87 300D Expansion tank flow issue - PeachParts Mercedes-Benz Forum

Chris_87_300D · #1 11-11-2008, 06:16 PM

I've got a strange one - no luck finding a similar situation on the forum. I've got a new-style expansion tank (a couple of years old) with the silica pack inside. The tank appears to be compartmentalized - the coolant level sensor is on a separate section of the tank than the fill spout (top) and main connection to the cooling system (bottom).

Root issue that got me into this is that the coolant level sensor was bad but when I went to remove the tank and sucked all of the coolant out of the main section, the section with the level sensor remained full. Similarly, after replacing the sensor, the section with the sensor in it now doesn't fill up (and, so, my light is still on). Anyone know what the inside of one of these expansion tanks looks like? It appears that there's some sort of blockage. I've got a great clean engine and never had oil, etc. in the coolant - it's approaching 2 years old and still looks good as new. Not excited about buying a new expansion tank until I know that it's the problem. I'm tempted to try and drill a hole in the wall between the sections but not sure that I can be certain to get all the plastic bits out if I do that and/or if I'll screw up the flow of this rig.

Jeremy5848 · #2 11-11-2008, 07:37 PM

When I installed the new-with-silica-pack expansion tank in my '87, I kept the old one, p/n 124 500 09 49. There appear to be some passages inside – I can't tell where exactly they are – but if I pour water inside, it gets into the place where the float is. The only thing I can think of is something has stopped up a passage so the coolant can't get to the float chamber.

Your float valve should not have died after only a couple of years – sounds like you have some problem inside the expansion tank. What kind of coolant do you use?

Jeremy

Chris_87_300D · #3 11-11-2008, 09:23 PM

You're right - it shouldn't die after 2 yrs (and it didn't) - I tested the old one that I removed and it works fine. I knew that I'd have to drain/remove the tank to get the switch out and I expected that to be the problem so I just spent the $10 for a new one before I tore it all apart. Bummer is that I probably have a ground fault on the line to the cluster - I'll need lots of luck to figure that one out...

I'm using MBZ antifreeze - PN Q 1 03 0002 (standard issue). And, like I said, there's no evidence of floaters, oil sheen, etc. in there so I'm really surprised that something could stop up the flow. I'm thinking that maybe the silica separates two compartments and it's acted as a filter for little particles that I can't even see. It also seems as though the design would put the float switch right in there with the main coolant hose that comes up to the bottom - in a total failure somewhere, you'd want to know right away, not after the coolant drains slowly from one compartment to another...

Just a couple of questions if you still have the old version out (although these may be moot since the design has changed) -
Are the passages relatively large between the sections?
Is there an easy flow from the float compartment to the main inlet/outlet?

Thanks for the quick reply!
Chris

Chris_87_300D · #4 11-12-2008, 12:55 AM

Ok - it gets more interesting... Appears that there's an issue with the tank. See the picture below - I filled the tank about an inch above the middle line when cold - no coolant made it over to the section of the expansion tank with the float in it. I ran it to temp and held it there for about 15 minutes (stayed right at 80 deg. C) When I squeeze the upper radiator hose, the water sloshes around by the float valve and the level goes up enough to turn off the low coolant light. When I stop squeezing the UR hose, it goes right back down again.

When the engine got to 80 deg, the coolant just about filled the tank but not quite (see red line near top of left half). 3/4 of the tank is full to the top but the remaining 1/4 (as you can see, red line on lower right) doesn't fill. In fact, it decreased by just about an inch (the level sensor was initially off / floated but the level dropped as it got hotter). Even more strange, the main line to the expansion tank (at the bottom) is not in the 1/4 section that won't fill up (it's almost directly below the cap, on the 3/4 full side). So, why does the float section slosh around when I squeeze the upper rad. hose???

My current theory is that there's an air lock. I.e., air pressure is keeping the 1/4 side from filling up because there's no release for air trapped in the upper part of that section. Perhaps there was a vent hole that clogged? Here's my plan - through the vent cap, stick a hot soldering iron in the tank and burn a small vent hole in the partition near the top. I break the air lock and since it melts the plastic (instead of cutting/drilling), there is no risk of any shards of plastic getting into the system. Any feedback from anyone as to if that would somehow screw up the design of the expansion tank and make the problem worse (or not resolve it)?

Chris

Jeremy5848 · #5 11-12-2008, 01:35 AM

The only passage I can see is at the bottom of the "wall" between the right hand quarter in your picture and the larger section on the left. It's rectangular, about 1/2 inch tall and 1/4 inch wide, maybe a little less. There must be other passages by the old tank is yellowed with age and hard to see into.

The overflow tube (rubber hose attached to nipple just under the pressure cap) runs to an overflow tank located behind a plastic filler panel in the right front fender. It NEVER gets serviced. If it gets plugged, it could indeed create an air block that might keep coolant out of the right hand quarter. To test, remove the overflow line and blow into it, see if there is pressure. If very adventurous, suck, and see what comes out of it. Confirm by running the car with the overflow line removed, see if it behaves properly. I have a thread here somewhere that shows how to remove the inner fender liner and remove the overflow tank and vacuum reservoir, which live there in the 124 cars.

Jeremy

gsxr · #6 11-12-2008, 12:07 PM

Some of the small openings between the compartments must have gotten plugged up with something or other. Each chamber is interconnected to other chambers. If your tank wasn't fairly new, I'd say toss it and buy a new one. But yours looks good, so I'd try to fix it. Here's a comment from Joe Knight via email...

Just confirmed that there is indeed a vent at the top of the partition between the fill section and the float section. It's at the top, about 3/16" wide x 3/4" high. I can feel it with my finger inserted thru the pressure cap opening and a wire tie wrap confirms that it is indeed an opening. Don't see how it can have gotten plugged though in the ordinary course, but if it's there a stiff wire might serve to clear the obstruction unless it's a manufacturing fault in which case the hot iron trick might avail. If an ordinary soldering iron can't be made to fit a suitably shaped piece of bar stock heated to cherry red might serve.

gsxr · #7 11-12-2008, 12:09 PM

BTW, Chris, since you own a 1987 300D/TD... you may want to read the article at this URL, if you haven't seen it before:

http://www.w124performance.com/docs/mb/articles/124.1x3_buying_tips.txt

There's also a bunch of info on my website, links are below my signature.

Chris_87_300D · #8 11-12-2008, 01:04 PM

Thanks all! Not sure why but my specific (OEM MB) tank appears to be flawed - I can see the passages described by gsxr and Jeremy at the top of every partition except the two that wall off the 1/4 section with the float switch. Mine has a small plastic block attached to the side and the top that appears to have no purpose (on the side of the float switch section, close to the fill top) - perhaps it was a mfg defect and should have been a vent. Although that wouldn't explain why it worked fine for the previous 2 years...

In any case, a red hot metal rod made a nice 3/8" hole in the partition via the fill spout (per Joe Knight's suggestion) and the level promptly equalized. All in all, 2 hours to do a 30 second job but the experience will live on!

gsxr - thanks for the link - that reads alot like my service history but there's a few that I wasn't aware of. 95% of the work, tips, tricks, recommendations, etc. that I've done are due to this forum so thanks again to all. Also, I always appreciate the brand recommendations - it's a nightmare to do a big job and then a year later have a weak replacement part fail again.

Chris