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560 SEL (1991)
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Discussion Starter #1
For some reason that I can not figure out I have little to no heat at higher RPMs as well as a cooler running engine at times.

Normally my gauge reads above 80. Right now it seems to get there but once it does it may drop a little or drop a lot to below 80. Its really not terribly consistent, but generally it runs below 80 now.

In the morning commute the engine warms up as normal and I will get initial heat. But once I get on the highway its gone 90% of the time. Only a couple times have I had heat hold fine. Even in the city I will lose heat if I get into the higher RPMs.

Under any situation heat is regained once I come to a stop or very slow speed. Such as the end of an exit ramp. Once I get moving, heat will go away again.

It seems to be related to engine RPM. With a warmed up car I can sit in my driveway and rev the engine over 3k RPM. When I do, no heat. I have NO viscus fan installed and the aux fans do not kick in.

The car never gets critically warm unless I hold the higher RPM for a while while sitting. As expected. Not quickly, so the pump is certainly circulating water.

I just replaced the fairly new t-stat today. No change. I also replaced the expansion tank cap, twice. One a generic and today a new OEM unit. New mono valve also installed. Old was was torn around base edge. With all these new parts, no change at all.

I suspect water pump. But the engine seems to run cooler, not warmer. If the pump was bad I would think warmer temps would happen and not a loss of heat. The other issue may be a head gasket. Maybe exhaust is getting into the cooling system. This might cause air to get trapped in the heater core and maybe explain colder water temp - that is if air is hanging around the temp sensor.

Really confused on this one. Google shows that a few others have had this issue but they failed to update their posts.

Any thoughts on this one would be appreciated!!
 

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1987 W201 190E 2.6
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179 Posts
For some reason that I can not figure out I have little to no heat at higher RPMs as well as a cooler running engine at times.

Normally my gauge reads above 80. Right now it seems to get there but once it does it may drop a little or drop a lot to below 80. Its really not terribly consistent, but generally it runs below 80 now.

In the morning commute the engine warms up as normal and I will get initial heat. But once I get on the highway its gone 90% of the time. Only a couple times have I had heat hold fine. Even in the city I will lose heat if I get into the higher RPMs.

Under any situation heat is regained once I come to a stop or very slow speed. Such as the end of an exit ramp. Once I get moving, heat will go away again.

It seems to be related to engine RPM. With a warmed up car I can sit in my driveway and rev the engine over 3k RPM. When I do, no heat. I have NO viscus fan installed and the aux fans do not kick in.

The car never gets critically warm unless I hold the higher RPM for a while while sitting. As expected. Not quickly, so the pump is certainly circulating water.

I just replaced the fairly new t-stat today. No change. I also replaced the expansion tank cap, twice. One a generic and today a new OEM unit. New mono valve also installed. Old was was torn around base edge. With all these new parts, no change at all.

I suspect water pump. But the engine seems to run cooler, not warmer. If the pump was bad I would think warmer temps would happen and not a loss of heat. The other issue may be a head gasket. Maybe exhaust is getting into the cooling system. This might cause air to get trapped in the heater core and maybe explain colder water temp - that is if air is hanging around the temp sensor.

Really confused on this one. Google shows that a few others have had this issue but they failed to update their posts.

Any thoughts on this one would be appreciated!!
Funny, they sound like symptoms of a stuck thermostat, or no thermostat at all, but since you mentioned changing it we can rule that out...
 

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560 SEL (1991)
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42 Posts
Discussion Starter #3
Ya. I will add that late last year I found out I was driving with one that was stuck open. Or mostly open. Probably since the day I got the car since it never really got above 80.

But during those times heat was okay. Not blazing hot, but not an issue to raise a red flag.

Now even when the car reads above 80 I can have no heat at all. So its an odd one. And its not consistent.

Does the aux-pump cause any issues like this if going bad? Maybe I can by-pass it to rule it out.
 

