1987 300d fluctuating tempature on the highway.

[Bolognablake]

Hi,

I have a 1987 300d with 214k miles.

It runs warm in traffic because the auxiliary fan doesn't work. Easy to explain.

But the issue right now is that the tempature fluctuates between 83ish and 95ish on the highway. Its weird, it just kinda goes up and down.

Any ideas? Thermostat? Ive already bought one, but I've heard it was a bear.

Thanks.

[JPUTT]

The aux fan will not come on until it gets to 115-120. Jump the temp wire to see if it works at all. If you are running in hilly terrain or heavy on the throttle you will see fluctuations in a diesel. I see 85-100 running in the mountains of NC depending on the outside temp and A/C running or not.

If your temp is jumping around or gets hot quickly and then suddenly drops you might have a thermostat problem. It is not too hard to change at least on a 603 engine. Make sure you don't have some air in the system after flushing or messing around with the coolant level.

[Bolognablake]

So I took it on a trip, about 3 hours each way, over somewhat hilly terrain.

It builds up heat while under load, and cools down on downhill/flat gound.

It got close to 100*C.

So I flushed the radiator yesterday. The antifreeze was super clean, didn't appear to have any oil in it (I really should have saved it). After the flush solution, nothing came out other than clear water. Also it looked like the radiator was fairly clean and new. It was a Behr, and couldn't be more than a couple years old. I'm almost certain there's no air in the system.

Didn't help. The temp is still averaging around 90*.

So, in addition to the aux. fan not working I noticed the the fan clutch didn't have much resistance at ~90* (after I shut the car off). It spun pretty freely. I know this shouldn't make a difference at highway speeds, but I found it odd.

Also, I was reading that I have to remove the water pump to access the thermostat. Is that true?

Thanks again.

[bobodaclown]

Originally Posted by Bolognablake

So I took it on a trip, about 3 hours each way, over somewhat hilly terrain.

It builds up heat while under load, and cools down on downhill/flat gound.

It got close to 100*C.

So I flushed the radiator yesterday. The antifreeze was super clean, didn't appear to have any oil in it (I really should have saved it). After the flush solution, nothing came out other than clear water. Also it looked like the radiator was fairly clean and new. It was a Behr, and couldn't be more than a couple years old. I'm almost certain there's no air in the system.

Didn't help. The temp is still averaging around 90*.

So, in addition to the aux. fan not working I noticed the the fan clutch didn't have much resistance at ~90* (after I shut the car off). It spun pretty freely. I know this shouldn't make a difference at highway speeds, but I found it odd.

Also, I was reading that I have to remove the water pump to access the thermostat. Is that true?

Thanks again.

Sounds normal. Above 100 and you've got issues from what I understand.

[winmutt]

I rarely get over 84. Check your fan clutch. Get a new tstat. New radiator. Use Zerex G5. Overheat a 603 and youll need a new head.

[aaa]

Originally Posted by Bolognablake

Also, I was reading that I have to remove the water pump to access the thermostat. Is that true?

No you don't. Start with replacing that, then move onto the radiator.

[babymog]

I believe that the clutch fan kicks in around 105F, you might want to check on that though.

[bobodaclown]

From the manual:
A distinction is made between the bypass mode (up to approx. 85c, mixing phase 85-100C and the operating state above 100C.
Up to a coolant temp of approx 85C the main valve is closed and the bypass disk is fully open.
At coolant temps of approx 85-100C the main valve as well as the bypass disk are more or less open, depending on engine load.
At coolant temperatures of more than approx 100C the mail valve is fully open. The bypass passage is sealed off by the bypass disk. The entire quantity of coolant has to flow through the radiator.

The fan cut's in at an air temp of 71C (71 degrees Celsius = 159.8 degrees Fahrenheit), or the coolant temp is between 90-95C.

Still sounds like normal operations for your setup.

[Bolognablake]

Thanks for the info guys.

I'm going to slap in a new thermostat today, I'll report after.

[Aquaticedge]

on the same note, I'd like to verify my fan works what do I need to Jump to see?

[babymog]

Electric fan? Jumper the high-pressure switch on the A/C filter/dryer for low speed, jumper the temperature sensor on top of the water outlet on the head (the one that has the hose to the top of the radiator) for high speed.

[Aquaticedge]

should it kick on with the AC going?

[babymog]

It should come on low when the pressure rises enough in the system to trip the high-pressure switch.

[Bolognablake]

Question about the thermostat.

According to the FSM, it appears the thermostat is on the bottom passenger side of the motor, on the water pump housing. Is this correct?

Also, what's the best way to get at it? The turbo and associated parts are obscuring the view from up to.

[Aquaticedge]

Originally Posted by babymog

It should come on low when the pressure rises enough in the system to trip the high-pressure switch.

then my fan is bad.