Can someone please help me with my 1995 3.9 V8 Discovery, when it is under load it starts by losing power then begins to misfire and escalates until it backfires, I know it has to be something electrical but I can't figure out what, I have replaced the dizzy cap, rotor arm, and coil, the silicon leads are only 10-11 months old.

Has anyone had a similar problem and can tell me what it was without me having to change everything to find out what it is??

Any help will be great, cheers.

 

Who's plug wires and plugs are you using? How many miles on the engine, what octane gas are you running?

 

Thanks for your reply Mike, the plugs are NGK, the car has 200,000 kms on it , the petrol out here in South Africa is 98 unleaded I believe, I don't know the make of the plug wires and at the moment it is pouring eith rain so I will have a look tomorrow, but I know they are a fairly inexpensive brand.

 

You get 98 octane, here in Colorado we only get 91 octane, boy are we loosing out.
Just a thought, have you replaced your fuel filter? Check for crack for failing vacuum lines or seals around the intake manifold. As for4 electrical things, the only thing thats comes to mind is a failing coil.

 

Too much Mexican food ;p 98 octane? Sheesh, best around here is 93, or one Sunoco that has 104 ;-)

 

Timing could be off, timing chain could be on it's way out or burned valve.

 

Hello Mike,

I have been away for a few days so unable to reply sooner, I have been having a nightmare with this, in the last day or so I have tried 2 more dizzy caps, 1 being a Lucas and the other a blue after-market one, 2 other rotor arms, another set of plug leads, checked the timing, changed the spark plugs.

A couple of things I have found out, firstly there doesn't seem to be any vacuum from the plenum chamber to the dizzy - when you take off the pipe from the plenum chamber nothing happens to the idle, but if I suck on the pipe the idles does increase, yet I have checked the nipple on the plenum chamber and it isn't blocked!

Secondly when I changed the plugs, numbers 5 & 7 where quite black and I know 5 & 7 are next to eachother on the dizzy cap.

If I am on level ground the car will excelerate perfectly so fuel starvation is out the equation this only happens under load ie: up hill and then runs lumpy for a short while.

Any suggestions other than melting it down for Coke cans will be greatly appreciated!!!!!!!!!!

 

One thing you could check,, lift your distributor cap and make sure your rotor shaft feels spring loaded, if you twist the rotor you should feel the mechanical advance springs , movement of about a quarter of an inch , if it dosent move at all or binds , you need to remove the dist and rebuild it.
lack of mechanical advance will cause those symptoms , and dont use anything but the Lucas cap.
Re check the firing order too, 1-8-4-3-6-5-7-2

 

Cheers DirtyDuk, have checked the dizzy, the rotor does spring back to position, 1 thought I have just had, I had the vacuum pump repaired, could the material used for a new diaphram be a little too thick and strong for the suction from the plenum chamber, other than that I am just gonna get the timing chain replaced.