Okay, I'm at my wit's end with this one. My '84 300SD with 331K miles is always running at 98C at highway speeds (75 MPH and 3000 RPM), and at about 95C in stop-and-go traffic.

Within the last week, I have replaced the thermostat, engine temperature sensor on the driver's side of the block, a brand-new Nissens radiator, brand-new Meyle water pump, and brand-new Meyle fan clutch. Additionally, I have done a complete flush (not a citric acid flush) and am using Zerex G-05 antifreeze with a 50/50 mix. The coolant that came out when I replaced the radiator and did the flush was extremely clean, with no scale or particulates, even from the engine block drain plug.

The system holds pressure quite well, though I'm not sure the exact pressure it is maintaining. Even an hour after running the engine, removing the cap on the coolant reservoir yields an audible venting sound of the system pressure.

Outside temperature does not have any affect on the engine temperature. Freeway engine temperatures are a consistent 98C, whether the outside temperature is 23F or 78F. I have verified that my cluster temperature gauge is accurate with an infrared non-contact thermometer shot at various locations on the head (about 1C difference between gauge and IR thermometer). A spare cluster I have lying around shows the exact same temperature as well.

The only consolation is that the engine temperature NEVER gets hotter than dead-even 100C on the gauge, even while driving up moderate grades. While driving DOWN a grade (minimal/no load on engine), the temperature drops to about 90C, but then levels off at around 98C once the terrain does. Turning on the heat will cause an immediate drop in engine temperature down to about 80C, but it then slowly rises back to 98C and maintains there.

Aside from a Shout/citric acid flush, I don't know what else to do to get the engine temperature down to where it is supposed to be, and I'm not convinced that the acid flush would cause such a substantial drop in temperature, but correct me if I'm wrong. I'm open to just about anything at this point. Please help!

Sorry for the long post.

Thanks in advance.

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1984 300SD
351,000 miles

Sounds like the thermostat is doing a great job of controlling the temperature, but at a level that's about 15* too high.

I think I would subject the thermostat to a stove top actuation test.

Exactly my thoughts. I boiled the thermostat before installation and it began to open at 82C, so I figured it was OK to use. When I noticed my high engine temperatures, I pulled out the thermostat and drilled a 1/8" hole in it and re-installed it. Aside from taking a little bit longer to warm up, engine temperatures were the same. I replaced the drilled thermostat with a brand new one that I tested pre-installation and I'm still getting the same temps.

Hmmm.

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1984 300SD
351,000 miles

As mentioned in the original post, I have tested the temperature of the engine with an IR thermometer. Also, perhaps I wasn't clear, but the water pump is brand new, installed yesterday.

I have verified that the thermometer is installed correctly, and with the arrow pointing skyward.

The temperature drops momentarily when the heat is turned on, but once the heat is pulled out of the coolant via the heater core, the temperature once again rises while the heat is still on. After turning off the heat, the temperature doesn't spike, so that leads me to believe the coolant flow is at least consistent.

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1984 300SD
351,000 miles

Perhaps your water pump is getting weak or your gauge is a bit out of calibration.

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85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.

Hmmm....when you mentioned that turning on the heat lowered the temperature, it makes me think that there is a possibility of two things causing no flow through the radiator.

1)Thermostat is not opening

2)Thermostat is installed backwards. (Believe me, you aren't the first that has happened to. Don't ask me how I know that.)

I will attach a couple of pages from the FSM if that doesn't help iron things out.

Attached Files

20-005 Coolant Circuit & Engine Cooling.pdf (181.4 KB, 156 views)

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )

check the overflow tank cap, should have 100 stamped on it

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1983 300SD (daily driver)
1991 420SEL (work in progress)
1979 300SD (future replacement for 83 SD)

RIP:
1983 300SD (totalled)
1986 420SEL

Any possibility of an airflow issue with the grille or condensor?

I just ran out and checked the cap. I don't see any 100 on the top, bottom, or any of the sides. Attached is a picture of my overflow cap. Is that the correct one? It has now been 3 hours since I shut off the car, and there was no pressure when I removed the cap (but then again, the engine is cool now, so...).



Good suggestion. The grille is clear, and the condenser is, in my opinion, quite clean: no bugs and just a few rocks (the are behind the aux. fan is clear too, as is the back side of the condenser and the front of the new radiator). Attached is a picture I just took of the condenser.

Attached Thumbnails

   

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1984 300SD
351,000 miles

maybe replace that one with an OEM one, 16 lb comes out to about 1.1 bar. the OEM specifies a 100 cap (1 bar). worth trying since you've done/tested everything else. the 100 cap will make the cooling system run at a little lower pressure

__________________
1983 300SD (daily driver)
1991 420SEL (work in progress)
1979 300SD (future replacement for 83 SD)

RIP:
1983 300SD (totalled)
1986 420SEL

What is the best source for the 1 bar (14.5 PSI) cap? The ones at Auto HausAZ are 1.2 and 1.4 bar. Napa and AutoZone only show 16 PSI caps (1.1 bar).

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1984 300SD
351,000 miles

There is nothing wrong with a 16 PSI cap.

take out your thermostat and check your temp. Verify your gauge....mentioned it before.

__________________
85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.

Are you suggesting that I test the thermostat opening temperature by boiling it and seeing what temperature it opens at? As mentioned in an earlier post, I've done that and it opened at 82C while heating it in water on the stove.

Also, as I mentioned in the original post, I have verified the cylinder head temperature with an IR thermometer and an additional instrument cluster, which reads exactly the same. I specifically went and dropped $60 on an IR thermometer just this week so I could confirm that my gauge is working right. There is only about a 1 degree difference between the gauge and the IR thermometer.

If I'm not understanding your suggestions, or if I'm not being clear, please let me know. Thanks.

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1984 300SD
351,000 miles

I meant that you might take the thermostat out and running your car without it. Was a bit ambiguous. The thermostat is quite a restriction to flow. I have solved heating problems in two engines by taking the thermostat out.

Because your water temperature runs at a certain temp does the cylinder head? I would doubt that the water temp is equal to the temp of the cast iron. It has to adsorb heat from the metal and probably is slightly lower than than the block/head. Maybe you could point your ir meter at the coolant hose flowing out of the engine or at the coolant sensor. Maybe you have done this.

Are the passage ways inside the block corroded and restricted? Is your engine running inefficiently causing lots of heat.

Lots of good ideas from others as well.

My car with 200 on it ran at 95deg I changed the thermostat (both were labeled as 80deg) and it runs now at 85.

__________________
85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.