Background: 2000 Disco II with 111,000 when symptoms started. At 97K truck had valve job, head gaskets, valley pan, intake, upper plenum gasket changed, new coils, new plugs, new wires. Fuel injectors individually cleaned ( ultrasonic ). I noticed the rough idling after gassing up. OBDII was consistently P0303 ( cylinder 3 ). Coolant needed topping up 6 to 8 ounces every 2 to 3 weeks. No coolant on ground or under the hood. Car would idle very rough when cold and if immediately driven would prompt MIL flashing and MIL display. If the car was allowed to idle about 2 - 3 minuted before driving the truck would drive well to the point that the MIL would eventually extinguish after several days. Cold starts would always result in rough idling and if driven before warm-up would cause P0303 MIL to reappear. My suspicion was a blown head gasket causing coolant to enter cylinder 3 on shut down when there is pressure in the system. When the truck is running for a while and has burned up the coolant in the cylinder that has accumulated after shutdown and cooldown, coolant leak into the cylinder is very minimal or nonexistent due to cylinder pressures. Otherwise rough idling would continue and coolant consumption would be higher than what it was. White exhaust was not remarkable. I wanted a quick fix till the weather was more conducive to car repair.
Poured Prestone Super Sealer directly into a hose and reconnected rather than into the overflow tank..... and drove the truck for 20 minutes. The following morning, upon starting, the engine idled very smooth when cold. Today the MIL went out. Can't wait till it gets warmer. Will probably torque down the head just a little bit more and flush the system. Don't like that Magnesium Aluminum Silicate and Gluturaldehyde in my cooling system.
Ed

 

Have you looked at the spark plug for cylinder number 3. A combustion chamber and a cylinder that consume coolant are usually very clean and the spark plug would show that.

If there was a problem with the head gasket that allowed coolant to seep into the cylinder, you will notice pressure buildup in the cooling system also.

I have seen a similar problem and the cause was a micro crack in the cylinder head that started behind the intake valve seat and went inward, which allowed coolant to gather overnight behind the intake valve.

I hope that the prestone sealer took care of it.

 

I think I might be having a similar problem on my no. 4. Hoping it's the old wires as mine are stock (119k), but worried it's more serious. Any follow up on how your treatment faired would be appreciated.

 

Why don't you pull the #4 plug when its cold and possibly get more information on the problem? Inspect it for being "steam cleaned" or cracked. It will take you all of 5 minutes.

 

My current reason is that I'm sitting at the office worried that instead or pulling my new-to-me boat to the lake this weekend, I'll be landlocked pulling heads with a po'd wife. Believe me, though, that 4 plug is coming out as soon as I get home. How cold does the engine need to be though? Will the 10 min drive home heat it up too much?

 

You'll get varying answers on this, but here's mine and I'm conservative with these things: I do not work on aluminum motors unless they are cold. Removing fasteners is one thing, putting them back in completely something else and much more risky (hot aluminum expands, so you are risking stripping threads).

If you have a 10 minute drive home, I'd give it an hour or so at least. You want to be able to comfortably put your hand on the head and be able to leave it there.

Can you get away with working on something hot? Sure. I've done it. But only out of absolute necessity.