Hi, I have a Discovery 3.9 v8i 1998. The problem apart from it's very large thirst it has an irregular tick over once it is hot and appears struggle for air every now and then.Can any of you shed any light on what maybe causing this problem?

 

Just so we are clear, "tick over" means idle. (most of use Yanks do not know what that term means)
Now we need to clear up the model you have, do you have a DI or a DII?
Does your heater have rotary ***** or is it electronic?

The first thing you need to do is check the condition of your spark leads, spark plugs and dizzy.
Then clean your throttle body and idle air control valve.
How many miles are on the truck?

 

Thanks for your reply, I appologise for not being clear with my description. The Discovery is a Disco 1 with a 3.9 petrol engine. I have recently changed spark plugs, HT Leads, Distributer cap and coil. I have adjusted the ignitiion timing to 4 degrees BTDC and when it idles it runs a smooth as you would like, it's not until it heats up to normal working temperature that it becomes eratic and it is if it is struggling for air it gives a surge and then it idles eratic. Thats the problem I have apart from it's enormous thirst....

 

I am leaning towards the idle air control valve being wonky.

 

Thank you for your advice Spike I will try the idle air control valve, just one last piece of advice could the fuel pump cause this problem as I think it is playing up on occasions.When I am driving the car it appears to die on me then it picks up, do you think this could be related to your diagnosis or something else. Cheers.

 

If you are cruising at a steady speed and then the engine "dies" but then comes right back, and it only happens every once and awhile, it is usually the throttle position sensor (TPS).

The idle air control valve only works at idle.
To figure out if the fuel pump is at fault you need to install a fuel pressure gauge, you can usually rent them.