I have 230k Miles on my 1990 300SL.
Overheating is an ongoing issue.
I have changed New parts as noted: NEW Behr Fan Clutch [changeout 2x], NEW OEM Radiator[2x], NEW water Pump, NEW 71C Thermostat [changeout 3x], NEW Hoses, NEW Sensors [Temp. Aux. Fan ], NEW Coolant, NEW Cap, Later Added a NEW SACHS Tropical Fan Clutch ($195.00) in lieu of the previous NEW Behr Fan Clutch.

Previous Modifications Completed: Added Manual Switch to Turn On Aux. Electric Fans, Added Upper Rad. Hose Tee Fill Cap with inline Temperature Gauge, Flushed Coolant System, Pressure Washed Existing A/C Condenser Radiator Fins, Modified Fan Clutch [added 1+oz. Silicone Oil - 500cst, Shortened Valve Pin by 2mm [Recommended to Early Lock Fan Drive to Driven upon Earlier Hot Ambient Temp.]. When Observing, via Dash Gauge, Temp Rises upon Stop/Go slow traffic, the Manual Switch [for Aux. Fans] is used.

Note: No Oil in the Coolant and vice versa. Using 15w-50 Mobile One oil, All recent replaced ignition components [platinum plugs, rotor, wires] and Passed Smog, fully functioning A/C System. Aux. Fan System is working but may be "Lazy" to activate when necessary.

Symptoms: Ambient Temp. 70 deg. F, and on a 40 mile run . . . Driving city[35mph]/highway[70mph] on slight hills and flat road (in the San Francisco CA Bay Area Region) . . . the 1st 20mi. Temp reached 85C-95C. After some city driving. Before return, Temp. begins to rise to 100C to 115C on the return trip [A/C NOT ON.]. Temp. is dramatically increased if A/C is ON.

I've done just about every thing, [[but am going to Replace some MBZ Coolant from the Prestone I am now using]] . . . . . ANY CONSTRUCTIVE SUGGESTIONS? . . . . other than getting rid of it??
HZ.
Thanks in advance . . . for any help. 033125

 

1] Correction Fan Clutch Silicone Oil added = 5000CST, 1oz approx.
2] Also replaced fan with 9 blade OEM vs existing old 6 blade fan.

 

Has this car ever had uncared for coolant fluid in it to the point of developing rust?
If so, that is your culprit and the coolant passages are blocked between the head and the block. That could be the case.

There has to be a blockage somewhere in which you are not getting enough circulation and cooling by the radiator.

When I used to visit the local pick and pulls, I used to come across situations where someone had pulled the head and you could see all the rusty crud in the water passages to the block.

You probably do not need to pull the head to find out blockage issues. You could loosen the block drain and force water from the thermostat housing side to see if you can get a good flow of water.

If you have an IR sensor you can measure temperatures at the inlet of the radiator and the outlet, and various other places to see if you can spot a blockage. That is another option.

Anyway this is what comes to mind reading your post.

How long have you had the car (how many years and miles prior)?

 

d:
Thanks for the incite of possible existing theory.
I am the 2nd owner of this 35 year old 300SL purchased around in 2003 with approx. 78k original miles.
The cooling system has been maintained to a fair amount of coolant changeouts [every 3 to 4 years] over the total time I've owned it. There had never been an excess dark old coolant color when evacuating. Old coolant removed always remained as the yellow-green [more faded/tainted than original . . . ] shade of Antifreeze, This had never been a daily driver as I, at the time, was rotating 6-8 cars in my "fleet" of use vehicles. Fast forward to today my vehicular rotation is down to 4 cars.

Since your comment, and prior to . . ., I was thinking that this 35 year old car, with it's original untampered block for the 230k miles, had developed, in the cooling ports/walls, excessively insulated with crud film/corrosion/build-up layers that may be preventing adequate heat transfer via coolant . . . . thus the elevated temperature. I was going to try a bottle of "Water-Wetter" that promotes better absorption of block to coolant heat transfer. I used this on a P38 Range Rover I owned with little success is why I hesitate. My attempts at flushing with parts house products maybe insufficient. And maybe commercial coolant flushing automotive services would better help this dilemma. >>> My next step.

Hz,

 

Is the car actually overheating? I dont like to promote my auto repair business here but I would like to send you to a shop who's owner actually DRIVES one of these. Import Connections in Campbell California, you'll usually see my E550 parked out there at night at the restaurant next door

 

Thanks Chris, for the referral. I'll give Import Connections a call, to see what recommendations. Campbell is about 40miles from me and an hour drive with heavy traffic usually occurring. I'm going to "overheat" going there.

