I'm afraid I may already know the answer to my question, but figured I'd check here with some more mechanically savvy folks before proceeding.
I recently replaced practically my entire cooling system; radiator, water pump, thermostat and housing, clutch fan, serpentine belt, etc. and flushed the system. After doing this, while my temp gauge never showed overheating, as soon as I'd stop and turn off the engine the coolant would begin boiling over out of the reservoir and is a rusty brown color, despite being brand new coolant. This only happens intermittently.
In the process of trying to track down the problem I noticed that despite continuing to add new coolant to top it off, it keeps coming back out with a brown tinge.
Does this mean I'm getting oil in my coolant, or is that even possible? What should I do next to track down the problem?
I've already checked that my thermostat and fans are working properly and my lower hose isn't collapsing.
Any advice would be greatly appreciated. Thanks!!

 

If the cooling system had a lot of rust in it, you sometimes have to flush it several times before it comes clean, or do a bunch of drain and refills with distilled water. On my Dakota the cooling system was so bad I flushed it out then filled it with distilled water and some cleaner, I drove it for a week like that, then I had to drain and refill it with distilled water 4 or 5 times, driving it a few miles between each drain and refill before it came clean. Then I drained and refilled with antifreeze.

 

Is there anyplace near you that can check the radiator cap? I tried 3 places near me, none of them could. Read a thread here a while back that if your cap is not operating right it can cause all manner of issues. Should be releasing the fluid to the overflow bottle at 16PSI.

Try a new cap if you can't get yours tested, and get all that rust flushed out of your system too.

 

Thanks for the advice!
Will giving the system a better flush and decreasing the rust help fix the overheating issue? Or does it sound like I have bigger problems causing that?

 

You need to flush the heck out of that system until you're getting no more rusty brown color. Only then can you determine if there's something else. If your temp gauge is reading normal temp, I have to agree that your cap probably needs replacing. Fortunately this should be a cheap and easy thing to do. Get a quality one and see if that helps.
You can also rent the test equipment at your local parts store to check for exhaust gasses in the coolant. Might be worthwhile to grab that while you're there - tool should be net zero to rent, but you will have to buy the chemicals which aren't too pricey.

 

Ok great, thanks. I’m in the process of doing another flush so I’ll replace the cap after and see what comes next.

 

Eggstackly.


Since you don't live where there are freezing temperatures, just refill it with tap water (yes, I said tap water, it won't be in there very long) and toss in a tablespoonful of Dawn dishwashing liquid of any of the HE rated liquid laundry detergents. Drive around for a few days to a week. Drain, rinse, repeat as needed.

One more thing to check - exhaust gasses in your coolant. Might be a head gasket issue.