'83 500SL Thermostat Location

[Diseasel300]

I pulled the SL out of storage today only to have the car troll me with thermostat problems (I swear the car knows when I want to go for a drive and promptly breaks down). Car got very hot very quick, then rapidly dropped. Engine temp keeps climbing to 100-100ish, then dropping quickly to 80, slowly climbing, then suddenly dropping repeatedly. Before today this wasn't happening, the temperature was always pretty consistent in the 85-90 degree range. Plenty of coolant, pressure cap is good, water pump seems to be pushing, fan clutch is locking up (passes the newspaper test). Everything points to thermostat issues.

Now the question - where is it on this car? The internet provides conflicting information, is it in the fitting on the side of the water pump or is it in the outlet fitting on top of the water pump? I don't want to pull the wrong fitting off and risk screwing something up or needing a gasket I don't have when the new thermostat comes in!

[rowdie]

Top.

84 degrees C.

[Diseasel300]

Excellent. The side fitting doesn't look fun to get at. Parts ordered, hopefully will be here by next weekend so I can finally take the car out! Fixed the brakes, now it's the thermostat. It's always something...

[meltedpanda]

Just be careful with the bolts/studs holding it all together. Soak them good with PB blaster or similar. Retapping those is no fun. Trust me...

[rowdie]

Originally Posted by meltedpanda

Just be careful with the bolts/studs holding it all together. Soak them good with PB blaster or similar. Retapping those is no fun. Trust me...

Good tip. Also after soaking try using a punch and hammer to "shock" them.

[Diseasel300]

Very good tips. I took the car for a short drive today to track the temperature behavior and I'm thinking the thermostat is stuck halfway between open and closed. It took forever to reach operating temperature (we're talking 15 minutes of driving before it got above 80C), held, fluctuated a bit, coasted up and down, speed, driving style, etc had no affect on temperature, it just did what it was gonna do. Parked until the parts come in next week sometime.

[Diseasel300]

The scary part is over with. Soaked the bolts with a 50/50 mix of Acetone/ATF and let it soak over night. Gave 2 firm whacks with a punch to each bolt and they came right out. Judging by their looks, they haven't been out in a LONG time. I had the intention of doing the job tomorrow afternoon, but they're closing the road to my storage building for paving. It's always something!

[Diseasel300]

As luck would have it the paving machine broke down so I had access to the car after work and did the job. The thermostat appears to have been the original, still had the Mercedes part numbers and everything on it. I found the little bleeder port in the thermostat housing clogged and luckily was able to clear it out and free up the little ball that lives in there. I suspect this was the actual issue causing an airlock. The car is rarely driven and the clog was not bad.

New Wahler thermostat went in, block got flushed and system refilled with fresh antifreeze and distilled water.

No more overheating! Temperature needle stays dead center in the gauge and doesn't budge. Makes me a happy camper!

[meltedpanda]

great, glad you got it sorted !

[rowdie]

Love it when the plan comes together.

[Diseasel300]

Well that was short-lived, back to overheating. Exactly the same behavior prior to the thermostat change.

Engine is running good, slightly rich actually.

Timing is good, no detonation or pinging, vacuum retard disabled.

Fan clutch is locking up at operating temp, shreds a rolled up newspaper.

I'm getting frequent high excursions of temperature on the gauge, greater than 100˚, but less than 120˚. Once the engine has fully warmed up, it refuses to run less than 100˚, even on a cool day. Using an IR gun, my inlet temp at the upper radiator neck is 220˚F, but the outlet at the bottom is only 160˚F, that seems like an unusually large temp split to me.

When I had the thermostat housing apart before, the little "burp" port on top was clogged shut. I managed to get it cleared out and can only assume it's clogged up again. Looked online for a replacement housing, and it looks like they're either NLA or astronomical in price! If that burp port is clogged, would it make sense that the thermostat is getting airbound and not functioning correctly? Anyone had any success cleaning one out and keeping it working properly? Am I barking up the wrong tree?!

Until I get this hot running issue straightened out, the car is staying parked, I'm not interested in blowing head gaskets!

[meltedpanda]

Seems like I recall that you have to have the thermostat in the correct orientation. Mine had an arrow on which way to position it in the housing
Also. how did you clear the port? Calcified or even structural changes in the port cannot just be cleaned and often need to be reamed out to return to original dimensions

[Diseasel300]

The thermostat in this engine only fits in one way, it's impossible to insert it any other way:



The port was fairly clean, but there is a little ball or something up in there that was "stuck" in place. Spraying with some CLR and prodding with some wire got it freed up and was passing air freely. I'm not entirely certain how that part is supposed to work, there doesn't seem to be any info on that fitting or it's function anywhere I've searched.

[meltedpanda]

Can you post a picture of the port

[Diseasel300]

I haven't got the part removed from mine, but you can see it on the one in this ebay auction:

https://www.ebay.com/itm/371818637727

The port in question is the little small part above the temperature sender threaded port. When mounted on the engine, it is at the highest point in that thermostat housing and has a hose that connects with the vent line from the top of the radiator to the expansion tank.