Today I replaced the Idle Air Control Valve on my 87' 4runner. When removing coolant lines, I noticed the tips of them had what looked like brown sludge coming out of the tip, but all the coolant seemed to be a clean bright green. What can cause this?

 

Pictures would help but probably rust mixed with other contaminants.

 

You can try and flush the system with the Preston Flush kit. That can help get some of the gunk out of the block and such. The radiator you need to take to a professional shop and have them to power flush it. The Preston kit wont get the gunk out of it. If it is the original radiator, these things get a lot of build up in them. The cooling systems in these trucks can get bad.

 

+1 on rust, Prestone Flush and having radiator professionally cleaned (OR replaced.)
When using Prestone Flush, pay attention to instructions on the container. Mine described a "heavy-duty" flush - Rinse cooling system very well, refill with water and flushing solution, drive several miles for a few days, drain, rinse thoroughly and refill with coolant. Gave me a good excuse to on a 400-mile road trip
When I drained coolant from my daughter's 2007 Corolla, coolant was perfectly clean and looked new. However, after the "heavy duty flush", this is what came out.... Imagine what you'll drain out of your 30-YO truck...

I have resolved to do heavy-duty flush on my truck every 2 years.

 

I wouldn't tackle this myself either. Get it flushed professionally, and that does not mean at a quick lube. Go to a radiator shop or a good mechanic. I would replace the rad, and have both heater cores flushed as much as possible. Refill with TOYOTA Red or ZEREX Asian Formula.

 

I agree on rust unless somebody I once put some dexcool in it. That junk likes to turn into mud. The Preston flush works good. One really good one is muriatic acid. Just don't leave it in there too long as it'll start eating at any aluminum components. The best I've used is a BG flush kit. We use them at work and the flush kit send to get rid of everything. Pretty much the only way to acquire it though is from a shop that uses BG products.