I have a 1997 3500 7.4L 2wd with 158,000 miles. Yesterday I took it to the shop to get the oil changed and radiator flushed. The owner showed me that the fluid in the coolant reservoir was black, but when he opened the radiator cap, the coolant was green and appeared fine. When the tech drained the old oil, he didn't mention any irregular color. I don't see any smoking or smell any burning. What could possibly be the issue? Thanks in advance for any replies.

 

This happened to me on my 99 Silverado 5.3l LS1...believe it or not after trying all sorts of funny stuff it turned out to be a cracked intake manifold. After replacing the intake manifold and seal all was well. Now I have a bad rod bearing and am in the middle of an engine swap.

I had a tough time trying to find someone to help me with this issue at the time. Even the mechanic was a bit suprised that was the culprit that was causing my truck to run rough at stopped idle.

 

I would remove the coolant bottle and clean it out. If oil was getting in the coolant it wouldn't really be black it would look like chocolate milk. If the truck is running good and you aren't losing any fluids. You should be fine. Also if someone has used some of that block sealer or radiator sealer it can turn the coolant real dark. Then if you flush the radiator and not the coolant bottle it will have that dark look in there. Do you know if any of that has been used on your truck?

 

Actually I did use some radiator seal about a year ago. After that didn't work, I had the radiator tanks replaced. So maybe some of that old stop leak got dislodged and disciolored the coolant in the reservoir? Also the engine seems to be running fine with no leaks.

 

I bet that is the discoloration. I would flush it out just to be certain though.