overheating 300sd - PeachParts Mercedes-Benz Forum

petesbluebenz · #1 08-21-2006, 04:27 PM

Hi fellas, I have an '85 300SD which i love. Problem is, after driving, especially with the air on, after i shut off the car, steam and coolant come out of the overflow tube. Have a relatively new radiator, new factory stat, new coolant resevoir, water pump and hoses in good shape. Temp guage rides right around 80 even on hot days, with the air on, and temp sending unit is good. Aux fan works. yet every morning I have to add anywhere from 1/2 to 1 gallon of coolant because it boils it out after i shut the car off. Doesn't do it nearly as much when i don't use the air. But the thing that frustrates me is that the temp guage never goes high unless it's low on coolant. Other that that, it starts and runs like a charm, with 291 well cared for miles. Doesnt even leak oil, if you can believe that. HELP!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

pawoSD · #2 08-21-2006, 04:35 PM

You could have a bad radiator pressure cap. Try replacing that and see if it stops the problem, they're only like
$4 or something. If it claims to be running at the right temp then there's definatley something up with the cap, the spring may have weakened allowing fluid/steam to escape at relatively low (normal) pressures.

jrgslg · #3 08-21-2006, 04:37 PM

Quote:

Originally Posted by pawoSD

You could have a bad radiator pressure cap. Try replacing that and see if it stops the problem, they're only like
$4 or something. If it claims to be running at the right temp then there's definatley something up with the cap, the spring may have weakened allowing fluid/steam to escape at relatively low (normal) pressures.

Ditto,looks like on your list that has not been replaced,Hope it works,Johnny

petesbluebenz · #4 08-22-2006, 11:21 AM

thanks for the replys guys. the cap is new as well, and it's the correct factory cap. sorry i forgot to mention the new cap, i've just replaced so much on this car trying to solve this problem. i'm praying it's not a blown head gasket, although i have no coolant in oil, no white smoke, and no oily residue in coolant.

petesbluebenz · #5 08-22-2006, 04:17 PM

quick update to my previous post. when i run the car up to temp, with the cap tight, and then shut it off, the bottom 2/3 of the radiator is cold, the top 1/3 is red hot. (radiator is new). now, if i loosen or remove the cap, the radiator temp is even from top to bottom. could a weak water pump cause this? just unable to maintain proper flow under pressure? god i hope it's not a head gasket. local mercedes mechanic quoted me about 1900 plus extras if it needs a head gasket. car isnt worth putting that kind of money in.

dieseldiehard · #6 08-22-2006, 04:31 PM

That is normal, the top is always hotter because the hot water comes into the radiator and its cooled across the length of the tubes as air flows by the fins, provided the tubes aren't plugged up, a radiator always works like that. Drain the water and it will start out warm and get hotter as the water at the top finally gets to the drain.
Are you sure the thermostat is good? There have been odd cases where a brand new (dealer!) thermostat was defective right out of the box.
And is it mounted the right way? Some have discovered that they installed it up side down, in the later engines they made it impossibkle to do that but in the 617 its possible to install it the wrong way.
Try running without any thermostat (some will tell you not to do that on a engine at least on a long term basis) if its still steaming then your water pump must not have all its impeller blades (ask gsxr what that looks like!)

$1900 for a head gasket? too much! Its doubtful that would be the case anyhow, unless you are losing coolant (I mean not out the overflow pipe)

Old300D · #7 08-22-2006, 04:35 PM

I've had fairly new thermostats stick partially shut. It sounds to me like your thermostat may be doing this.

85chedeng300D · #8 08-22-2006, 08:15 PM

even if you have a perfectly good working thermostat, but if the system has an air lock in the engine block and cylinder head, the thermostat will not open because there is not enough antifreeze to trigger the thermostat to open and circulate the antifreeze all around the entire cooling system, therefore the bottom of the radiator will be too cold compared to the steaming hot upper radiator temp. be sure to burp the system and get rid of the air inside the engine block and cylinder head to ensure the proper operation of the thermostat and the circulation of the antifreeze over the entire cooling system.

petesbluebenz · #9 08-22-2006, 09:25 PM

thanks for the quick replys guys. i will try all your suggestions. i refuse to give up on this car,, i absolutely love it. tomorrow i'm pulling the pump to check the impeller blades. will give you an update

stephenson · #10 08-22-2006, 09:45 PM

I don't think impeller blades check will provide much info - waterpumps either work or start to leak around the seals, then eventually will fail ... I don't think I've seen a "weak" one.

If you don't have good pressure in the system due to a bad seal on the waterpump it can cause strange things to happen - check the waterpump weep hole to see if you are getting any seepage.

SD Blue · #11 08-23-2006, 12:07 AM

but this really sounds like a large air lock in the system.

If so, try this: put the front end up on ramps/jack stands, while cool, and disconnect the upper radiator hose. Then fill both the radiator and upper hose till they can't take any more. Reconnect the hose and run till warm then top off.

petesbluebenz · #12 08-24-2006, 12:14 PM

First, thanks to all you guys that replied. I appreciated your suggestions. Still couldn't understand why the benz ran cool, and rad temp was even from top to bottom, as long as i left the cap off the resevoir. Put the cap on and run the car down the road, and the bottom 2/3 of the rad would be cold, and the top hot, then on shut down, coolant would boil out of the overflow hose. Guess what the fix was? I had questioned the water pump. so i pulled it out, and the impeller blade fell off and hit the ground. So, the impeller would turn on the shaft as long as there was no pressure on the cooling system. As soon as i put the cap on, and the system built up pressure, the impeller would just stop spinning. and the pump shaft would spin within it. So, water pump fixed it and my baby is back on the road!!!!!!!

bbeardb · #13 03-31-2007, 01:27 AM

Quote:

Originally Posted by SD Blue

but this really sounds like a large air lock in the system.

If so, try this: put the front end up on ramps/jack stands, while cool, and disconnect the upper radiator hose. Then fill both the radiator and upper hose till they can't take any more. Reconnect the hose and run till warm then top off.

Is this burping?

I'm having the exact same issue. Very hot top hose and 1/3 radiator, bottom hose and 2/3 radiator cold as can be. It's a new radiator too.

psfred · #14 03-31-2007, 02:16 AM

There is an easier way --

Squeeze the upper radiator hose flat, then crimp the small line to the reservior shut and let go of the upper hose. Coolant will be pulled into the lower radiator hose and rad.

Repeat until no more coolant pulls out of the reservoir. You will likely have to add more several times.

I believe there is a coolant bleed bolt on some heads, too.

617 engines have thermostat in the upper radiator hose, but 60x engines have the thermostat in the side of the block where the lower hose comes in, makes it easier to get them full with this method.

Usually fills the heater core pretty well too.

At least you aren't putting a new head on like I am....

Peter

hey_allen · #15 03-31-2007, 05:32 AM

Has anyone else been tempted to put a bleeder plug on the top of the thermostat housing?

Looking at the one on my 300SD, it looks like they designed a point in it to allow that, with what looks like a reinforced boss to tap for pipe thread, just in front of the thermostat.