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  #1  
Old 06-07-2002, 05:40 PM
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Why Is My 1991 350 SDL Running Hot?

Have had the car since new, 142K miles now. Car always ran a little over 80 degrees C in the past.

Jumped all of a sudden to just under 100C. Low speed electrical radiator speed electrical fan on all the time now, high speed electrical fan never comes on - it used to cycle frequently in the past even at the little over 80C temp. When I rev up the motor good with the hood open in the driveway the main engine cooling fan does not seem to change it's speed/pitch and I can't hear it
make a "whoosh" sound as I think it should.

My mechanic says he's stumped after changing the engine heat sensor,thermostat and coolant. He says the engine cooling fan is "operating normal at high speed" (am I wrong in my observation above?) He says the high speed electrical fans kick in as they should at 105 degrees C. He wants to change the engine water pump next.

My main questions now are:

1. At about what RPM should the main engine cooling fan clutch kick in and should it be obvious to see and/or hear it when it does?

2. What could be the reason(s) the high speed electrical cooling fans are not kicking in and cycling below 105 degrees C.

In the past I had a problem with the high speed electrical fans being stuck on high speed and replacing the main air conditioning relay fixed that problem. Sorry ror the long post. Thank you.
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  #2  
Old 06-08-2002, 12:53 AM
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Check the radiator temp. Are there any cool spots? If so the radiator may be partially plugged up.

Is there a lot of debris preventing proper air flow through the condensor and radiator?

If the engine is hot and you can stop the fan clutch with your hand, its not working. Also even if the fan clutch is not working the engine temp should come down when the vehicle starts moving.

The low speed fans should not cycle all the time. They should run with the A/C on and should cycle with A/C pressure.
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  #3  
Old 06-08-2002, 02:19 AM
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Has it always run 80 for you before? Even in the summer? with the weather heating up it may make a difference, you gotta warmer air comin in to that intake.
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  #4  
Old 06-08-2002, 02:43 AM
TANK
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I think it could be clogged. A clog somewhere in the coolant system that is. Have it flushed thoroughly yet? If so, maybe the water pump is not spinning properly.
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  #5  
Old 06-08-2002, 09:03 AM
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Radiator, water pump and condesor have about 40K miles on them. Car has previously always run 80-85C. The jump in temperature actually took place on an interstate trip at 75 mph, one minute temp was normal, next minute it jumped up and stayed up.
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  #6  
Old 06-08-2002, 03:15 PM
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Hi CHOWPIT:

With motor temp up to normal and beyond I can stop the fan at idle with my hand and the low speed electrical fan is on too. At what RPM should the fan clutch engage or is it strictly based on temp?

Thank you
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  #7  
Old 06-08-2002, 07:43 PM
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Did you really stick your hand in a moving fan ?
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  #8  
Old 06-08-2002, 09:48 PM
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Hi Mike, Well yes I did, veeery carefully, takes as special touch, you had to be there, now I'm not afraid anymore. I catch knives too (just kidding about the knives) Besides I'm Irish so they let me get away with stuff.......
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  #9  
Old 06-09-2002, 01:36 AM
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I'm a Mick as well. Guess I need to buy some knives.
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  #10  
Old 06-09-2002, 10:36 AM
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Same car, same effect ... different circumstances.

I recently replaced the fan clutch and water pump on my 350 SDL ... it, too, had always run at 80-85C - always! ... after change it now runs there during low temps and/or limited load, but when load increases, creeps right up to 100C ... with AC on at 85 MPH it runs near 100C ... used to run 80-85 during same conditions.

Is the viscous clutch the issue? Am replacing thermostat when it gets here ... eliminating cheap and easy first ... only other thing is radiator, but what is the chance the radiator goes clogged in a binary sense ... don't they usually degrade a bit first as a warning?

The viscous clutch is a neat idea, but would a plastic deformable blade running unclutched work effectively, too?

Ideas?
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1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg
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  #11  
Old 06-09-2002, 11:37 AM
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Is there a way to somehow test the water pump for proper pumping short of just replacing it on a trial and error basis?

BTW my radiator looks like new inside (as far as we can see) the coolant has been replaced every 20-30K miles.

Thank you.
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  #12  
Old 06-09-2002, 11:59 AM
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ezrider and stephenson,

If the operating temperature suddenly changed, it is unlikely the radiator is the source of the problem unless there is something obvious, like a plastic bag covering it, or it is clogged.

On my 350SD I had the fans all come on driving over the mountains on I-90 in Massachusetts a few years back in the middle of a blizzard. The car actually started to get hotter than the normal 80 or 85 degrees C, so I stopped to check things out. The entire radiator and the electric fans were packed in snow. Once this happened the radiator became less effective and the car started getting hotter, approaching the red area on the gage.

