Thermostat replacement OM603 - PeachParts Mercedes-Benz Forum

cmac2012 · #1 12-12-2019, 09:26 PM

My engine doesn't go over 70, at least not until my recent radiator blowout. I read people saying that overheating issues usually aren't stat related but I'm thinking running too cold probably is.

I pop the hood, thinking I'll knock this out in 30 minutes. Oopsie, maybe not. I'm not even sure I've found the housing yet. One guy on some thread talked about a need to remove the intake duct for the turbo.

I'm not seeing a turbo air intake anywhere close to what surely must be the thermostat housing. In the pic, the arrow points to the closest candidate I can find and it doesn't even look that much like a thermostat housing. The entire intake manifold out would likely be fine and dandy but I'm not going to do that - not sure what the aforementioned guy was talking about.

I already bought a NAPA thermostat, they get good reviews in numerous places. About now I'm wishing I'd bought the online item with pdf how-to. Not your normal thermostat replacement.

Diseasel300 · #2 12-12-2019, 10:38 PM

Thermostat is on the intake to the water pump. Follow the lower radiator hose. Where it connects to the engine is the thermostat housing. 2 bolts and that cover comes off. If working from the top, you do want to remove the air box and rubber hose to the turbo.

cmac2012 · #3 12-13-2019, 02:26 AM

Thanks for that. It's an unusual engine somewhat.

Are you saying there's an option to work from the bottom? I'll look it over soon, I imagine the answer to that question might be obvious. OTOH, you did say 'if working from the top' so I'm guessing there is an option.

83w126 · #4 12-13-2019, 03:05 AM

It would be way easier to reach from the top, but yeah its where the lower radiator hose connects to the engine. It's not as easy as some cars but it was definitely not anything too difficult when I did mine.

cmac2012 · #5 12-13-2019, 10:49 AM

The business at the end of the upper hose looks formidable. Was not going to dig into that without more information. Didn't have the right shape as well. I'm more than a little relieved to learn it's on the bottom hose.

Mxfrank · #6 12-13-2019, 04:41 PM

The thermostat isn't in the upper hose neck, it's at the lower hose connection. It's very simple, two bolts and it's out. You can probably get away without even undoing the lower hose. This is what the housing looks like:

https://www.peachparts.com/shopforum/attachments/diesel-discussion/138159d1472570223-603-coolant-change-forcibly-opened-thermostat-bypass603_5.jpg

cmac2012 · #7 12-13-2019, 09:23 PM

Thanks. This is a wild engine.

dlssmith · #8 12-16-2019, 11:02 AM

You'll definitely want to remove the turbo intake hose. And while there you might consider changing out the temp sender.

Diseasel300 · #9 12-16-2019, 12:15 PM

The temp sender on the passenger side of the engine is for the glow plug controller. No point changing it if it isn't physically broken. The '86/87 cars don't even have afterglow.

cmac2012 · #10 01-02-2020, 04:30 AM

Anybody know what the little brass bumpout is for? My guess is it allows a small amount of fluid flow w/o appreciably altering the function of the thermostat. Perhaps having the thermostat on the lower part of the block necessitates this feature. First time I've seen it. There's a tiny steel ball in the little brass cage, stainless steel no doubt.

No surprise, this thermostat was no longer functioning, you can see it open in the photo. Standard operating temp was about 70. Maybe less.

Note to self: buy fluids and American car parts at NAPA, nothing more for Mercedes. The thermostat I got from them, supposedly for the SDL, has no dimple for mounting one way only and no tiny fluid vent.

Mxfrank · #11 01-02-2020, 08:40 AM

This is called a ball valve or jiggle valve. When you first fill the system, there's a possibility that an air bubble will form behind the thermostat. The valve allows this air to bleed out. Once the system is full and the engine is operating, coolant flow forces it closed. You can read more about this subject here:

https://www.coolcatcorp.com/thermostats/Air%20Bleeds.html

cmac2012 · #12 01-02-2020, 12:07 PM

Great write up. I had a feeling that's what it was doing. I didn't realize that it would close once the air was bled out.

I had heard that patience us needed when filling the coolant system on that car, this makes is clear why.

Diseasel300 · #13 01-02-2020, 01:54 PM

Filling with coolant can be GREATLY sped up by filling through the upper radiator hose. Fill until it overflows, reattach to radiator, keep filling the coolant bottle until it won't take anymore. It'll get you really close, maybe a pint or two low once it burps the whole system.

cmac2012 · #14 01-02-2020, 05:34 PM

Great advice, I've been doing that lately. Seems like extra insurance. You're getting a lot of coolant into the head and block right off.

I was almost tempted to make the bleeping NAPA thermostat work. Not worth it. Engineers sometimes overengineer but too big a gamble for saving a day or two and pennies. I have one on the way. The nearby dealer wants bank and it would take them two days to get it!