Coolant level question - PeachParts Mercedes-Benz Forum

cgryphon · #1 08-03-2006, 11:03 PM

The 87 300SDL low coolant light came on tonight after driving about 10 minutes. I had just gotten on the highway, so I had given it a hard run up to ~75MPH. A/C on full blast and engine temp running around 90C.

Before I left the driveway I had checked the oil and looked at the coolant tank and brake fluid reservoir, as is my habit. All fluids were looking good, so I was surprised that the light came on.

I pulled over, and with the car running took a look. Sure enough, the coolant level is down near the sensor. Nothing wet under the car or around the engine, no steam, and the exhaust was normal. Left the car idling while I looked all around again, nothing obvious noted. Engine temp. had climbed to ~ 100C while sitting there.

Decided to run over to my “real” gas station where they still have some mechanics around. About a 3 mile run over there, engine temp steady at 90C with A/C on, no smoke, running great, but light still on.

Pulled in to the station, shut it down, waited a couple of minutes for one of the guys to come over, popped the hood and………coolant level normal. Re-started the engine, increased the RPM while watching the tank and sure enough the level started to drop.

What’s going on? Is the coolant level supposed to rise and fall in the expansion tank depending on if the engine is running or not? I think not!

Please help me understand this.

Thanks,

Clay

kmaysob · #2 08-03-2006, 11:09 PM

yes the coolant level will rise and drop. if its dropping that much i would think maybe and air bubble or sumthing?

cgryphon · #3 08-04-2006, 12:54 AM

Looks like the level is moving around 1.5 to 2 inches on the side of the expansion tank.

Seems like if it was supposed to move this much the owners manual would have said something like "only check the coolant level with the car off" or they would have two lines marked on the tank.

It is like the coolant is taking a little trip somewhere else while the engine is running at higher rpm's and then returning to the tank while idling or off?

I am confused. Where is it going?

Thanks,

Clay

sixto · #4 08-04-2006, 02:13 AM

There are a couple of sensors on the hump where the upper radiator hose attaches to the block (looks like a thermostat housing but the thermostat isn't in that area). Remove one of the sensors and fill the reservoir until coolant leaks out of the hole. See if that improves anything.

Sixto

vstech · #5 08-04-2006, 07:26 AM

look over your hoses and see if one is swelling.
air can do this, but operating temp should push it back up...
seems ominous.
John

newptken · #6 08-04-2006, 07:51 AM

What’s going on? Is the coolant level supposed to rise and fall in the expansion tank depending on if the engine is running or not? I think not!

I had a 91 SD that the coolant in the expansion tank would rise and fall, I got information on a forum that it could be my water pump. Replaced the water pump problem solved. Hope this helps.

TX76513 · #7 08-04-2006, 08:42 AM

You may have a bad coolant sensor (located at bottom of reservoir). The rise and fall of the float is activated in the last 1 3/4 of the bottom or tank. They do go bad as mine did. Not an expensive part or a hard DIY.

I strongly suggest you do an oil analysis on your next change to see if you are getting any anti-freeze in your oil. Good piece of mind for $20.

cgryphon · #8 08-04-2006, 09:42 AM

OK, I've left it overnight to cool, so I'm going to have a good look around it today and see what I can find. I was thinking it might be a hose also and not something worse.

Thanks for the ideas so far.

PaulSpringmann · #9 08-04-2006, 01:06 PM

My SDL does did this as well this past spring; the low coolant light would only come when driving a freeway speeds, and then go back off when coming to a stop.

I initially thought the sensor was the problem, but checking the level immediately stopping, did show the level significantly down. However, the level would slowly rise back up.

I added an additional pint of coolant and the light has not come since.

I imagine that the level still fluctuates, but I don't know if this is normal. I keep my eye on it all the time for any other symptoms.

as ever
Paul

kmaysob · #10 08-04-2006, 08:56 PM

coolant level rises and falls. why do you think new cars expansions tanks are labeled with hot and cold fill lines?

Dervman · #11 08-05-2006, 01:34 AM

My SDL did exactly the same thing soon after I purchased it. Gave me quite a scare what with all the concerns about overheating heads and everything.

After checking all the obvious things and asking a few questions here, I concluded it was air trapped inside the cooling system. I raised the front of the car up on ramps overnight, then next day removed the rubber hose between the expansion tank and the top of the Rad. at the tank end then held it up and used a funnel to top up the system with coolant. I was surprised how much it took to expell the air that had accumulated in the top of the rad. After that I had no problems.