Doing a routine look under the hood, I noticed some gobs of gunk in the coolant resevoir, and around the lip under the radiator cap. It's not black, nor oily, just kind of gunky, so I'm hoping that it's not a sign of the head gasket approaching detonation, but maybe just a coolant flush is needed. Any similar experiences? Is this a normal thing? Car is a '97 OBW, 2.5L, 122K miles.

Yep, that snot is perfectly normal! All cars do that.

 

I would recommend draining the coolant out of the engine and overflow tank. Remove the overflow tank and rinse it well, plenty of gunk in the bottom of it! Mount it back in the car, and fill to the FULL mark with water.

Fill the engine with pure water - demineralised please - and run it for a few miles to dissolve the gunk in the system. Then drain that out, and re-fill with a coolant that can handle the temperatures you'r expecting. 50% glycol should cover most ranges.

If you're unsure about purging the coolant system of air, do a search on the site, plenty of advice already given out.

 

I would stay away from "coolant flushes", they are caustic and God knows what they do to aluminium and rubber seals..

 

While you're at it, you might want to replace the main coolant hoses, and the radiator cap.

Good advise above. It doesn't look like that frothed tan foam, does it? That would be a problem.

Yep, that snot is perfectly normal! All cars do that.

 

I would recommend draining the coolant out of the engine and overflow tank. Remove the overflow tank and rinse it well, plenty of gunk in the bottom of it! Mount it back in the car, and fill to the FULL mark with water.

Fill the engine with pure water - demineralised please - and run it for a few miles to dissolve the gunk in the system. Then drain that out, and re-fill with a coolant that can handle the temperatures you'r expecting. 50% glycol should cover most ranges.

If you're unsure about purging the coolant system of air, do a search on the site, plenty of advice already given out.

 

I would stay away from "coolant flushes", they are caustic and God knows what they do to aluminium and rubber seals..

 

While you're at it, you might want to replace the main coolant hoses, and the radiator cap.

Great advise but do not forget engine block plugs also for a true flush.

Hear you on the plugs, but I have an irrational fear about messing with them :-)

Hear you on the plugs, but I have an irrational fear about messing with them :-)

They are really easy!! plus it flushes the block also!!

Thanks for the advice! Yeah, no foam, so that's a good sign. Hoses look pretty new as well, so no need to do those now.

Remove the lower hose to drain the engine as fully as possible.

 

To avoid the really massive air-bubbles in the flat engine, detach the upper hose and fill the engine slowly. Once the hose starts to fill up, attach to the rad, and fill through the rad opening.

They are really easy!! plus it flushes the block also!!

 

Are those two drain plugs 13mm hex or 14mm hex?

I believe that on a regularly maintained engine the "gunk" is only normal for those makes that have stop-leak tablets added at the factory as a preventative measure, in particular the US Big Three. My old "other" car had it, as well as its current replacement with low miles.

The stop-leak forms a hardened deposit in the area above the normal level in the expansion bottle, exposed both to the splashing cooant and to the air.

But if you have the gunk it might merely mean that the coolant change is overdue.