I do have a nearby Indy at $80/hr that is very adequate [servicing MBZ, Porsche, BMW and some exotics] and have yet to approach them due to $$$ constraint. DIY until I've exhausted all options and I'm nearly there.

With my driving use, I had NOT allowed the Dash Gauge temp. to go into the 120C RED mark . . . . Use of the manually switched Elec. Cooling Fans, prevents increase and a max. of 115C,. At stop light traffic, I'll place the Trans. in neutral position and keep engine running at 1500 rpm, until the green stop light to go is present. . . . Such a hassle to contend with, especially without A/C use. And with ambient temps hovering around 69F to 75F, this high coolant temp is unacceptably too high.

When summer heat arrives I will NOT drive more than 20 miles with this existing condition.

I do mostly local, stop/go in-town driving and will experiment with a LOCKED [bolted] Fan Clutch. Also, . . . . I was also thinking if I can locate a 19" fan opposed to my 18", 9 blade fan. The fan shroud has an opening of 20" dia. A closer tolerance resulting of 1" clearance may increase volume suction of air through the grill and possible increased cooling?

HZ.

 

Have you verified the sensor is working correctly and the water temp?

 

Indeed, with such a car, I would verify that the temp gauge is telling the truth. That is also why I suggested an IR gun to find out the real temp in and around the cooling system.

In it's current state, at what gauge temp does the aux fan turn on?

I have two M103 cars both around and a little over 200K miles. One is original to me from '89. Neither car has ever had this condition you are describing.
I do maintain both cars and have not been to an indy in 8 years. Coincidentally I'm only 4 miles from the indy shop mentioned but that is not so relevant.

These are all theories/suggested without knowing your car but it seems you have replaced just about every component of cooling in the car.

So I would first make sure the temp sensor is working properly. You can also check resistances of all the sensors to make sure they match and also are in spec.
You have 3 sensors to work with and have all the same resistance spec (gauge sensor, sensor for ECU and aux temp sensor)
Then check for hot/cold spots all around the coolant system.

Talking about dead horses, the M103 engine is pretty trouble free until one repeatedly has excursions into the 120C territory.
And that may cause head gasket failures after too many excursions.

At the risk of stating the obvious, you are doing the right thing by not driving it much. This comment is not for you, but for others who maybe reading your post.
You appear to be a car person and probably you are well aware of what I'm stating here.

What you are describing (hot coolant temp) on the highway is certainly very unusual for this engine. In heavy traffic and only for 90+F ambient is more of a problem that is remedied by lowering the aux fan turn on temp down to 100C is what takes care of that but yours, if true, is pretty catastrophic.

Now I should also add that when thermistors age/go bad their resistance tends to go up not down. Which means they sense lower then actual temps. So opposite of your case. You should still check the temp senders against their spec. So I would trust but verify.

 

I previously changed [within 10 months, 200 miles] out all the cooling system sensors that I researched on this car is so equipped with and purchased replacements from FCP Euro Parts House. I have NOT checked resistance of these sensors assuming new and from a reputable supplier. Further researching, I changed new the R15 Aux. Fan Resistor along with the A/C Sensors. With these changes the fans do not activate repeatedly automatically, as before . . . thus . . . manual switched activation based on viewing temp. gauge. These sensors are for the automated operational electronic protection of the cooling circuit. The thermostat, fan clutch, coolant flowing through the radiator are NOT dependent on sensor monitoring, let alone even their existence. Great when fully functional for ONLY in supporting an automated system A/C and Cooling circuitry, combined.

What points on the block and engine area do you suggest to take IR readings? I will, as suggested, check those varying block locations with a IR gun, at normal operating water temp.
Placing that 2nd Temp. Gauge in the top Rad. hose, eliminated air bubbles and confirms what the Dash mounted gauge is showing. I previously was using an IR gun to check temp. at top Rad. hose for comparison.

I'll post what the results discovered.
Thanks,
HZ.

 

I only asked about the aux fans turn on temp just to remotely gauge your coolant temp at the cylinder head. They turn on at ~103C and turn off at ~95C.
But seeing that is inoperative, that is moot. And you believe the temps are really reaching 120C, so only thing left us is the why are you not getting cooling.

I would measure at the head were the sensor is, at the thermostat, inlet to the radiator and the outlet. With good circulation those numbers should be close to each other, with the outlet of the radiator being slightly lower of course.

If you take the measurements and post them here with pictures of where you are taking them, I can take the same measurements with my two M103 cars and see if we can determine your situation remotely.

- Cheers!

 

I will take IR temp measurements tomorrow at Points A, B, C. Are there any other ones needed?
HZ.

