The past few weeks I've noticed a coolant seepage from the rear end of the block on the drivers side.

I immediately wondered if this was the head gasket failure I've heard so much about on Discovery 2s. I had one severe over heat event after I bought the truck a few months ago (Dash temp light came on) However after doing some more reading I'm not so sure.

SYMPTOMS:
1. Lots of white smoke on startup ( its been cold in my area (freezing temp and 0 F past month) and there is no upstream 02 sensors on this truck so I know its running very rich, Knock sensor recently replaced and wires repaired)
2. Coolant seepage around the engine rear drivers side
3. White smoke mostly clears after the truck reaches operating temp and driving for a few minutes. (I keep an eye on my temps and they stay at ~194 whether idling or driving)
4. No rough idle, I haven't drove a 'perfect disco' but it drives about how I would imagine a 24 year old truck drives. (Gets up to speed fine, cruises at 60mph without issue)
5 I didn't measure how much coolant I added to the tank over the past week but it was pretty minimal.
6 Have not seen coolant in the dipstick/ under the oil cap or oil in the coolant (I check every time I drive the disco)

I've read that the Valley Pan gasket can seep from the same general location as the head gasket, my plan is to add UV dye, clean the engine bay well, drive a few days and check. If this is is a 'Valley Pan' gasket issue rather than a head gasket, how urgent is it to replace?

I'm planning to get a coolant tester to see if I have combustion gases in the coolant, but will this definitively point to Valley Pan or Head Gasket?

 

I would expect the white smoke is combusted water, head gaskets can leak in many places. I would get a leak down test, borrow one from an autoparts store. Pull your plugs and look for white reside on the electrodes. might as well put a scope in the whole to look for steam cleaned cylinders. you can get one that plugs into your phone for $10 on amazon. If you cleaned up the back of the engine real well you may be able to put a scope or a mirror back there to see a leak. The dipstick only tells you if the water is getting in the oil, not if it is leaking externally.

 

It is true those leak spots are close to each other, but should be able to visually pinpoint the leak: Valley pan gasket leak is at the coolant passage from the side of the cylinder head to the intake manifold, Head gasket leak is right at the bottom of the cylinder head at the coolant passage from the block to the cylinder head. There are lots of photos on line of the Disco cylinder heads for reference, so you can see the coolant passages. Sequence of disassembly goes to valley pan removal before cylinder head removal, if you end up troubleshooting by removing parts. Small leaks can become larger leaks over time due to localized corrosion, but it's a bit of a judgement call about if it's urgent or not. Some mechanics do not mind using stop leak tablets or chemicals, others don't like them at all.

 

add dye and pressure test the cooling system. You will know right away.