Summer has finally hit the east coast. Driving down the road the engine temp guage sits about 90. Stop and go driving quickly sends the needle to about 110-120. It acts like it is low on coolant, but the level is correct in the expansion tank. It has the correct 50/50 mix. One other key that leads me to beleave I have air in the system is shortly after shutting the engine off I hear a boiling sound from the area around the battery. Sounds like it is coming from under the dash. It was my understanding that the system was self bleeding. Or is there a procedure to ensure all the air is out of the system.

You could have air. What year?? check the level when cold. remove the cap from the expansion tank and let the engine run up to temp. Turn on the A/C. Watch the temp and make sure your fans are coming on. Rev it a bit and listen to see if the fan clutch is OK. If the level is correct and it doesn't boil over w/o the cap, depending on the year, you may have crud in the rad or a reluctant thermostat.

Ken:

I seem to recall you owning a 300-SEL. If I'm wrong, ignore this post.

I have a 300-SEL and had the same problem you described. The first time it occurred, I too suspected air in the system. Reference task 20-010 - Draining, pouring in coolant. This task is on the 126 CD-Rom. Step 7 is very interesting. I've NEVER seen this mentioned on this or any other site. Following the procedure in 20-010 will eliminate trapped air.

The gurgitation in the overflow tank recently reappeared. It turned out that the pressure cap was bad AND that the expansion tank was leaking, thereby not holding pressure. I replaced both along with all hoses.

My cooling system is once again doing fine.

Regards.

My first inclination was to point fingers at a faulty fan clutch. But this revelation of fact does lead one to believe its an air-pocket which must be eliminated as suggested by Mike.