Can't Leave Well Enough Alone - Cooling System

Last summer, I did a lot of cooling system work on my '91 350SD. New parts/services included:

- New Aux Fan (only one bad)
- 2 Complete System Flush
- New Thermostat
- New Fan Clutch

The system was neglected and needed to be cleaned. An overheating 350SD was cured. She did run a little hotter than I would like on a summer day, but it was much, much better.

This spring, I just changed my radiator. Put in a new Behr and pulled out the original. FIgured it was good insurance.

While, she hasn't overheated, she seems to get hot quicker with loads now. Her cooling behavior is not the same. I am thinking of putting the old radiator back in. I'm fed up. Should have been an easy change.

Is it possible that I could have air pockets somewhere in the system. I have run her at high RPMs with the defroster on Hi (and sun roof open).

When not under load, she does cool down. Sometimes she seems to get a little warmer than she did before around town. The fan clutch still stops on a dime when the car's temp is at 90 or 100.

Can you offer any help? Please.....

By the way, the coolant is approximately the same mix as before also. Good till about -27. My Indy doesn't keep it any stonger than that for cooling purposes he says.

Do I have to bleed the system? Changing back to the original radiator is easy enough - just messy. Really, with the plastic, would have preferred to refresh the unit. It's 13-14 years old.

[Sooty Taillight]

Maybe you have a defective thermostat?

Replace the radiator CAP!
spo out

[TonyFromWestOz]

Pull the thermostat and check its operating temp. There have been cases where the thermostat, rated @ 85'C doesn't open until 95'C or 100'C.

I swapped out radiator caps with my other 300SD (same part). Also, with regards to the thermostat - why would it act up immediately after R&R the radiator?

[Sooty Taillight]

The last thread here from down under explained the thermostat pretty accurately, I thought. Try another one and if that doesn't solve your situation maybe you have a temp guage with a mind of it's own. I had two Volvo's that used to scare me half to death only to pull over and find nothing amiss!

Also, there is another thread on this diesel forum right now explaining nearly the same as here!


Sooty Taillight

I guess the questions I am asking is why the thermostat would start to behave weirdly just as a i changed out the radiator. It's odd that the temp was fine until I did that.

PS the thermostat is on the bottom radiator hose and a real pia on this model.

[Sooty Taillight]

Richard, I wish I could help you with that but yes, it is odd that installing just the radiator, a new one at that, should cause higher temps. One would think it the other way around!

Maybe you got an obstruction inside that new radiator. Wonder if there is a way to time how long a gallon of water takes to run through.....You could time the old one against the new one?????

Anyway, just a thought.........Good Luck


Sooty Taillight

A radiator cap makes NO difference in the temperature as long as it's below the boling point of the coolant. It will allow the coolant to boil at a lower temp, but the rate of heating will be unchanged until you reach the boiling point of the coolant.

The OM603 engines are very prone to air locks following coolant changes and if the cooling system isn't full the engine WILL seem to heat too fast and sometimes a bit erratically as low coolant levels allow boiling in some spots of the engine where the system is less than entirely full! If the pressure cap is not tight, this will be exacerbated.

Marshall

[Sooty Taillight]

Good point. I've been scratching my head on this one and hope that you have identified Richards heat problem. I should be more tuned for some answer, I deal every day with engine heat situations ranging from 16, 12, 8, 6, and four cylinder, 2 cycle and four, marine diesel engines that pass a lot of fresh and salt water.

An air lock does happen and can mimick what sounds like what has been described.

So now, what should be a procedure for unlocking air within the cooling system.

1. Fill the system full as you can get it.

2. Leave the cap off.

3. Run the engine.

4. Turn the climate control knob all the way into the red. (hot)

5. Run the engine well up and into operating temps.

6. Drive the car around the block. The cap off is not going to be any problem. You want to "burp" the system and with the cap on will hinder your objective.

Try that, and let us know!

[romansek]

After changing the coolent in my 603 I drove it for 3 weeks with the temperature around 100F. Than when I replaced the temperature sensor on the top hose connection to the block I discovered an air pocket. I had to add about a 1/2qt. thru the sensor opening. With added coolent the temperature droped to 90F.

Roman
----------
87 300SDL 148K
86 300SDL 210K
86 420SEL 225K
83 300D 222K
81 300D 148K

[Sooty Taillight]

My temp guage shows that I run average about 87C (about 1/4 the way to the next line on the temp guage from 80C) and has never traveled more than 3/4 the way to the that next mark on the temp guage, I guess represents 100C as it is not numbered.

I've climbed some pretty steep grades with the pedal all the way down and with the tranny downed to third gear!

Today, this same OM603 was installed from the 87 SDL into the 87 300D and I have driven the 300D about 65 miles this evening as a shake-down cruise and the temp guage reads exactly as it did in the SDL's temp guage.



Sooty Taillight:p

[FrankM]

your symptoms after replacing your radiator would be consistent with a defective radiator or a restriction in your coolant or air flow.

First choice, as already mentioned is an air pocket. We have at times had to raise the front of a vehicle as high as possible to allow the air to come out of the open cap/reservoir. Yours should be on right side so that may not work..

Usually though after a number of hot and cold cycles the air will be displaced anyway..