2001 M6, base.

so Im driving to work (about a 6 mile trip) and noted the temp gauge is beyond 230. Turn on the heater. Once I start rolling, temps drop back to normal. Next light, same behavior.

Get back home, crawl under, no debris.
Also There are no obvious coolant leaks.

Checking the tank, yup, antifreeze is low. Top off.

Continue the leak check. I notice a fitting high right (passenger side) .of the radiator. 1/2” dia. Has a pale green plastic screw-in plug, but the fitting also has external threads. The green plug is loose and kinda falls out into my hand.

first though is external threads...what happened to the cap for this hole, and could this be a leak point?
But there’s no crud, discoloration, that would indicate a leak,

2nd though is this fitting for the auto transmission cooler line I dont have, And the green plug is just a cover. Since I dont have a A/T, it makes no difference?

As an aside, the radiator was replaced (dealer) a bunch of years ago, so AFAIK, it is a GM radiator, not aftermarket.

Your thoughts?

BB Bob : Thanks!

As a follow on for other readers, I just finished letting it come up to temp.
Fans kicked on at 225, brough the temp back down in rapid fashion.


Looks like a light seep at the overflow tank cap, so that might explain where the coolant has been going over time..I just never smelled/noticed it, and was insufficient amount to leave spots anywhere.

I checked again this morning.

The area around the nipple is clean and dry, the clamp is tight

THe crud at the cap is only in the rear side groove that runs parallel to the engine, not in any of the other grooves molded in the top of the tank.

But on the ground, There's is a leak spot

So back to close examination.

There’s no wet lines or spots on the body, and all the hoses are the normal uniform dry grey color from the top.
Bottom side of the hoses are clean and dry... except the reservoir return line hose

From the bottom, I can see damp sticky crud at one point on the bottom side of the tank hose. from that spot on the hose, it is straight drop to the ground, and corresponds with leak spot

There’s no wet line from the radiator to that point on the hose, nor is there a wet line from the tank to that point. Manipulating the hose does not show an obvious hole, so my logic says it must be a very tiny split or pinprick that only opens up when the hose is hot and under pressure..

So...looks like I‘ll be off the the local parts place(s) for a cap and a hose.

Thanks for the additional suggestions!

Follow up.
Had to special order both the cap and the hose. Both replacements are Delco/GM
topped off coolant, replaced the old 15 psi cap with a new 18 psi cap, replaced the reservioir hose.

Temps seem to hold stable at 192 driving around my neighborhood.
Back home, I let it sit and idle. Temp went to 226, paused, dropped back to 217 and held for a minute
Climb back to 230, pause, drop to 217. Repeat the 217/230 cycle.

Interestingly, the fans didn’t step up to high until I turned on the AC.
Temp dropped to 209 before the fans changed back to low speed.
Turned off AC, Back to the 217/230 cycle.

Follow up 2. Found it

While the car was idling (30+ minutes) nothing dripping, etc.

2 hours after shutdown, there’s the spot.
dripping from the bottom side of the water pump housing area.

So much for an inexpensive fix....

New pump is in. About 3 hrs or so. Have not refilled coolant yet. Gotta find the how to bleed air FAQ.

Much easier reinstall if you take out the MAF.

Wearing for anyone that is reading for how-to hints - the thermostat housing bolts do not like 22# on the torque wrench. Sheared the head off one of them.
Thank (insert favorite deity here) there’s a countersink on the holes. Managed to catch the one protruding thread with a pair of wire cutters and was able to get the rest of the bolt out.

On reassembly, its easier to put the help on the tensioner first, push really hard and slip it under the water pump, rather than try to put iit under the pump and then over the tensioner