350 SDL normal temp range

They do that. Being in the red is ok. STAYING in the red for a while is not. Having an aluminum head you only get to do this once, if it overheats, it's dead, Jim, unlike the 5 cyl motors that can overheat and live.

Flush the cooling system and use redline water wetter. And keep an eye on the temp gauge.

-I have had a 91 350 SDL for some years now (from 106k to 182 k). They have numerous clever ways of overheating.

- AC charge low. This seems to cause excessive heat discharge by the ac in front of the radiator.

- Use of MB coolant. I think this stuff leaves deposits in the cooling system. Citric acid cleaning helps.

- I have heard partial plugging of the mufflers can cause the engine to run hot.

- Inoperable aux fans (one comes on when at 110 the other when the ac pressures are high)

- check the temp sensor of course

- low oil level is not good

- fan clutch failure - not as uncommon as some think

- fan belt failure

- Idler pulley or dampers worn out. (this and the ac charge made the biggest difference for me)

In North Carolina mine will often move up over 100 in city traffic but comes back down to 85 on once traffic flows steadily. One exception is driving in the mountains. Sustained heavy acceleration has caused it to go up to 110 (before some of the fixes mentioned above).

AND BE CERTAIN TO DISABLE THE EGR VALVE. Mine was passing black sludge to the intake. This is not good for pistons.

Chris

As Richard said extreme high temp is sure engine failure.

It is possible your aux fan is not working when AC is on... I would first buy a infrared thermometer and aim it at the brass sensor to get an accurate reading.

On older cars, the temperature gauge do go bad and can be off as much as 20 degree.

The temperature sensor is easily seen... they all have one or two prongs... one is definitely blue plastic with two prongs... this is for AC. Sometime you will have one prong with read coloring... this also related.

Best bet is go to the dealer and they will tell you which controls which.

The single black one is for temperature gauge... sometime this is replaced by 4 prongs round black sensor.

Just to throw in my two cents. It seems that most 350SDLs vary their temp around the operation range. I disagree that operating above the upper white line of the operating range should be considered acceptable. Operating in red zone will not cause immediate failure, but it is best addressed when the temp starts getting above the operation range.

I replaced fan clutch, water pump (water pump was bad though and flowing backwards sometimes), several thermostats all to no real avail. The two things that really messed up our engine was 1) a burned through head gasket probably caused by running in the red zone for too long and

I've had the AC system low and empty and for us it never really affected the operating temp, but I can absolutely believe that it will. I've never understood why that happens but it seems to.

In other words, do what Chris said, and don't discount the exhaust. A clogged exhaust can cause bad heating up and low power.

How does an exhaust get clogged?

This is not a problem one typically sees on older cars. Keeping them from falling off is a more tyical scenario. :-)

This is for a W126 but you'll get the point:

Simply remove the vacuum hose from the EGR and insert something like a golf tee to plug it.

Chris

It's a quirk of this car. Oily crap comes out of the cylinders and builds up in the can in the midpipe causing an obstruction in the pipe and there is a power loss and the engine runs hot. It goes right up there with the bent crank arms, which are also a quirk of this car. Personally, once the burnt through head gasket was replaced, I've pretty much turned up the radio and ignored a great deal of what is going on with the 350SDL.

Um, it's not exacly legal to block the EGR and a golf tee is a dead giveaway.

A ball bearing in the hose is a but more covert. Of course you'd only do this for off road and racing purposes.

Yes, the 350SDL is known for its tremendous hill climbing AND racing ability. :)

(Sorry, the picture of it made me laugh).

Ummm, I have no good answer for that. We found it out by replacing the exhaust stem from stern because it was cracking, and the side effect was an engine that ran much cooler as well as more powerfully.

Sorry that I can't help. I just know that this is what happened to us, and I have read similar experiences from others over the years who have had the same thing happen. Have you tried searching this groupd on the subject? Something useful might pop out at you. Best of luck - H

The 1991 350SDL is also a w126. With the same engine they later put into the w140.

cp