V8 astronomical fuel usage

[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.  

[ibruceuk]

Hi,

yeah I've cleaned the sponge thing, (well replaced it), but there is still a vacuum, but this may be quite normal, anyone able to check?

I'll try adjusting the timing, but the engine seems to have plenty of power, and the EFi has no lambda's so it wouldn't get feedback about overfuelling, it would just run on the Non-Cat map as according to the tune resistor.

I'm gonna try and check the injectors this weekend, but from what I can see of the LPG system it only kills the injectors so the fuel rail stays pressurised so it can't be a leaky injector.

The timing is BTDC, I too would have thought the CO would be lower on LPG, but I can't weaken it anymore, but this may be related to the high crankcase vacuum...

[Range Rover Blues]

You have to change that timing before you damage your engine.  Over-advancing will make it much harder to start too.

YOu could have a leaky injector, that bit about waiting for it to start, well the fuel pump primes every time you turn on the ignition then cuts out untill the engine is running.

Are all your spark plugs the same colour?

[Budgie]

It could also be related to the timing. If it's taking in more fuel than it can burn then the excess will go out of the exhaust and register as high CO, same as on petrol.

By altering the timing you're compressing the gasses that little bit more and giving a better burn rate when the spark fires. You may not notice any increase in power from the engine but it should use less fuel and burn it cleaner.

Just to confirm about the second breather, it's at the front of the  righthand rocker cover, where the oil filler is on the left bank, and has pipe coming off the top of it into the inlet manifold and looks like this. It's got like wire wool inside and can clog up.  

[Range Rover Blues]

Just re-reading some of this.  If it had cats id had lambda sensors, have thse gone too? if so you need to change the tune select resistor to tell the ECU they are missing.  Your car may just be in limp home mode.

On a 93 that must be a hot-wire EFi right?

A very slight vaccum is a sign of a good engine, it's there to protect the seals on the carnk and prevent any oil fumes escaping, they should all be burned by the engine.  Check that the pil fume pipes on the front of the plenum are all ok thoough, one of them has a very narrow bore inside.

[ibruceuk]

You're right, it did have lambdas but I removed them along with the cats but I did change the tune resistor to the Green one when I did this.

I can't seem to remove the flame trap on the right hand rocker cover, can someone just confirm that this does just unscrew, I don't want to apply too much pressure if this isn't correct.

The pipes are clear, I cleaned them a few weeks ago.  I'm wondering if the previous owner who installed the LPG may have screwed something up and possibly joined both banks of injectors together, i'll try and check that tonight.

and yes it's hotwire.

[Budgie]

Yes the flame trap does just unscrew, standard righthand thread. If it's not coming then try when the engine is warm.  
There is an o-ring under it to form a seal so WD40 won't help.  

If you can't find a problem and altering the timing doesn't do anything then a trip to an LPG specialist could be the way to go, if only to make sure both the LPG & petrol sides are both set correctly.

[Range Rover Blues]

If you check the resistance of the injectors at the ECU multi plug that shgould tell you how many injectors are connected to each bank.  How did the installer fit the injector cut-out? is it emulators? or a cut out relay in the injector power supply?

What colour are your spark plugs?

Have you checked for a fuel leak?