040125

 

Sounds good. C is the inlet of the Radiator. There should be a return hose as well. In mine that is below C . Not sure where it is in your radiator. But that would also be important and tell us how efficient the radiator/fan is.
After you send me the values, I can take mine at similar locations tomorrow.

 

Check fan shrouds are close to fan blade tips and sealed tight to radiators to ensure air flow over radiator.
Check temps at top and bottom radiator hoses there should be a difference if it is cooling.
I prefer to use a BBQ instant read probe thermometer and put tip on the neck of hose connections - not the rubber hose
I find laser thermometers can read erroneously if they do not have proper settings for material you are scanning.

Anything up front blocking air flow or changes air flow?
Splitter?
Any extra coolers in front of radiator like PS cooler, tranny cooler, engine oil cooler, etc?

Has engine ever been FLUSHED via heater hoses?

All it takes is a little restriction in a head coolant passage to get over heating.

There are the dumb things to check - sorry but have to ask.
Are all fans moving air from front inside engine bay?

 

ygmn: Thanks for comments and answered as follows:
Check fan shrouds are close to fan blade tips and sealed tight to radiators to ensure air flow over radiator. [[ Exist. Rad. Fan = 18" Dia. Fan Shroud opening 20" Dia. Am trying to locate 19" Dia. 9 Blade Fan.]]
Check temps at top and bottom radiator hoses there should be a difference if it is cooling. [[ . . . will do so . . . soon ]]
I prefer to use a BBQ instant read probe thermometer and put tip on the neck of hose connections - not the rubber hose [[point taken . . . will acquire a contact temp. probe]]
I find laser thermometers can read erroneously if they do not have proper settings for material you are scanning. [[maybe OK for my purposes as this is NOT a critical point resulting in a possible catastrophe . . .?!]]
Anything up front blocking air flow or changes air flow? [[NONE]]
Splitter? [[NONE]]
Any extra coolers in front of radiator like PS cooler, tranny cooler, engine oil cooler, etc? [[NONE]]

Has engine ever been FLUSHED via heater hoses? [[only used otc chem. fluids and opening heater controls in hopes of cleaning too. I will contact local indy to get professional Flushing . . . as I once did so on a 55 year old truck I had with success.]]

HZ.

 

Operating Water Temp at IR an Probe Temp Measuring = 88C.
.
Test IR: pts. A = 85C, B = 81C, C = 74C, D(at Lower Rad. Hose) = 71C.

Test w/ Contact Probe: pts. A = 75C, B = 68C, C = 74C, D = [NOT measured]

HZ.

 

Your contact measurement is not accurate. Unless that probe is embedded in the measurement area (which is impossible) you will get lower readings.

Your IR measurements match my contact probe measurements (done with a thermo couple and DMM) more or less. My IR measurements are pretty close to my probe measurements.

I measure 85c at the temp sender area.
82c at the input of the radiator
72c at the radiator output.

You are also getting a 10c difference between input and output of the radiator.

All this time my cluster gauge was registering about 85-90C which matched all these measurements.

What was your cluster gauge at when you took your measurements?

 

your fans are hooked up correctly and spinning the right way?

 

>> My cluster gauge reads identically to the hose mounted gauge[ . . . at the time of measurements,Operating Water Temp = 88C].pictured. The measurements are what is as noted. I measured 2 and 3x to reconfirm my previous ones to keep track.Possibly some error, though, from what I've measured, what is your opinion on the functionality or non-functionality of my cooling system as it stands to date? The comparison of your vehicles' state, being your own standard of correctness . . . is just that. I again, appreciate any tips I can gain to improve my cooling system, based on the data I've presented.
HZ,

 

Thank you for that tip . . . . And . . . YES, I did confirm Fans rotational correctness as one of my FIRST steps on checking my system's operation.

Hz.

 

Mine is a 2.6L not a 3.0L however the cooling dynamics should be the same.
As for my car, its cooling is near perfect. It never overheats. I do have the aux fan switch trip point lowered a bit to ~98C and even so, it only turns on if I am idling at a very hot 100F day.
On a normal 90+F day, may gauge is always around 90C +/-3C.
And I have never seen the temp go over 100C, while the car is running.

Since both of our cars get a 10C drop across the radiator and yours is overheating, that could only mean that yours is just not getting enough flow of water to cool your engine.
That is my guess. Not sure why that is.

A thermostat does restrict the flow a bit. What I would suggest it to remove the thermostat as an experiment and see if that fixes/improves the overheating.
If it does, than you know you have a flow issue. If it does not, that would be non-deterministic because you could have a restriction of flow or not enough flow for another reason.

It is a worth while experiment for you.

I know you replaced your thermostat, hopefully you used an MB one. But not all thermostats are created equal. Some open more. some open less if they are aftermarket.

