3.5L V8 hesitant and jerky, Poss MAF problem??

Short fat bloke · Apr 15, 2010

Good Evening all,

Im hoping you might be able to help me?

Ive had some poor running problems with my 1992 Disco V8, its the baby V8, 3.5L but fuel injected, Im guessing one of the 1st of the fuel injected ones?

The problem I have is its very hesitant and a bit jerky when under throttle, its far worse when its cold, but is def still there when its warm, just not quite as bad?, its ok ish if your running below 2000rpm but not perfect. I have no idea if it feels under powered as Ive only had the car a week?
Also the idle seems to vary, one minute its ok, sitting at about 800rpm and the next its at 2000rpm and pulling you along quite well by itself? Again worse when cold? It never seems to stall tho?

All the above is running on Petrol, I do have a single point LPG system on the car but never use it as it hardly runs on it so I dont bother using it at the moment. I want to get it running perfectly on Petrol before I start looking at getting the LPG going.

Im told that the problems Im having could be the MAF sensor?? Anyone got any ideas other than that suggestion? Im a bit new to this V8 Lark and need some guidance!

The MAF sensor I have on the car is a Lucas Hitachi AFH55-1

Many thanks

SFB

V8_Disco · Apr 15, 2010

what checks have you done already before we start throwing things to do at you

fett · Apr 15, 2010

Might be the timing also, especialy if it was set to run with a bias towards gas.

POAH · Apr 15, 2010

check the CO trim setting on the AFM it should be between 1-1.5 volts with a volt meter (Blue/red wires on the AFM plug, just pull back the black cover a bit).

1992lse · Apr 15, 2010

change all the plugs leads rotor arm and cap if you aint done so in last 24 months

Short fat bloke · Apr 16, 2010

V8_Disco said:
what checks have you done already before we start throwing things to do at you

Ive Changed all the plugs, leads, Dizzy Cap, Ive got the rotor to do this arvo, but the old one looks like new, so dont think thats the problem?

fettRe: 3.5L V8 hesitant and jerky, Poss MAF problem??
Might be the timing also, especialy if it was set to run with a bias towards gas.

What should the timing be set to?

Short fat bloke · Apr 16, 2010

POAH said:
check the CO trim setting on the AFM it should be between 1-1.5 volts with a volt meter (Blue/red wires on the AFM plug, just pull back the black cover a bit).

Cheers will check it this arvo, how do I adjust it if it is wrong, or is it even possible to adjust it?

POAH · Apr 16, 2010

Short fat bloke said:
Cheers will check it this arvo, how do I adjust it if it is wrong, or is it even possible to adjust it?

there is an adjustment screw (hex bolt) in a small circular recess. you can see it here at the top right of the AFM. this only adjusts the bas value though but worth while checking the voltage.

you can also check the airflow by probing the Blue/Green wires signal should be between 0.2-0.7 with no air flow.

Check both these settings with the ignition on but engine not running

Short fat bloke · Apr 16, 2010

POAH said:
there is an adjustment screw (hex bolt) in a small circular recess. you can see it here at the top right of the AFM. this only adjusts the bas value though but worth while checking the voltage.

you can also check the airflow by probing the Blue/Green wires signal should be between 0.2-0.7 with no air flow.

Check both these settings with the ignition on but engine not running

Thank you very much for that POAH, much appreciated! Will get on it this arvo/tomorrow. Hopefully its just a case of adjustment!

fett · Apr 16, 2010

Dont fook with those screws too much or you will be in a world of hurt!

Beast o Bodmin · Apr 16, 2010

slipping liner syndrome here we come..

fry baby fry....

classic kev · Apr 16, 2010

POAH said:
check the CO trim setting on the AFM it should be between 1-1.5 volts with a volt meter (Blue/red wires on the AFM plug, just pull back the black cover a bit).

Careful, cat or non-cat?

If AFM was playing up, EFI warning light likely to come on. If motor is hesitant would first check ignition timing. It would also be worthwhile making sure vac advance and mechanical advance in dizzy are working correctly.

Next thing might be to take off the plenum and clean it, check/set throttle disc and then make sure there are no intake leaks.

Have you checked all the values for the efi components as per Haynes or similar?

Hope that helps ya.

fett · Apr 16, 2010

fett said:
Might be the timing also, especialy if it was set to run with a bias towards gas.

Yes

Short fat bloke · Apr 16, 2010

classic kev said:
Careful, cat or non-cat?

If AFM was playing up, EFI warning light likely to come on. If motor is hesitant would first check ignition timing. It would also be worthwhile making sure vac advance and mechanical advance in dizzy are working correctly.

Next thing might be to take off the plenum and clean it, check/set throttle disc and then make sure there are no intake leaks.

Have you checked all the values for the efi components as per Haynes or similar?

Hope that helps ya.

Cheers, I look at this tomorrow

Short fat bloke · May 2, 2010

Evening All, Ive been tinkering,

New Leads, Dizzy Cap, Rotor arm, Plugs, Coil and MAF,
Cleaned Engine earth point and put new terminals on the coil.
Also lifted the plenium and squirted some carb cleaner in there,

The result,
every thing I did seem to make a little difference, but its still not quite right, still a bit hesitant, and every now and again if you give it more than about half throttle it'll cut out all together for a moment, lift off a bit and it'll come back? If you do get it to rev it then feels very flat towards the top end of the rev range, The idle still plays up to! more so when cold! Its now driving me bonkers! :mad2::mad2::mad2::mad2::frusty::frusty::frusty::frusty::frusty: Help!!!!

fett · May 3, 2010

Timing!

classic kev · May 3, 2010

Short fat bloke said:
Evening All, Ive been tinkering,

New Leads, Dizzy Cap, Rotor arm, Plugs, Coil and MAF,
Cleaned Engine earth point and put new terminals on the coil.
Also lifted the plenium and squirted some carb cleaner in there,

The result,
every thing I did seem to make a little difference, but its still not quite right, still a bit hesitant, and every now and again if you give it more than about half throttle it'll cut out all together for a moment, lift off a bit and it'll come back? If you do get it to rev it then feels very flat towards the top end of the rev range, The idle still plays up to! more so when cold! Its now driving me bonkers! :mad2::mad2::mad2::mad2::frusty::frusty::frusty::frusty::frusty: Help!!!!

You don't say if you have checked the mechanical and vac advance on your distributor, what is your engine timed to? Have you checked and cleaned the stepper motor. You say you lifted the plenum and just squirted carb cleaner in there - did you use a brush and/or cloth to properly clean it, did you check the throttle disc setting and are you sure there are no inlet leaks. Have a careful look at all the vac hoses connected to the plenum.

Why did you change the MAF? Have you checked that the replacement is correctly set?

If I have any other thoughts I'll post it here but let us know how you get on.

3.5L V8 hesitant and jerky, Poss MAF problem??

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