I cleaned out the snow and the temperature came down, but within about 15 minutes it started climbing again. Clogged with snow all over, so I cleaned it out again. Did it twice more before I got home. The car used more of the temperature gage range thereafter, and I presumed it was the operation of the fans that was the problem until I ran the car idling (used to idle at any ambient temp and stay at 80 degrees C) and watched the fans work as the car heated up to 105 or so C. In the end I traded the car in and never solved the problem, but now have a similar situation on my 1982 240D, which I will address this summer. I am convinced it is the thermostat, based on threads on this and the Diesel Discussion boards on the subject.

For the radiator to suddenly plug up, a hose or some other part has to start to disintegrate and spew large enough particles in sufficient quantity to make the radiator stop flowing the nominal designed fluid by a significant margin. A gradual build up of chemical crud on the inside is not going to suddenly either reduce flow or change the heat transfer characteristics of the radiator enough to make such a change.

I think the number of reported radiator change outs with no effect on the operating temperatures validates this, and takes the likelihood of an internally disintegrating part out of the picture as well. A new radiator would not immediately plug up with particulate, unless the hose or whatever was also showing signs of failure, like bulging or leaking.

The same is true of the number of water pump changeouts to address the cooling issue, with no positive or net effect.

The least expensive, and most critical element of the temperature control that is active at engine temperatures of 80 degrees Celsius to 85 degrees Celsius is the thermostat. The fans don't come into play until the temperature gage reads over 100 degrees Celsius. So they cannot be the reason the temperatures that used to stay well under 100 degrees Celsius now creep up to those temperatures.

In normal operation the thermostat is never "still" as it adjusts the mix of hot bypass flow and cooled flow from the radiator to suit the load and ambient temperature and air flow conditions. Consequently this device is subject to wear. It is also substantially more complex than most thermostats as it has two moving flow control devices that sit in hot coolant from the engine on one side and cool fluid from the radiator on the other. It has been reported that one individual on the Diesel Discussion board got three or four non-functioning thermostats from the dealer in his area, and it was not until he got one that worked that his engine operating temps went back the 80 degree Celsius mark and stayed there. He even pulled the head and changed the head gasket looking for some reason for the problem. He has some head work that was due at the time anyway so he did not take the head off for this problem alone, I believe.

Since the thermostat has a number of functions, it is hard to check by boiling it and visually determining what is up. It should be feasible to boil the new and old thermostats, recording the temperatures each unit responds with motion of the two flow control elements to, and then compare the responses. Responses being rate of movement and range at the temperatures. By the time the thermostat is at 100 degrees Celsius, it should be in its full cooling capacity position, as the fans start coming on after that to increase capacity.

I hope this helps, Jim
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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
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  #13  
Old 06-09-2002, 12:36 PM
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Posts: 338
Why is there a fan clutch? Why not a fixed fan? Why not modify the fan so it is fixed and eliminate the clutch action altogether?Mine was replaced three years ago (40K miles) and it looks like it's bad again. Why not just drill holes and bolt the pieces together?

Shouldn't the thermostat be able to handle things just fine with a fixed fan. All my driving is in Florida anyway.

Better yet is there a fixed fan replacement available somewhere?

Thank you.
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  #14  
Old 06-09-2002, 04:50 PM
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Location: Old Lyme, Connecticut
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ezrider,

I think you would be better off permanently activating the clutch by some means. Maybe take it out and blow a hair dryer on the bimetallic spring element and see what it actually does, then mechanically make it do that all the time.

The fans should be less in demand at higher speeds of the car as the air being forced through the radiator at higher speeds is probably being held up by the fan, since it only runs to 3300 rpm (see my other post on the viscous fan operation). So, if you ran the car at higher engine speeds and got over much over 3300 rpm on the fan, it might decide to come apart.

I think faking the mechanism out to think it is hot is better and safer than mechanically locking the clutch mechanism. By the way, what were the symptoms of the first viscous clutch failure and did they go away when the clutch was replaced? Good luck, Jim
__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
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  #15  
Old 06-09-2002, 05:09 PM
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Join Date: Jul 2001
Location: Old Lyme, Connecticut
Posts: 3,596
ezrider,

I forgot to address the question of why a fan clutch directly. I believe it is related to need and economy.

The fan is loaded on what is essentially a cubic curve, meaning at twice the speed it uses 8 times the power. If the fan were locked on all the time and ran linearly with engine speed, it would use more fuel, and at higher speeds this would be significant. For example, as a fraction of a horsepower at idle or low rpm city driving might be several horsepower at 80 mph. Additionally, a fan designed to run at 2 or so hp would have to be redesigned to run at 6 or 8 hp, meaning the fan would have to be quite a bit more robust and cost more initially, and then use more fuel all the time.

The other factor is that you should not need such a fan. The machine at higher loads should be running fast enough that the air being forced through the radiator is more than adequate to remove the heat from the radiator without a fan at all. An exception might be towing a heavy load up a hill where you are in a lower gear and not moving that fast. So, considering the permanently on concept gives you the most output when you need it least or not at all, and cost extra money to buy and operate, it is not used by MB or any other automobile manufacturers these days. Good luck, hope this helps, Jim
__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
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