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1990 300SL
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Discussion Starter #1
Hi. I have a 1990 300SL (r129) 137K. I bought this car less than a year ago and have been battling the cooling system ever since. I made some big mistakes thinking I was doing the right (or ok) thing. I knew the car was leaking antifreeze somewhere since I could smell it but could never see a leak. The shop who did my oil change when I bought it said it had very little antifreeze in the system. So I drained off some coolant, added a stop leak : ( and antifreeze to the system. Being the uniformed newbie, I put in the wrong kind of coolant. So needless to say, I have been replacing the most of the cooling system. I have replaced the waterpump, most of the hoses, thermostat, a/c temp switch/sensor, camshaft position sensor, and bypassed the washer fluid heater when it failed. I replaced the temp sensor because the aux fans were not coming on though the fuses and relay were all intact. It still isn't working properly so I have bypassed the sensor as well to keep the fans on whenever the car is on.

I am still overheating. I have flushed the radiator many times now, but still the gunk comes out and still overheating. The thermostat is working, the waterpump is working, the fans are working (sort of), and the coolant seems to be circulating through the radiator. I have used Zerez super flush twice. It actually ran cooler when it ran only pure water. Makes me think the previous owner with almost no antifreeze might have been onto something.

And there appears to be no issue with the head gasket either (also a reason for the head gasket sealer I used)

A friend suggested running it without the thermostat as a test or temporary solution, but then it overheated faster. (as a note I do not let it exceed about 110C) What else can I try short of replacing the radiator? Is there a way to make sure it isn't clogged and is working properly? He suggested I run CLR through the system to clean the engine passages, but I have read really mixed reviews, some say ok, other say NO!!! I have also read about using citric acid or vinegar or detergent as a cleaner. But will anything get this super tenacious, super sticky additive out?

Everyone I have talked to is at a loss. Any insight would be greatly appreciated. Sorry for the REALLY long post, but I figured better to answer the questions now. (And yes, I know that this car runs "hotter" than the average car but it should not run at 95-100C on average, and up to 110+ at idle, consistently.)
 

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02 SL500 SA, 03 Mercury Marauder, 89 Mercury Grand Marquis Colony Park
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131 Posts
You can pull the radiator and have it tanked buy a radiator shop. I usually do not like to recommend aftermarket but you may want to enquire of your local parts store what is available in radiators.

Alternately, a crash supply shop may have the best deal on a radiator. You could ask your body shop where they get their chinese parts.

You must get the system clean in any case.

Has anyone checked the fan clutch? (Assuming the car has one) General rule of thumb, have someone start the car while you watch the mechanical fan. Have them shut the engine off and keeo your eye on the fan. If the fan rotates more than one revolution after the engine stops, the clutch is shot and needs replacement. Like I said that is a general rule of thumb, not SL specific.

Jau
 

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1990 300SL
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Discussion Starter #6
Well guys, I guess it is time for a new fan clutch as well. Though it appears to work fine, I can keep turning it around by hand. A previous post I read said I shouldn't be able to turn it more than twice.

Does anyone know what I can use to clean the system better than Zerex super flush? It's just not doing the trick.

Thanks,
Carla
 

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1990 300SL
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Discussion Starter #7
another question

BTW...does anyone know why it would run hotter after the thermostat was removed? Shouldn't it have heated up more slowly and run at a lower temp? What might this mean? :confused:

Thanks everyone!
 

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IT IS PROBALLY NOT A RADITOR OR HEAD GASKET PROBLEM. IF YOU REPLACED ONE TEMP SENSOR YOU NEED TO LOOK AT THE OTHER TEMP AND AC SENSORS TOKIX THE PROBLEM. LOOK ON THE CYLINDER HEAD LEFT FRONT FOR OTHER SENSORS. THEN ON THE WATER PUMP. ALSO THE PRESSURE AND TEMP SENSORS ON THE AC RECIEVER DRIER ACTAVATE THE AUX FANS IN FRONT OF THE CONDENSOR. IF ANY OF THESE MALFUNCTION YOUR COOLING SYSTEM MAY NOT WORKCORRECTLY.
 

