I've noticed a black residue on the sides of the overflow coolant reservoir of my '88 vert. There also seems to be a slight oily film on the surface of the coolant in the reservoir. There's no sign of coolant in the oil.

Are my engine seals shot or could this residue be from rubber in the system breaking down. I've been told to remove the cap over the thermostat and look for bubbles in the coolant with the engine idling, but when I try to remove the cap, the coolant just spews out! Any suggestions?

Rocketman

 

To do the "cap test":

1. Do this with the engine stone cold
2. Remove coolant filler cap
3. Start engine
4. Look for bubbles
5. Rev engine, does coolant gush out?

If you have a steady stream of bubbles coming from somewhere in the engne, and coolant gushes out when you rev, this is an indication of a coolant seal (o-ring) failure.

Oily film is fairly normal in coolant, as well as some blackish deposits. These are caused by the rubber in the system breaking down (as you said), but also stray hydrocarbons pushed into the cooling system on startup. Don't worry about it unless the car uses coolant and the deposits are excessive.

 

Thanks for the words of reassurance. It sounds like I'm in the not too serious category.

Which fill cap do I remove? The one over the thermostat/water pump, or the one over the radiator. (it's a N/A engine)
Won't the thermostat be closed with the engine cold (preventing the bubbles from getting to the filler cap)?

 

Remove the one at the thermostat. The thermostat has a "jiggle pin", which still allows a small amount of coolant to flow even if it is closed.

 

Well, I tried the test and it worked very well.
NO BUBBLES! and no spewing when it revved. When I let the rev's drop after revving it, the level rose a bit. I assume this is nothing.

Thanks for all the useful help
Rocketman