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1987 W201 190E 2.6
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Someone correct me if I`m wrong but the aux pump only pumps water to the heater for better efficiency and would not have any significant effect on overall coolant flow. If your heater has intermittent heating issues, I`d look at that pump or the temp dial on the climate control unit or the mono-valve, which you have done. The reason I would have though thermostat is coz both your heater AND engine are cool, particularly at higher revs. Perhaps it`s more than one problem.
 

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560 SEL (1991)
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Discussion Starter #5
Even when the engine is hot, the HVAC output is cool at higher RPMs. So engine temp and having heat do not always correlate.

I am certain HVAC control is working. When I no longer have heat the fan will start speeding up as it tries its best to get me more heat. But it just gets cooler, causing the fan to move more air. If I turn down the dial the automatic fan control slows. So it matches the temp setting very well. It was one of the first things I looked into - thinking the cabin temp sensor was bad, etc.
 

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MB 300SE '90, anthrazite grey, black leather
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186 Posts
I have similar problems; at highway speeds I get cold air inside, once the speed slows down, it gets warm again. I have replaced the aux water pump (new part), no help there. I have also tried two (used) ACC units, no significant difference. And I have also tried a couple of new monovalves...otherwise I have no problems regarding the coolant temperature, just with the temperature regulating inside the cabin (I have new cabin sensor and the motor connected to it to rule those out).

Could the whole monovalve magnet be the culprit then, or the vacuum pods...I do not think my ACC has worked properly during the almost three years I have owned my car, sometimes it had seemed so but wait just a while and something comes up...and troubleshooting here now in winter time is no fun either ! If I ever find the solution to this I will be sure to post it !
 

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1991 560 SEC 1994 E500 2014 E350 Cab
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so perhaps the heater core is plugged up, only allowing coolant to flow thru small portion? How old is the radiator? Could you have air trapped in the system? Have you ever done an effective citric acid flush? Have you used Zerex coolant since you owned the car? How much coolant does the vehicle take to fill the system? Just spitballing things you may have thought of, not thought of etc. Sorry if I am captain obvious, just seems to be a circulatory thing. If you were leaking into the head, would you not see smoke issues....glycol should crystalize like sugar in the head under heat I would think and that damage would be quite evident.
 

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560 SEL (1991)
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Discussion Starter #8
Zerex all the way, coolant is fresh. I dont think there is a clog, if there was I would not have the intense heat at idle that I do.

Im am going to pull the plugs and do a compression test. I did one last year, so I have some baseline readings.

The vacuum pod idea - thats an interesting thought. For me the issue is at higher RPM, which is when vacuum is weak. If there is some other part of the system, besides the mono valve, that can control heat output and it vacuum controlled, that may be the issue.

Willing to try anything at this point. By the way, I have unplugged the mono valve too. With a good valve it should be full heat all the time. I sure have uncontrollable heat at idle, but none at higher RPMs again.
 

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The engine coolant should remain a constant temperature once warmed up and that should be a little above 80 C.

And the only thing which regulates the engine coolant temperature is the thermostat. It decides to send coolant to the radiator for cooling or not.

-If the engine is cold, no water is sent to the radiator.

-If the engine is a bit too warm, then just a little water is sent to the radiator.

-If the engine is hot, then all the water is sent to the radiator.

When the car is not moving, no air is flowing through the radiator (if fan not on). Thus not much cooling of the water going through the radiator. However if the car is moving, then air would flow through the radiator and cool the coolant.

So something is fishy with the thermostat. Maybe installed backwards. Maybe too cool of a temperature of a thermostat (opens too soon due to its design). Maybe wrong thermostat and it can't shut off the flow of coolant to the radiator (water flows around it when it is closed). Or maybe large "particles" are circulating in the coolant and one of these got stuck in the thermostat and it can never completely close.

Unlikely but... Maybe there is a blockage somewhere where the water circulates through the engine and water does not circulate through the engine as it should - thus a LOT of heat is created near the thermostat causing it to open? (Engine would be more hot on one end than the other.) This would be a last resort nothing else is the problem thing. But check that water can flow through the engine unobstructed.

