300SDL overheats
 

Overheating.
My 87 300sdl overheats. AC works great, 35F at vent.
Car runs great, plenty of power and good mileage.
History:
Original thermostat came out in pieces.
*replaced cracked cylinder head (#14) with #20, super clean from machine shop.
*replaced coolant tank.
*replaced coolant tank cap (20lbs pressure type).
*replaced water pump (beck arnley)
*replaced thermostat (MBZ 80 degree C from dealer)
*replaced radiator ( Behr)
*replaced top and bottom radiator hoses.

I define overheating as:
Ambient temp is 85-90 F.
* during 4 mile climb (highway 50, Bass lake grade) temp with ac on is 105 C.
* during a 3 mile grade at 25-30mph with ac on temp is 110 C.
* On flat freeway, ac on, temp is 85-90 C. Going downhill it drops to 83C.

I have removed the upper radiator hose, while parked on a hill, and filled the hose...
several times.
The coolant tank is full, slightly above full mark when at operating temp.
I have 2 electric fans and a custom shroud instead of stock fan.
Plenty of hot air being emitted from fans.
I have not flushed cooling system...yet.
Heater works ok after about 20-30 minutes drive.
Today, I drove my usual route home and temps exceeded 110C. When I got home I let car idle,
until temp gauge showed 95C. I used infrared non contact thermometer to measure head ( at injector 2)
to be 235F.
Radiator hose from cylinder head measures 227F and lower hose measures 222F. With old radiator,
I measured a 45F difference from top hose to bottom hose.
The overheating problem has gotten worse with the new thermostat and radiator.
It appears that the coolant is not going through the radiator enough to generate a
significant temperature drop.
With ac off, temp drops by 5-10C.
I'm stumped. Since the ac is cold, I would believe that the fans are doing their job.
But, obviously I'm missing something.

OVER HEATING
 

1984 300D TURBO 153K
REPLACE THE MONO VALVE AND SMALL WATER PUMP AND FAN CLUTCH DIY AND HEAD BY PASS SMALL LINE ON FRONT OF HEAD TO BLOCK 5 IN. LINE REMOVE AND CLEAN, YOUR HEATER IS SLOW TO INCREASE HEAT

Tstat could be bad right out of the box....

I'm not a thermodynamics physicist, but in my mind if there isn't much difference between the temp across the radiator, it would seem like more of an air-flow problem than a liquid flow problem. If the coolant was flowing slowly through the radiator, there would be more time for heat transfer and would seems like temp differences would be relatively large.
I'm thinking it may have to do more with your shroud/fan setup that may actually be hindering air flow at higher speeds. Hope this helps.

I haven't done anything with the monovalve or aux pump, I'll remove them and check them.
I thought that when the monovalve was OFF (12v applied to its coil), the coolant was blocked from the aux pump and heater core.

You bring up good points. I was guessing that my low ac temps were an indication of sufficient air flow at idle. I will install the original mechanical fan and shroud as a test.

thanks.

Agree with LoosBenz. Full temp drop (from 227 to ambient) would indicate a coolant flow problem. Minimal temp drop indicates an AIR flow problem. You need more air. Does the car still have any noise panels underneath? How many amps are your fans?

Remember that the AC condenser is warming the "ambient" air consumed by the radiator from 10-40 dF, depending on all the variables. Stick a thermocouple betwixt the condenser and radiator, then at the radiator outlet.

The monovalve should always be open/flowing and have 12 VDC on one side (if the fuse and wiring are good). The climate head supplies a GROUND to make the valve cycle/engage and block coolant flow. Polish wiring on a Kraut car; common political border? The 126 mono is a weird one that tees to allow the snow-melt windshield heater to function.

Sometimes rads are bad right out of the box as well... Sounds like a rad problem if your heater core set to max with the fan on max shows a cooling difference from the in and out line of about 28-30F...

http://www.peachparts.com/shopforum/showthread.php?t=280871

Have a look at the info here. Not 100% sure this will all apply to a diesel, but it's a good basis....

Thoughful tutorial, I'll go through the list. Thanks.

This sounds like what my car is doing, except that I get heat after about 5 minutes on cold days. It only seems to heat up on slow climbs at low RPM, like the one to my house on a mountain. It is worse with the AC on.

A few years ago I replaced the radiator and it is better than it was before.

My prime suspect at this point is the fan clutch which I will refill soon. I have stock electric fans that work great.

I just replaced my thermostat because it was leaking! I found a tiny chunk of plastic in the old one, but I don't think it was interfering with its operation.

What is a MONO valve and where is it?

Placerville...

Your car wouldn't happen to be black with silver lower panels would it?

No, his name is "Whitey", arctic white.
Sometimes he sports a roof rack carrying lumber.

Lol - awesome. I'll keep my eyes open.

Reason I ask is that I passed a guy in a black 300SDL going up Bass Lake grade a few weeks back, then saw him later on Pleasant Valley Rd. Stood out because it's exactly the same color scheme as my car - even had the same wheels!

If your ever out and about in P*Ville and you see a tall bald guy in a black & silver 87 SDL with a Biodiesel emblem on the trunk, feel free to say hello.

Incidentally, I'm having the exact same overheating issues you are. Just ordered a new Behr radiator and I'm hoping that takes care of it. Only things that won't be new after that is the water pump, monovalve and temp gauge.

I'm also missing the noise panels that are supposed to be under the engine. No idea how that affects cooling.

Could altitude have anything to do with it? We're not that high up, I don't see why it would.

I replaced my water pump because it had bad bearings and it didn't make a difference. A new radiator helped, but didn't totally fix it. I still have the cover on the bottom.

My next step is the fan clutch.