3.5v8 disco drinking fuel like a reet good un'!!

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dirtydiscov8

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Location
Hertfordshire
Mates got a disco of a fine 1990 vintage with a 3.5 running petrol an on lpg. It's started to run super duper black smokin' rich! And has a small missfire.

So far swapped the coolant temp sensor
m.a.f
tps checked timing
fuel pressure regulator
plugs and leads...

...it's running richer than an i've seen a rv8 before £30 petrol for 35 miles driving! Strangley also seems to be bad on lpg.

Can't find anything wrong with it, thinking a badly worn cam could be a cause?

Any ideas chaps?
 
Have you done a compression test?
Have you tested the air flow meter - CO Trim and Air Flow Signal?
Have you tried a substitute ECU? - It may be nothing to do with the engine itself.
Is the Disco running with or without cats? If with cats, check O2 sensors.
Check the ignition timing, make sure the cap and rotor arm are clean.
Check battery voltage and that the alternator output is strong - weak electrics are known to sometimes cause rough running symptoms.
 
Kev, compression test was done, all around was al within 135-140 psi
got the right trim value on the maf and air flow voltage also seems right
not tired another ecu yet but will do soon
non cat car
timing was checked and was at 10btdc and went up fairly smoothlt to just over 30 all in at 3200rpm and the cap and arm are clean and genuine lucas items.
Battery is all good too!
 
I'm thinking possible worn cam too. His doesn't seem to pull too well but i'm used to my slightly modded 3.9. Might have to break out the dti gauge and see what valve lift it's getting!
 
dirtydiscov8 said:
Kev, compression test was done, all around was al within 135-140 psi
got the right trim value on the maf and air flow voltage also seems right
not tired another ecu yet but will do soon
non cat car
timing was checked and was at 10btdc and went up fairly smoothlt to just over 30 all in at 3200rpm and the cap and arm are clean and genuine lucas items.
Battery is all good too!
Click to expand...

Basics sound O.K. except that ignition timing. I guess the timing has been advanced up for the LPG, standard would be 4 deg +/- 1 BTDC at 800rpm dynamic vac disconnected.

Worn cam/lifters is a possibility but I would have thought if there was that much wear in the valve train a compression test would give some indication of possible issues, how many miles has the engine done? Try a substitute ecu before tearing in to the engine though.

What is the state of the battery and alternator? Carefully check the wiring to the coil and the ignition amplifier, check the coil as well.
 
Last edited:
Just had a quick look at compression figures; they could be a bit low.

I don't have figures for 3.5 but for 3.9 I find the following:

8.13:1 compression ratio = 150 - 160psi
9.35:1 compression ratio = 170 - 180psi

If readings differ by more than 10% rings or valves may be at fault. Low pressure in adjoining cylinders may indicate head gasket.

HTH
 
classic kev said:
Worn cam/lifters is a possibility but I would have thought if there was that much wear in the valve train a compression test would give some indication of possible issues, how many miles has the engine done? Try a substitute ecu before tearing in to the engine though.

quote]

Not true, if the cam/ lifters are worn the compresion will be fine as the valves will seal fine but will be opening incorectly.

BUT i think kev could be on to somthing with the timing.. might be worth running throught the timing procedure to make sure its set right
Click to expand...
 
My thoughts were hat the timing was advanced to run the lpg. It's obviously meant to be mainly run on lpg as the main fuel tank has been replaced by two lpg tanks and there's just a 25 litre tank in the boot for petrol. Timing hasnt been touched and the dizzy is tightly clamped so it cant really have moved.

I would have also though that a worn cam shouldn't affect compression test too, just the valves wouldn't be opening much so not letting much air in and making it run very rich!

When he bought the disco he was told it had a 2nd engine fitted at some point and inside the rockers look clean and it has good oil pressure...the disco itself on 215k miles!! so i would have though it would be pretty dead by now.

Got a spare maf to try on it again and will double check all the coil connections and amp. Doesnt seem ignition related though, get a good whitee/blue spark that will jump to an earth when a plug is removed
 
you can always mark up the the dizzy position and retract the timing a bit, there is a bolt that pushes a plate to hold the dizzy in position which can easily be loosened.

make it so its just lose enuf to turn by hand
 
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