I would post a picture of the thermostat you purchased or the brand/part number. Maybe someone here knows if that is the correct part or not?

There could be other problems, but I would work on getting the engine coolant temperature to be stable first. Then see if there are additional problems.

Note: There can also be a problem with the temperature sensor from which your dash engine temperature gauge gets its information. Sometimes these will read cooler or warmer than the actual temperature. Thus it is good to get a "second opinion" on the engine temperature. Place a thermometer on the engine in various places.

Also when first starting the car, you can place your hand on the radiator and see when it starts to get warm. That means the thermostat is open and allowing water to flow through the radiator. Compare this with another car as it warms up. Notice the radiator is cold until the engine temperature gauge reaches a certain point, then the thermostat opens and allows water to flow through the radiator.

Another thing to look for is where the thermostat "seats". (Inside where the thermostat goes.) Be sure there are clean flat surfaces (not bumps and crud in there which might press on the thermostat once installed).
 

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560 SEL (1991)
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Discussion Starter #10
I agree that the t-stat seems fishy. That is why I replaced it yesterday. Its a german brand in a Gates box. Whaler or something like that. It was the same as the old, even though the old was boxed in another brand. Both were marked 80 degree celsius. I cleaned all seating surfaces. And it can only install one way due to the housing design.

I will check when the radiator heats up this morning, but I know that the car heats up to temp normally. Unlike before when I had a bad t-stat and it would take a while.

I will also replace the temp sensor or install a secondary. It could be possible that the temp sensor failed just when I happen to have HVAC issues. Stranger things have happened.

Thanks for the tips.
 

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Another test you can do (trick used to charge A/C systems in cold weather) is to place cardboard over the front of the radiator so no air can flow through it.

Be VERY careful doing this - keep an eye on the temperature gauge so your engine does not overheat!

(Also you can use a half covering of the radiator or 3/4 - 1/4 depending on how cold it is there.)

Then use that test to get your engine temperature up to 80 C. And see if your heating then works.

That would rule out any problems with your HVAC system (if your heating then works) and you could then concentrate on the engine coolant problem.
 

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P.S. We are really getting into "thermostatology" here!

Perhaps in addition to what temperature the thermostat opens at, it could also be how much it opens? Maybe some thermostats allow more water to flow (open more at a certain temperature) than others? (I don't know.)

And here is a Mercedes coolant system diagram which shows two opening/closing valves and seating with the thermostat. I don't know if this is the same as for your specific model? But explains how it should work.

http://www.mcclearyconsulting.net/matthew/cars/mercedes/W123 service manual/Engine/617/20-005.pdf
 

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...I will also replace the temp sensor or install a secondary...
One more thought for various brand vehicles in general... Many vehicles have two engine temperature sensors. (To my knowledge our Mercedes has just one?)

But on vehicles with two temperature sensors, there is an "engine temperature sensor" and an "engine temperature sender". The former going to the engine computer and the latter going to the instrument cluster gauge.

So as a "double check", it would be a good idea when replacing the temperature sensor to disconnect it, then see that the instrument cluster temperature gauge has no reading or is maxed out. Then you know for sure that is the correct sensor.
 

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1986/1990 W126
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I had a temp sensor become inaccurate around the time as my stat wasn't great either. Confusing..
Replaced the stat as little heat, temp reading low. Stat was indeed faulty, but car gauge still read low. Changed the sensor and all good!

From reading on forums it would seem that even MB thermostats are faulty on a surprisingly frequent basis, new.from the dealer. Could be that's all it is.
 

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560 SEL (1991)
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Discussion Starter #16
Two bad stats though would be odd. I dont think its air. If it was then I would never have heat, I get PLENTY at idle. I have done many things to make sure I dont have any air in the system.

At this time I am leaning towards a vacuum issue with the HVAC with a possible faulty temp sensor. The sensor is original so I may as well replace it. Its a cheap part.

I still will do a compression test since the HVAC can be more trouble to dig into.

