you have a separate fuel tank...

Graham132white 12-05-2012 10:59 AM

Yes I have the original petrol tank and a spare wheel well lpg tank. I guess he may have programmed the lpg to fuel the engine based on throttle positions and not using the maf?! I'm really not sure, I'm just so lost on how it can run perfect on lpg but not petrol.

The backfires and misfires are really bad on a cold morning start but get better when fully warm and turn into a massive burbling flat spot on petrol. It take about 15 mins to get 100 yards down the road when it's freezing out side in the morning, can't get over 10mph until it warms up.

Zulu95 12-05-2012 11:15 AM

From what I just read the the LPG uses a secondary mixture computer which "intercepts" the engine management signals and uses those for the LPG system. I wonder if it is possible you have a torn air boot on the MAF to throttle-body connection that is throwing the codes for the petrol powered system but the LPG has more computer controlled adjustment so it can keep running without problems.
The condition you described certainly sounds like how our X was running when the rubber boot slipped off of the MAF (on the side going to the engine).
Hope it helps!

Graham132white 12-05-2012 11:33 AM

Thanks a lot mate. I think you may have hit the nail on the head with how it runs and the way it intercepts the ecu. If this was the case it would explain my symptoms. I am off work tomorrow so I'll do a strip down and check the inlet pipe thourghly and keep you posted.

Again, thanks for the help. I love the X but the way it is right now I feel like burning it out!

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