Why Is My 1991 350 SDL Running Hot? - Page 2 - PeachParts Mercedes-Benz Forum

ezrider · #16 06-09-2002, 04:57 PM

Jim Smith, when clutch was replaced it cured my hotter than usual operation, tech just said it was bad and I let him replace it, glad I did at the time. It cost me $382 three years ago.

stephenson · #17 06-09-2002, 05:01 PM

JimSmith,

Issue on why I replaced the viscous clutch my 350SDL was pretty straightforward ... was operating fine (temps staying always in 82-85C range), but the bearing was shot ... started wobbling and getting too close to the cowl ... that was how I first noticed it. I think the waterpump bearing was also going as it was still a bit wobbly (not as bad, though) after I replaced the clutch.

Like I said, I always look for changes to occur after changes have been made ... and, mine immediately starting to run at a new temperature "profile" of 82-85C under light load and 100C plus under heavy load (high speeds in warm weather (80F) with AC on or in traffic with AC on or off) ... my change has been very noticable.

Wish I had saved the old clutch ... would put it back in and then see if it changed the profile.

stephenson · #18 06-09-2002, 05:06 PM

Maybe I should ask a very binary question on this issue ... should the ram air across the radiator at 85 mph in 80F weather maintain the minimum temp of the thermostat with the air conditioner on?

Sounds like some believe this is the case ... if so, then neither the viscous clutch nor either electric fan should have anything to do with the increased temperature "profile," right?

Ergo, it is either the thermostat not passing enough coolant, or the radiator not cooling enough coolant, or the pump not pumping enough coolant, right? So, as I replaced the pump (not likely this is an issue since it looks and is geared the same as the one I took off), only the radiator and tstat could be the issue, right?

JimSmith · #19 06-09-2002, 05:24 PM

stephenson (George),

I have tried to make that point. If the car is operating at 80 degrees C plus or minus a few degrees steady and you are driving at 60 to 80 mph (in my Diesels and gas cars, speed has little to do with operating temperature - it usually more sensitive to a high speed run followed by stop and go traffic or sitting idling with the airconditioning on in slow moving traffic), the fans do not play a role. And if the airconditioning system needs no more than the ram air, the radiator should also need no more than that.

There are a lot of these overheating threads posted and some guys have gone nuts trying to (edit to correct spelling) fix the issue. Replacing radiators, head gaskets, heads, pumps, hoses, fan clutches, sensors and so on. The only items that really work are, when you have a cracked head or blown head gasket and see higher operating temps in conjunction with these failures, replacing the head or head gasket. But if you only have elevated operating temperature, it seems changing the thermostat is the only fix that restores the original operating profile. Because the thermostats have been unreliable, one guy reported three or four bad thermostats from the dealer before he got one that worked, the thermostat is often changed with no apparent benefit too, leading to confusion and trying other solutions.

This over simplifies the issue as leaks from hoses or bad radiator caps also cause overheating and when they are replaced the problem usually goes away too, but the issue there is not just sudden higher than normal engine operating temps - there is a leak or lost fluid too. Things like water pumps fail due to seals (I have never replaced a water pump because of reduced output, for example) and overheating is caused by lack of sufficient coolant.

So, I am leaning towards the thermostat as the problem, as it is normally moving to respond to load changes and keep the operating temps even. After a while I think they just wear out and cannot regulate fast enough or thoroughly enough. Then you see higher temps than are normal for your car. When you start looking into the issue you hear from others or read in the manual that operating up to 120 degrees C is ok. But it isn't normal if the car ran for a hundred thousand miles or two at lower temps. And you should be able to reestablish the old normal behaviour.

So, this is frustrating. But if we can figure it out and post the answer or answers, many of the members will be relieved. Jim

edited to correct spelling and grammar/diction

joebnz · #20 06-09-2002, 06:45 PM

Pick a warm day, remove the thermostat, go for a drive and see where the temperature goes. This will give you a go-no go on the thermostat.

joebnz · #21 06-09-2002, 07:06 PM

My mistake, German engineering. Please retract my suggestion.

JimSmith · #22 06-09-2002, 08:04 PM

joebnz,

If you take the thermostat out, the by pass line letting fluid from the engine bypass the radiator, will be open all the way and you will have no control over engine temperature. The pump will draw fluid from the path of least resistance, and in this cas it will likely be mostly from the bypass path as the radiator introduces some extra resistance. The manual very specifically states removing the thermostat will not correct an overheating situation. If the temp stays the same though, it might let you know the thermostat is not doing anything. Jim

stephenson · #23 06-24-2002, 07:15 PM

Wanted to follow-up with my results from replacing parts.

My 350 SDL's problem was covered early in this thread ... it simply started getting hotter (never overheated, but would rather easily run to the point that it would kick on the electric fans ... 105C, I think?) than it every had, and, this corresponded exactly with replacement of the viscous clutch and coolant pump - I had replaced the viscous clutch to cure a wobble which was part viscous clutch and part coolant pump.

I replaced the tstat and put on another viscous clutch - not scientific since I should only replace one at a time, but I was not gonna tear all the stuff apart and drain it twice!

Result is that it runs just like before it started getting hot ... it appears to come up to temp faster, and then stay at 82-85C in most driving. Today it was 98F and I was able to make it move, but even in stop and go with AC running, with almost no airflow over the nose, I was unable to get it to 100C ... just would not touch it. As I started moving even a bit, it immediately would run back down to 82-85C.

I wish had more time and money to address the issue more completely - an item at a time - would be nice to KNOW. I suspect the problem was the tstat ... I don't think the viscous clutch should be a factor with ram air at 50 mph, regardless of the outside air temp. I boiled it to watch it open and close, which it did, but I don't think this is a valid test on this tstat.

#24 07-13-2002, 01:44 AM

email me I might have some insight for you the 350 is particular.
jwii@zoomtown.com

stephenson · #25 07-16-2002, 01:20 AM

jwii350sdl,

I monitored the other postings. Thanks for the information.

My 350SDL has been pretty nice so far ... only odd item is that I will occasionally get a brief puff of smoke and loss of power for a 1/2 second. I believe it is ingesting carbonated oil residue from the valve cover area that has become hardened- perhaps by the EGR input from the exhaust ... not sure, actually.

What a great car, though.

The cooling issue is still stable with 82-85C in normal conditions - warms up in 98F, but never above 100 even with AC on in stop and go.

Most interesting observation is that it warms up so fast - assume this is the new tstat.

Would appreciate any interesting info not already on a thread.

Thanks.