'84 300SD always running at 98C
 

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.

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.

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

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.:o)

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

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.

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.

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

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.

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

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.

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.

I have re read your post and it looks like you have done your homework before you posted.

Is your timing retarded or perhaps your car has lots of blow-by heating up the internal engine.

Ahh, okay. I thought that might be what you were getting at, but I wasn't sure because in the the FSM that SD Blue posted, it specifically says that, "It is therefore wrong to remove the thermostat for better cooling of the engine." While I never planned on permanently or semi-permanently using the vehicle without a thermostat, I wasn't sure what the objective of testing the temperatures without a thermostat would be.

The place I like to focus my IR testing is right at the area around the engine temperature sensor. Not only is it more accessible for the IR gun, it also seemed logical to test at/near the sensor reporting the temps.

From what I can tell, the engine seems to be running very efficiently. About 6 weeks ago, I rebuilt and balanced the injectors (using Monark nozzles, and getting all injectors within 3 bar of each other). I'm currently averaging 28 MPG with a 60/40 mix of freeway/city driving. I have not yet emptied an entire tank on the freeway, so I can't report on highway-only efficiency. My Mercedes-only (diesel specialist) independent mechanic tells me he's very impressed with the low amount of blow-by from my engine, especially considering its high miles.

I personally don't know how to measure blow-by and I haven't done a compression test, but the car has good power, doesn't smoke, and when removing the oil-filler cap on the valve cover with the engine hot and running, there is less blow-by than any of my 3 friends with the same engines in their cars. (How scientific, right?)

Lets see:

You have a car that runs strong, gets good mileage, doesn't smoke, and doesn't overheat (you're not overheated until the gauge hits the red-see page 54 of the owner's manual). I'm failing to see a problem.