Ive got dark white thin cream like sludge all over the inside of my oil cap car has been over heating heater core went out of it any idea what the sludge could be?cud it having anything to do with it over heating?

You need to flush the system. Even though it looks ok and cools it is slugging inside at the crossovers and purking up.

Mike

Are you seeing an actual leak on the ground, or are you just having to add coolant ?

I have the same problem with my Dodge Avenger. The issue is with the heater core. The company that made the heater cores for Dodge that year put to much flux in it. The flux comes loose and you get a sludge build up in the heater core that drops to the bottom of it. The heat for the passenger side blows on the bottom of the heater core. So that is why the air is cool on that side. Solution: you can try flushing the the system. If you take the cap off the coolant reservoir you will see the sludge build up. It has a reddish tint to it. Hope this helps.

dirty heater coil, clean it out so the hot air will travel thru

The brown sludge you're seeing is the result of GM/dealers/repair shops installing cooling system sealer tabs in the cooling system. This sludge is NOT related to the use of dex-cool coolant. The cooling system sealer tabs are brown in color, and resemble dirt after they are crushed and poured into the cooling system, and pretty much turn into nasty brown mud in the cooling system. I have seen this on many vehicles. The problem you are having with your heater is most likely related to the sludge you are also seeing in the radiator. The sealer finds its way into the heater core and plugs it up, resulting in cool air instead of hot air. The best way to clean it out is to remove the heater hoses from the heater core and back flush the heater core with a garden hose. You will see a lot of nasty brown junk come out of the heater core when you do this. I have found that cooling flushes that dealers and independent shops sell are not effective at completely removing this sealer, and may have to be done multiple years in a row to get most of it out. Even then, there may still be sealer lodged in the system.
Your description of the heating and air system defaulting to the defrost and floor positions tells me that there may be a vacuum leak in the vacuum supply hose between the engine and the interior control (HVAC Programmer, if I recall correctly). The default position is defrost when the system loses vacuum. You may want to have your local shop check that and make sure you're getting vacuum to the HVAC controls.

whats the oil look like and oil pressure --was their any sludge when you changed tappets --sludged up engine channels can restrict oil return and cause oil starvation same with old oil thins out when warm and oil filter clogged or collasp

The head gasket has gone, is it a 1,6 1.8 petrol? if so its the most common fault on that car. best way to tell pull the dipstick up and look at the very bottom if it looks creamy white then its the head gasket, also open the oil cap and look inside the head and on the cap any white creamy sludge means there is water in the oil, "head gasket"

Hope this helps

As the car was at 98 you will have warped the cylinder head so will need skimming costing £40 new head gasket fitted is about £170 do it your self for about £80

Was the sludge Orange or more of a tan or brown colour? If there is a tan or brown colour sludge in the rad, this usually indicates a head gasket leak. I would flush out the cooling system and be sure that there is good coolant flow through the rad ( you should be able to see the coolant moving in the rad with the rad cap off and engine running) as well as no air locks in the cooling system. Feel the heater hoses at the fire wall with the engine at opperating temp. The hoses should be hot and close to the same temp. If not then there is a flow problem or a plugged heater core. hope this helps..Good luck.

Heater core!! are your windows fogging up and when or if so won't clear with heater blowing on windshield you must wipe with cloth or tissues??? if so it will "more" than likely be your heater core big job!

A few tell tail Signs to look for are
1. Excessive Pressure in radiator and car over heats
2. Look under oil cap when antifreeze and oil mixes it becomes a cream-ish white like heavy cream almost. It may be under there.
3. White smoke all the time out the tail pipe
4.loosing antifreeze but there is no visible leak.
5. Antifreeze in oil or oil in antifreeze.