On the subject of fan size, you need to make sure there is at least an inch gap to the shroud all around. Because there needs to be enough tolerances to cover engine mount sag and general vibrations of the engine. I think out of the factory it is in the range of 1.5 inches or so.

- Cheers!

 

This is likely the culprit; I'd put money on it. I'd even put up a visit to my favorite massage therapist, lol.

 

Thanks for everyone's comments to my SL cooling deficiencies. I can't seem to leave this car alone and just sell it. However . . . attempting yet another fix towards road worthiness . . . I had previously mentioned of hacking my fan clutch, as an experiment, with bolts. I did just that and made a 50 mile run of freeway/city diving in 70F+ ambient temperature. It operated at 100C max throughout the trip and with A/C running. I do NOT intend to leave it that way. I will remove the thermostat, as "d" suggested and reinstall my "Tropical" $195.00 Fan Clutch and observe on another 50 mile test run.

I've changed New thermostats as follows: 85C, 81C and finally [twice 71C]. These were all aftermarket and I had a suspicion of possible fault. I've never test heated measured temperature water, immersing thermostat to view correctness of operational opening on any of these units. Maybe I should. If removal of the thermostat, in its entirety, is promising, I won't leave it alone as that . . . I will replace with a OEM MBZ one, and observe results. If negative results I may drill 5mm holes in the thermostat plate and observe that. Yet, professional coolant system flushing is still on my agenda.

>>> getting back to previous fixes . . . upon renewing past fan clutches and thermostats . . . results were positive for about less than 12 months [this is NOT a daily driver], before temps. creeped higher, again, requiring my attention. That's 3 New Fan Clutches and 3 New Thermostats and 2 New Radiators + Sensors, Fan Blade upgrade, Coolant changeouts, Etc. in a 5 year span . . . is apparently masking some hidden Gremlin. Noting, again, that there is NO Oil evidence in the Coolant and Vice Versa . . .

I'll post results . . . unless I succumb to Boredom.
HZ.

 

Please dont. The thermostat opens the large circuit and closes the small circuit. Without a thermostat, the coolant only flows the short way through the small circuit, and the engine overheats quickly and severely. Aftermarket fan clutches are all junk, most of the time, they are engaged and constantly spinning.

Is the radiator scoop completely in place and fits snugly against the radiator? Otherwise, the fan will draw air from the warm engine compartment instead of through the radiator.

 

A.H.
Thank you for the incite . . . and OK. . . I was thinking that it was NOT such a good idea also. I've had no experience in attempting such a mod. only some negative comments by persons not approving. As I understand, this is commonly done in extremely elevated geographical areas of the globe.

As far as my attempts at solutions, I'm thinking my changeout with New thermostats [4x] should quell the idea of "defective" goods. . . or is it my luck of a "bad repeated draw . . . ?

As far as the shroud [you referred as "scoop" . . . ]. It IS in the EXACT, Snuggly Fitted against the radiator position as the OEM installed designed. It is complete OEM [shroud, lock ring with all mount tabs intact], and NOT damaged nor deformed in anyway.

FYI: If you're wondering of my Mechanical abilities . . . I maintain a 1974 Porsche 911S and a 2015 G63-Wagon which are both are more complicated than my 300SL. I've totally rebuilt another Porsche 911 engine in its entirety, along with other motor types. Not bragging . . . just noting my aptitude on this subject for anyone reading this post and thinking otherwise.

I was researching a 19" Fan with a minimum of 9 blades. If a BMW or AUDI or Other fan with correct Fan Clutch, 3 point mounting and single hole for center bolt crankshaft mount + correct rotational design to suit my car, i would like to purchase. Had anyone of you experienced or persons done any research on such a subject? Please comment here and/or a note a link to a forum thread or parts site or any other hints to follow such a modification, I would greatly appreciate. Another idea would be to modify the OEM shroud to lessen gap of 20" to 19" opening to my 18" 9 blade fan. Any hints on this type of Mod.?

HZ

 

The OEM thermostat is made by a company named Wahler. If there's a small hole in the thermostat valve, it needs to be facing upwards. I'm not familiar with the 300SL; on the 300-24, the radiator inlets and outlets are on the same side, on the same water tank. To make this work, there's a separator plate in the middle of the water tank. If it's broken, the coolant barely flows through the radiator, only through the water tank.

Modifying the fan wouldn't eliminate the cause; I don't think that's helpful. The original thermostat opens at 87°C; there aren't any factory thermostats that open at 71°C. I would simply install a genuine or a Wahler one that meets the original specifications. If the small circuit isn't closed, the engine won't be cooled.