Coolant Temp Help, 1991 350 SDL
 

Around town, and on the freeway, my 350 SDL seems to be behaving well. Coolant temps 80-90 with AC on a 95 degree day.

As soon as I start climbing mountain passes, the coolant temps are jumping quickly to around 110 before I cut the AC and turn the heater on. Temps work their way back down quickly. If I just turn the AC off, it will float around 100.

Today, I pulled over when it was around 105 C, and checked what was going on with the fans. The mechanical fan, had a reasonable amount of air blowing strait up, meaning that if I held my hand directly above the fan, I could feel air. The electric fan on the driver side was running like crazy, but the passenger side was off. A few questions:

1. Because i have well behaved temps around town, and I can feel a reasonable amount of air moving from the mechanical fan, is it safe to say the clutch is working correctly?

2. At what temp should the second fan kick on?

3. Can anyone point me to wiring diagrams for the fans?

Im pretty sure 2nd fan should be running if 1st electric fan on.

I haven't found any wiring diagrams yet, but I did read a lot that you can unplug the coolant temperature switch, and both fans should run.

I did the test tonight, and only one fan runs. Can anyone confirm that this means I just need to buy new auxiliary fans? Without the wiring diagram I want to ask before buying.

Check the colors of the wires at both fans. If they are the same (black/white stripe & brown) you may conclude that they are wired in parallel, and should run simultaneously.

Can confirm that both fans are on the same feed wire, they should both be running if active. It is also likely there is a buildup of dirt/dust on the fins of all of the coolers after 34 years, sometimes a thorough cleaning helps considerably. I'd even pull the radiator, fans, and clean the condenser and radiator very well, things like to collect between them. Though a very thorough cleaning without removal may help a lot.

Don't forget to clean the oil cooler under the drivers bumper corner as well, this is often overlooked and should have a shield from road debris on the tire side that is sometimes missing, when it is it can just be caked in mud.

got the new electric fan installed yesterday and can confirm both fans turn on.

On a long mountain climb in 90 degree temps, my coolant temps like to sit at 105 and if I get stuck behind a truck that slows me down, it creeps up to almost 110 before I cut the AC.

It seems like 90 would be dead center on the gauge, and I see a range of 70-110 depending on the situation. Is this just how these cars are designed? Does anyone here have a 350sdl and climb mountain passes in summer?

I was thinking about doing a radiator flush and coolant change, maybe I'll just change the thermostat for good measure. After that Im at a loss of things to check.

I am kind of tempted to put a bigger oil cooler on to see if I can suck some heat out like that while maintaining a stock appearance.

How old is the radiator? How often is the antifreeze changed? When the tubes of your radiator plug up there will be cold spots in your radiator. With a heat gun or your hand. Look for cold spots & if you see a section of the radiator cold. New one

thank you. I will give this a try. I don't know the history of the radiator and coolant. The car has 117k miles on it and seems like it was maintained really well, but the radiator is original from what i can tell.

I used an infrared thermometer and didn't see any noticeable cool spots. Also, I pulled the upper hose off and tried to look inside. From what little I could see, I didn't see any build up on the coolant passages. I am tempted to take it off and give it a good cleaning and then have a shop flush it for good measure. With the age of the car, Im going to just go ahead and do a water pump and thermostat while the radiator is off.

What are peoples experience with the 2 or 3 pass aluminum radiators they sell on ebay? Did you notice any reduction in coolant temps using these over a OEM single pass? I'd really like to keep the OEM, but with the potential rod bending, I feel like keeping the engine cool is the biggest priority.

I’d recommend to stay away from Nissans, the last one I fought didn’t have the thins metal tubes that support the hose clamps on the lower and upper hoses.
The aluminum ones on eBay are nice, but the fitment is so-so

If you are going to clean the cooling system consider using CRC ThermoCure, I've had good success with the product. Temp climbing a little pushing a diesel uphill is normal, I wouldn't be too worried about some increase but 110C is warmer than I'd like.

Change the thermostat with a MBZ, Behr or Wahler, it may not be opening all the way.

Thanks for the tip. Im out of town, but after reading a lot of threads it seems like these cars tend run hot on uphill drives, but 110 is pushing it. Seems like a partially stuck thermostat was the culprit a lot of the time too.

I'm going to try that flush product, and remove the radiator and give it a really good cleaning. I'll do the thermostat and probably the water pump. If I still run hot after that, Im going to install an 1600 CFM electric fan on the oil cooler to see if that can help. Turbos add a ton of heat to the oil, so it just might be enough to keep the temp down on climbs.

Im also wondering if my cat is starting to get clogged up. The car seems a little sluggish, but then again, it always did.

ThermoCure works slowly over time and can be left in for days. Most of the Benz's have accessible block drains that really help with flushing the system, if possible remove it to flush vs just draining the radiator.

RedLine WaterWetter is another effective product to get lower temps, it is an emulsifier that reduces cavitation in the cooling system.

Owner's Manual: If the anti freeze is effective to -22F, the boiling point of the coolant in the pressurized system is 257F (125C). When driving in severe conditions, or in stop and go traffic, the temp may rise close to the red marking on the gauge.

I would pay attention to it, but not worry about it.

I had good results with Preston flush. It's gentle and cheap. I read you can cause leaks with heavy duty flushes.. back flush it from a heater hose by fire wall. Mine was disgusting and is staying cool. now.