True, unless the coolant system was just flushed. Thinking about OP's first post where he just had a shop flush the system. They almost surely did not remove and clean the overflow tank. Which is what I always do when I flush my systems. All that old crap in the overflow tank will be there forever if you don't.

I advise to remove and clean the overflow tank. I use Simple Green, a de-greaser. It takes me about 30 minutes to an hour to clean an overflow tank that is cruddy. I have done this on all three of my trucks after flushing the cooling system. After a few years the overflow tanks are still clean but only because the cooling systems are clean.

Remove overflow tank and flush with garden hose. Then put a strong solution of Simple Green in it and wait 10 minutes. Now take a rag and stuff down inside the tank. Use a long handled screwdriver to shove the rag around the sides of the tank. Take care not to damage the plastic inlet, it is quite restrictive. Repeat as needed, sometimes it takes 3-4 times. When done your tank will look almost like new, you won't get everything (especially high up where you can't reach).

But hey - obviously the state of the overflow tank represents the condition of the rest of the system, so as Brad said if the rest of the system is crappy it won't do any good to clean the overflow tank.

 

Well see how it all turns out. I have a tranny cooler on my truck and my buddy I originally talked to about this problem that is a Ford mechanic says its the cooler lines seeping out in the radiator but its very very slight. The tank is showing signs of something again on the sides at the top. 100 miles of driving since the shop flush. Its not the end of the world but I think I'm gonna, have to buy a new radiator w tranny lines built in eventually

 

Alright so I'm bringing this back up b/c over time there is now a thin layer crud that's came back in the overflow tank and I am gonna ask here but I am going to have a shop test if it's oil or transmission fluid. Personally, I do not think it's either and here is why although it does have the appearance of dark brown/black oil, and it sits on top of the fluid but if you add just a little water, it all goes away. Anybody that has any thoughts please chime in.

For one, there are no signs of anything bad in the actual oil, on the filler cap, or in the radiator or it's cap nor the transmission fluid. Two, if you stick a paper towel down in this stuff on top of the fluid in the tank (it's about a fingernail thickness worth) it does not smell like oil/gasoline smell. It smells like rubber to me so maybe it's stop leak that someone put into the system? Another reason it doesn't seem like it's oil is I changed the oil recently and the stuff that's coming in there is dark, not light like the new oil would be. I personally do not think it's tranny fluid either b/c my tranny fluid is very light pink and whats in the overflow tank is like dark brown/almost black. I don't want to throw money away at a new radiator although that's what one mechanic told me b/c the tranny lines are leaking in there. It's not tranny fluid IMO. Another mechanic said it's nothing at all to worry about, it's in great shape and that's just crud from 21 years. A 3rd mechanic just looked at it and stuck his finger on my paper towel and said "oh boy you are probably going to need a new head gasket" but he didn't smell or test what's in the tank and I think he just wanted to tear down the truck b/c hey I guess it couldn't hurt (wouldn't hurt his pocketbook but it would mine!) for a problem that may not even be relevant. It does seem like I might be losing some coolant so there may be a leak (so maybe it's stop leak in there) but I've flushed the heater core recently so that may be why it's low as I didn't pay attention where it was before. Anybody that has any input or thoughts is appreciated before I pay someone to diagnose. Does stop leak smell like rubber?

 

Rubber breaks down too. Could just be that. In either case, I believe you are fine. If it really bothers you, just wait until you have warm weather, drain all the coolant you can. Replace all hoses. Flush system, start with new fluid.

Drive it like you stole it and keep trucking!

 

How's your radiator and heater hoses look? If you think it's rubber then maybe the hoses are coming apart internally.

 

The heater hoses look good. Radiator hoses look fine on the outside but we all know they come apart on the inside first. I appreciate the feedback. I think I'm ok too and it doesn't bother me, except for the fact I want to confirm I don't have a serious problem and keep driving it that way. That's all I'm really concerned with.

 

So it sounds like I'm going to have to take my truck to a shop to confirm this is not a serious problem but I think I may be onto something. I am getting worried this is a more serious problem that may cause other damage if I don't get if fixed now.

But, something I noticed is the PO sprayed what I believe is rubberized underbody sealant all around the top where the heater hoses meet the heater core. I had to peel it off enough to loosen the hoses to flush the core. I don't know if it leaked or what but I don't know why else anyone would do that unless the clamps leaked or something.

I have not noticed it leaking or smell anything in the cab but it got me thinking I wonder if that is what is in the system since it does smell to me exactly like rubber. The only thing still perplexes me is that what seems to float up to the top of the tank is slick so I don't know know if melted rubberized spray would do that or not but I'm not exactly sure how it continues to get into the system either.

I'm going to have the coolant tested hopefully this weekend, and if it's not oil or tranny oil (praying for neither one of those) then I'll deal with the leak/new heater core from there but this is driving me crazy

 

Two words. Pressure test. Get that done and then post the results.. Any thing else is speculation.

 

Ok so im gonna let this topic rest. The mechanic shop said the truck is fine. Test ran at just over 16lbs which is what the cap is rated at I think . He said its probably just crud in the overflow tank itself but didn't really know. The coolant in the radiator looked good too. Said just drive it and nothing is wrong at all