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Greek God of the R129
SL500-500SEL-190E
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There is no need to scream. Lol.

Engine HOT at 600 rpm fan should turn tight.

At 2000 rpm it should kind a spin.

At around 3500rpm it should ROAR.

If it doesn't replace the fan clutch.

After that please put back what you have removed.
Your car is very well engineered. Lol.

Regards,
aam.
 

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90 300SL
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554 Posts
when you flush the system , describe the 'gunk', what color? If you are still getting gunk its gotta be come from somewhere , like the oil maybe.

You are sure about the gasket problem being solved?

As far as temp, does it move past 100 when you are moving as say 55 mph in 5 gear turning 2k (should be around 90-95) or does it only go over 100 when sitting still or moving slow in traffic. If the temp is over 100 while moving air through radiator then the fans don't matter, it's the head gasket that is overheating the coolant.

one other thing is the radiator cap sealing correctly? System relies on a cap that maintains pressure
 

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Outstanding Contributor
1997 SL600 Brilliant Silver Sport with grey/dark metal interior, 234K miles
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So, you used the orange Dexcool and found out that when it's subjected to air (in a non-closed, leaking cooling system) it turns into a brown thick passage blocking crud.

The coolant has to pass through the engine and radiator at a certain speed in order to both absorb heat (engine part) and release it (radiator part). If the coolant rushes through the system too quickly, it doesn't have time to do it's job and the engine overheats. The thermostat provides the regulation to coolant flow so that the engine gets cooled properly.

Good luck getting that stuff out of the engine and radiator. There are no tricks that I am aware of other than dissassembly/cleaning and replacement.
 

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SL55, 300SE, GL450, Daytona 955i
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A new Behr or factory fan viscous clutch (not Uro and eBay off brand ones) will solve 99% of cooling problems with high running temp or temp hunting (100-120*C) in M104/M119/M120

Uro ones come broken slippery from the factory, and I am not even kidding...
 

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1990 300SL
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Discussion Starter #13 (Edited)
Update to issues

Ok here's an update to what I did last night. Thanks to everyone who responded.

I haven't replaced the fan clutch yet and will do that here shortly, though the fans really do run at full blast seemingly. I did however flush the system multiple times for a very long time and this time I back flushed it. (No, I didn't previously use the orange dexcool...sadly ordinary green. But it was mixed with whatever type the previous owner used, because this dummy didn't flush the system first. Of course green is bad anyway but it was the mixing that was the problem and coupled with the stop leak stuff, added "just in case" that was the problem...bad news.) Anyway, the gunk I was referring to is a white, crunchy sort of stuff and some flaky stuff that is coated in stop leak, and also everything else is coated with the very sticky stop leak stuff.

But the good news is that I have gotten almost everything out (except the stop leak.) After flushing the crap out of the system with the thermostat out, I then ran 2 gallons of vinegar through it. It removed a lot of the deposits and maybe some corrosion. The smell allowed me to be completely sure it was all out. But I am not convinced that I am getting the circulation that I am supposed to and I will take a look at the tensioner to make sure the belt is tight enough, as that was also replaced in the first round of damage repair with the waterpump. Is the tensioner self-adjusting or do I need to do that? I followed someone's suggestion that I throw the thermostat into boiling water to see if it was still working right, but that proved nothing. I put it back in and it is working fine.

When I drained the fluids the first time, the antifreeze concentration was way too high, though I was careful to only put in what was supposed to be 50%-1.5gal of coolant. It was relatively clean when removed too. So I only put in a small amount of coolant, probably less than 25% and let it run. I ran it overflow cap off and on to remove any air. The system is pressurizing as it is supposed to now that the thermostat is back in (no seal before with it out).

The great news...I idled the car for about 30 mins with load and without...92-96C temp range! Best I've seen yet. True test will come today with road test and idling in traffic with 90 degree heat. Will let you know.

