Wife has a 1991 Seville with the 4.9 liter engine. A couple years ago we had a heating issue, and ending up having the heads machined.
About a month ago, I noticed the cooling systems fill tank was full of pasty rust. The coolant was brown, the radiator was also full of this rusty paste.
Wife took the car in to a radiator shop, they flushed it and said it was one of the worst they had seen.
I just checked again yesterday, after engine started to overheat (only 80 degrees outside), and I have the problem again.
What do I do to prevent this?
Thanks,
Richard

It's very possible that you don't have rust. These engines require a special coolant supplement sealer from GM that turns the coolant a dirty brown color.
Unless you've actually seen rusty scale, I'd suspect whoever last put in the sealant either put in too much, or didn't properly run the engine to mix it well, causing clumps to form.

If the radiator shop flushed the system with a chemical product, you may have lost sealant in crucial areas like the head and intake gaskets. I'd recommend going to any GM dealer to get the sealant, or go to an auto parts store and get Bars Leak Gold Seal Powder (nothing else). Buy yourself a gallon of 50/50 mix coolant while you're there.

After the engine has cooled, remove the radiator cap and pour in the sealer (if you got the tablets from GM, crush them first). Before replacing the cap, start the engine and let it run until the thermostat opens (about 7-10 minutes), then have a helper run the engine speed up to 1500 or so rpm. Top the radiator off and reinstall the cap tightly before the engine returns to idle.

Now go drive the car for 20 minutes to get the sealer to circulate. You should be good from there on.

Would this cause the system to plug up? I mean, the first time, it was really bad, the coolant looked like light chocolate milk, with a thick paste everywhere.
I pulled the cap off the coolant fill tank last night (after the engine was off), the tank was overful, and the coolant bubbled down for about an hour before returning to normal level. Do I need to get the system flushed again before replacing coolant/sealant?
Thanks,
Richard

I just did another look at what this brown paste looks like. I pulled the radiator cap, and all around the hole is a brown paste, that has the consistancy of a semi-dried rubber cement. There are large "gobs" of this everywhere.
I am sure that this paste has plugged up alot of the radiator, how can I remove this, so I can put in fresh coolant and sealant?
Thanks,
Richard

Go back to the radiator shop and ask them. If they did a complete flush and the system is clogged again, then there is something wrong. What that can be, I have no idea, but I do know that cooling systems don't clog with out a good reason. Possibly they didn't flush everything out, but only loosened it and now the balance of the crud is coming to the surface. I have seen some systems become so bad that when you finally clean them enough to get free flow, there are leaks that need to be dealt with.

Richard,

What is the history of the car? Didit happen (overheating) right after you bought the car? Looks like someone filled it with too much sealant to stop leaks or hide the problem. I would flush it with lots of water several times. And if it does not develop leaks and overheating, fill it with 50/50 green antifreez and distilled water and some two tubes of Bars Leaks Golden powder from Walmart for $1.44.

About a year after buying the car (about 2 years ago) it was overheating and had to have the heads done, and new gaskets.
About a month ago, I first noticed the build-up of the brown paste, and had a radiator shop flush the system.
Then, last night I noticed it again. It is almost as bad as when I saw it a month ago.
Thanks,
Richard

Huf is right. You need to flush the system with distilled water, maybe a few times. The radiator shop probably just flushed the radiator, and didn't clean all the goop out of the engine or heater core. Don't use a chemical flush product on your engine!

If you had to do head gaskets shortly after buying this car, odds are the previous owner knew they were leaking and dumped in way too much sealant, and probably the wrong kind. It sounds like old fashioned rabbit turd Bars Leak, which is the consistency of rubber cement.

If you're doing this yourself, here's the plan: Buy two big anti freeze drain pans , three gallons of water, the proper sealant, and two gallons of 50/50 mix coolant. Drain the rad. Remove the overflow tank, empty and flush it. Remove the lower radiator hose to get most of the water out of the engine. The system has about 13 quarts of capacity. Re-attach the hose and close the rad drain, fill with distilled water. Leave the cap off and run the engine for about 10 minutes. Let it cool, drain, and repeat the process until you get fairly clean water back.

REMEMBER THAT ANTI FREEZE IS TOXIC TO KIDS AND PETS, BUT IT TASTES SWEET LIKE CANDY. KEEP IT SEALED AND OUT OF REACH!

You might want to change the thermostat and radiator cap while you have it apart. When it's flushed, refill with your coolant and pour the sealer into the top of the radiator. Top off using the method prescribed in my previous post. Unless you have other problems, you should be good.

You should change the coolant every two years maximum on these aluminum engines.