Today I had heat one way traveling at about 60mph on a B road for 20 minutes. On the way back, nothing. But on the way back engine temp held above 80.

I am also going to look at voltage when I have a chance. Maybe the alt is putting out high voltage. That can cause some goofy stuff too.

So much to look at, I need to start a list!
 

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1987 W201 190E 2.6
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The climate control on my old 380 used to do all sorts of weird stuff such as heat, then no heat, then heat again but through different vents etc. so I pulled it out, disassembled it, inspected the circuit board and found that the series of soldering that connects to the side circuit boards were cracked and causing the relays to connect and disconnect randomly, notably when the buttons in front were pushed. I re-soldered all the cracked joints, as well as all the points where the relays sat. Behaved properly after that.
 

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84 500 sec black AMG
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lack of sufficient heat at speed

I still think you have an air pocket in there.
I've listened to so many people have a complaint of insufficient heat,[not no heat ] after changing the coolant. Every time I change the coolant,I get the front of the car up in the air. Pull the upper radiator hose from the radiator and pour the coolant mixture directly into the engine.The thermostat is not going to let much in,if any,but there is a bypass in the housing.It will take some minutes to fill the engine and the bottom of the radiator.Put the connection back together.Fill the expansion tank to the full line. Do not put the cap on yet.Start the car,massage the lower radiator hose,to move the bubbles.Top up if needed.While the car is coming up to temp turn the climate control to defrost,on high heat.
If you have a infrared thermometer point it at the bottom hose.It may take about 5 minutes to get a reading for the bottom hose to show an increase in temp.If the temp on the gauge is getting too high, 100c, shut the car off.Let it cool. You may need to top off again at this point.
Lower the car.After about 20 min of resting,check the coolant for full again.
If needed top to full.I would drive the car at this point with the defrost on full blast.Watch the temp gauge.If after all this and the bottom radiator hose doesn't get hot, you may have a thermostat that is stuck open.
Just remember if the bottom hose does not get uncomfortably hot to the touch of the hand,You've got air in the system,or perhaps on a more severe note,a blocked radiator,but I doubt it.
I hope this helps.
 

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'85 300SD
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I've had the same problem. I haven't looked into it yet as I have other bigger problems to fix, but as I was driving cross country in winter (Halifax to Victoria, Canada) I got good at pulling over, killing the engine, then starting it immediately. The heat would then come on and stay on until highway speed. With the key on engine off I would hear water gurgling behind the dash. No doubt the heater core being re-filled with hot coolant. I suspect low coolant levels or air pockets. My upper radiator hose is more or less permanently glued on with JB Weld because the neck cracked. When I get to it a new rad and careful fill will hopefully fix the problem. As others have suggested, I would start there. I once had a Ford with similar problems -- I installed a valve in the rubber hose going in to the heater core, pulled it up so the valve was at the highest point, and used that to bleed air out. Might do something similar with this one when I get to it.

I too have suspected the aux pump but haven't replaced anything yet. Still scraping the cash together for a turbo rebuild.
 

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84 500 sec black AMG
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Canada cruiser

It would be a perfect scenario to have a T fitting with bleeder valve at the return hose from the heater core, as it is positioned at least as high the expansion tank.Unfortunately we have that monovalve and it's connections in the way.All I know for sure is if that bottom hose wont get similarly as hot as the top radiator hose,you may get heat,but it wont be anything like you will feel when all is well.I have done quite a few different makes of vehicles.This is true of all of them.The hardest car to get air pockets out of has to be a VW cabrio. Probably the easiest is a Ford fusion.Sometimes geographical location helps.I have a friend in North Jersey who lives up in a very hilly area, the driveway entrance to his garage is probably 30 degrees incline and all we do is pour in as much coolant as the expansion tank will take.Run the car.Wait for the needle to move.Turn the defrost on full blast.Top up after about 20 min.Put the cap on the reservoir,and go.
Love Canada.Furguson lake near Calabogie is blissful.
 
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