Thanks everyone.
 

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2015 Jaguar XF R Sport Twin Turbo, 1992 500sl Pano, 1999 CBR 1100XX, Super BlackBird, Jaguar X Type
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We get radiators re cored over here if it there are cooling problems. All the centre pipes and cooling honey comb is replaced the header and footer tanks are blasted clean and the whole lot painted. This is relatively inexpensive compared to a new Radiator.
I would think all that anti leak going in would eventually restrict the waterways in engine and cooling system to some degree.
The biggest problem maybe getting the cylinder head coolant channels flowing freely when the rest is sorted, without removing them.
I believe the r129 cylinder bores maybe silicone impregnated which is great until they are overheated too much then the car would be virtually throw away if that is the case.
Mercedes seem vague about temp, they don't seem to mind hot running as long as the temp does not stray into the red. Mine gets hot in traffic but cools when it starts to run.
 

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1990 300SL
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Discussion Starter #15
I had forgotten about re-coring. I had that done on an old mustang many years ago. I would have to look into it but I suspect in the end it may wind being about the same as a new radiator (~$500). But I will check...it's a good idea if I need to change it.

Did the road test yesterday. Ran great for about 4 miles with the A/C on then got to about 105C at 65mph. The aux fans are not doing their job. I have replaced the sensor on the waterpump housing but cannot locate the others that someone mentioned here, apparently 3 others. I reconnected the connectors to the blue temp switch but apparently they are also directionally dependant. So I have changed them and now marked them to show which connector goes to which side but I have yet to do another test drive.

As I said previously, when connected to the sensor, the a/c will work but the aux fans won't. When by-passed (jumpered) the aux fans will run full speed, so they are functioning, but I just have to figure out what keeps them from working properly when only connected to the sensor. This is a new sensor too. For now I will try keeping them connected AND running the bypass wire too. See what happens. I have long thought this sensor/switch was not "switching" but it appears to be the right one and the only one I could find (OEM German one.) Maybe it isn't the right kind? So combined with the fan clutch, I am just not getting enough airflow.

Does anyone know if the tensioner is self-adjusting or can I tighten it?

Thanks,
Carla
 

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1990 300SL
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Discussion Starter #16
Let me rephrase that a bit...when the wires are connected to the a/c temp sensor, the aux fans do work to some degree. They come on, they change speed but they don't come on fully when the engine reaches 105 or as they do when bypassed.
 

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1996 SL320
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Good day, I also went through the same path. Do yourself a favor, stop by or use the internet Harbor Freight Tools and purchase an infrared temp guage 49.00 , you can measure the correct temperature at any point on motor, radiator ,near the temp. guages you will find the temps much cooler.
 

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90 300SL
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554 Posts
tensioner

your belt tensioner should look like this (attached) . Don't over tighten the rod adjuster, I've snapped one. there are posts on setting the tension.

Was the 105c you had in last post the highest it got ? What was the air temp at time. With the a/c on at +90F that may not be that bad.

Try it again in evening with ac off when the temps are lower.
 

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02 SL500 SA, 03 Mercury Marauder, 89 Mercury Grand Marquis Colony Park
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Well 105c is 221f so it is a little hot not terrible but a little hot unless it was really hot out.

Sitting in stopped traffic with 102f indicated on the dash my engine temp gauge reads 90-95c/194-203F with AC on.

Did you eyeball the fan clutch to see if it stops after 1 revolution when hot?

Jay
 

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90 300SL
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operating temps

per owners manual (my 90)

If the antifreeze mixture is effective to -37c the boiling point of the coolant in pressurized cooling system of your vehicle is approx 130c.

During severe operating conditions and stop-and-go city traffic, the coolant temperature may rise close to the red mark.

The engine should not be operated with the coolant temperature in the red zone.
Definitely should pay attention to a rising gauge but as long as it doesn't go into the red then don't panic?
 
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