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V8 astronomical fuel usage

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ibruceuk:
Now I know it's a V8 and thirsty but I estimate it's currently running between 2 and 4 mpg!

Put it this way, you need a gas mask to stand anywhere near the exhaust.  It has got LPG which comes out at about 11 mpg, but petrol is so much thirstier.

It runs fine on petrol other than the starting point below, and it seems to have plenty of power more than LPG anyway.

When starting it's a bit strange....  If you treat her like a diesel and wait a few seconds after turning on the ignition then she starts fine on petrol.  Whereas if you just jump and turn the key straight to start she fires and then cuts out then it takes about 15+ seconds of turning her over till she starts.

Has anyone got any ideas, or obvious pointers I can't afford an  ESSO tanker to follow me everywhere!

p.s. I can't smell petrol anywhere!

Cheers,

Ian.

Budgie:
What engine do you have, carb or EFi?
Is it manual or auto?
Does the exhaust clear once you switch to LPG?

It sounds like you have a carb motor by the way you describe the starting process on petrol. Check the fuel return pipe isn't blocked and that all the fuel pipe connections are secure.

Running LPG & petrol is a bit of a compromise (sp?) in that the timing setup for one tends not to be the right setup for the other. Although 11 MPG on gas and 2-4 MPG on petrol is not really good on either account.

Do you have an LPG garage near you that could take a look and setup  the system correctly? It maybe the quickest answer.  

Sorry I can't help further.

ibruceuk:
Thanks for the reply, it's an EFI engine sorry I forgot to mention.

It's 1993 3.5 Auto.  On LPG it all seems fine, just passed MOT with CAT's removed, emmissions on LPG were...

CO - 2.23
HC - 996

Didn't test on petrol for obvious reasons.  The timing is set at about 12 degrees advanced.  The mixture on LPG is set about as lean as I can get it without backfires.

I don't know if it is related but there is a negative crankcase pressure.  i.e. If I take oil filler off there is a definite change in engine note and a small vacuum wants to 'pull' the filler cap back in.  I've used a pin to make sure the breather on the LHB rear rocker cover was clear.

Cheers,

Ian.

Budgie:
Have you cleaned or changed the breather on the RH bank? Maybe worth a look.

12 degrees advanced on the timing seem alot to me, although I've no experience of running LPG so maybe that's where it should be.
Haynes is quoting post 1990 EFi with 9.35:1 compression ratio engines should be running at 4 degrees BTDC on petrol. (You can find the compression ratio where the engine number is under the LH cylinder head.)
If you're running 12 degrees then that could be adding to the problem as the gases wont be fully compressed when the spark fires and a load of unburnt fuel will go out of the exhaust. Just a thought.  :?

Budgie:
Just looking a your CO level in the MOT and I'm sure that's about what our 1989 3.5 EFi Rangie runs at on petrol and on a run I can get near 20 MPG out of it dropping to around 13-15 MPG for normal driving.

I would have thought a CO level nearer 1.00 would have been about right for LPG.

When you say the timing is 12 degrees advanced, just so as I'm clear, is that BTDC or ATDC?
If it's BTDC then try bringing the timing back to about 8 degrees BTDC and see what difference it makes on both petrol & LPG. On petrol you should find that the revs at tickover will come up and the exhaust should clear a bit.  :wink:

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