Engine running toooo cold
 

Car: 1991 560 SEL 103K miles
Problem: temp gauge never go above the tick between 40 and 80 C even after 30 minutes of stop and go.
1st Diagnostic: Thermostat stuck open.

Spend three hours to get that ^&*^%$ thrid screw to get the T-stat out. Put the unit in a pot and cooked it. The 1st stage open at 82 C and 2nd stage open at 95 C, working perfectly.

Now what? Please advise how to check the temp sending unit?

Low engine temp seems to contribute to my car running high idle. Engine keep wanting to go to a higher RPM to get warm.

You could verify the engine temp if you have access to an infrared thermometer.

It's just my opinion, but the cooking-on-the stove thing is not proof conclusive.

Buy a new t-stat and make sure it's a Behr.

I'm sure that your temperature gauge does not show the correct temperature. Engine combustion produces high temperatures which have to be cool. Just drive the car for 10 minutes, open the hood and touch the upper hose and if you can not hold it the temperature of the coolent is higher than 40-80C. I will look for the problem in the sending temperature sensor or wiring from the sensor to the gauge.

Roman
87 300SDL 152K
86 300SDL 212K
86 420SEL 226K
81 300D 139K

I did exactly that when I got home Thursday night (30 minutes drive). I could put my hand on the radiator feeling hot but not burn.

I have a behr T-stat coming Monday and we will see what happens after put it in.

air lock run the engine without the rad cap on so all the air gets out and watch for the cooling fan to cut in then all is ok

also ensure the heating is set to hot in the car for a full system air bleed

Thank U teky!!!

I was worried about how to bleed all teh air out. So what you are saying is to connect everything back with gaskets and everything. Then fill the expansion tank with MB coolant (44%). And just let the car run with heat at Max?

How could I know if all the air is out? Five minutes or 20 minutes?

If you had an air lock, you would be running hotter; not colder.

Good idea to bleed anyway.

If the upper radiator hose isn't too hot to touch, chances are the thermostat is opening too early. Otherwise, the thermostat housing and hose should be at 80 C or more, much to hot to touch.

To bleed, try squeezing the upper radiator hose flat, then crimp the hose to the expansion tank shut, the relase the upper radiator hose. This will pull the air out of the radiator, where most if it sits (no way to vent except to the tank). Repeat and fill tank until it won't take any more, then expect to have to refill it a couple times. If you have the engine running with the ACC set to full heat (defrost with the temp wheel at the red clickstop), you will get most of the air right away.

Peter

Hello Mike, I was referring to bleeding after I put the new T-stat on. There will be so much air in there and I was worried. So, you also agree that teh correct way to bleed is to let the car run while leaving the cap off?

BTW, why did you not like the "boil in a pot" test? I don't really know how T-stat works but it is surely amazing. Especially these MB two stage T-stat.

all hoses have to be hot when running this test, any cold pipe means an air lock

and its the lower hose you need to compress to get the air out, if it remains cold the stat wont cut in it sees the engine as cold

not as bad as the BMW 525i but its a sod

Hello Teky,

I would like to think that air lock = poorer efficience = higher temp. Since my engine running cold, I think it is either T-stat or sensor. I will put in the new T-stat and drive it on highway for 20 minues. If it is still at 60C, I will replace the sensor.

Test the sensor w/ohm meter.
Here is a temp/ohm chart for calibration

60C-110 ohm
80C -67
100C - 38

Most likely suspect is STICKING OPEN thermo.. don't replace w/whaler..

Peter's advise is what I'd use. If you follow his logic, the air is going to be pushed out because all escape routes will be shut off.

M.B.DOC once posted a technique used on some earlier 103 motors. The front end of the car was elevated to get the air out. The expansion tank cap was of course removed.

My experience with 103 motors has been that trapped air causes overheating. If coolant can get to the t-stat, it won't open and allow coolant to flow. Coolant then starts boiling in the engine block.

Hi Arthur, I just looked, the temp sending unit has only one pole? How do I meaure ohm on that one pole. I was thinking about dip it to hot water to observe the changing ohm.

<<Hi Arthur, I just looked, the temp sending unit has only one pole? How do I meaure ohm on that one pole. I was thinking about dip it to hot water to observe the changing ohm.
>>
The ohms reading is taken from the one pole to the case [ ground]
You can do it several ways, but the Easiest is to get the engine close to one of the specs on the chart and pull the wire and take an ohm value to see if it relates to the chart..
The Best way it is to get a 1% tolerance resistor [ RShack} of the proper value
on the chart and just hook the wire that goes to the sender to it and ground the other end of rsistor to the engine ground. I have one of these in my tool box, just to save time and you do not have to run the engine to do the test.. you are simply subbing a known chart value and the gauge has to respond to that value . the advantage to this test is you now testing BOTH the gauge and the sensor.....
Your water boil will work too, but then you have to remove it and then monitor the water in the pan , bla, bla..
this is why the laser temp units are so popular..............

Great Idea Arthur. Let me rephase it so I am clear. I will need a 67 ohm resister and I connect one end to the cable and the other to any metal part on the car. All I need is to turn the key half way to activate all gauges w/o running the car. If the gauge shows 80C, my sending unit is bad. If the gause still shows 60C, my gauge is shot.

BTW, I already got the T-stat out and it is at fully closed status. Boiling water test showed normal open and close for both stages.

Man, after doing all these work on this piece of *&^%$#@$%, I will be a decent mechanic. Probably make more money that way.

Correct ..

You can also get a trimming resistor and set it to each chart resistance value [ [as measured w/ohm meter] and check the full range of the units, but that is over-kill...

Are you sure?

I have a similar problem with my M103: warms quickly to 60C (mark between 40 and 80), runs like this practically all the time, only in stop & go traffic raises to 80C.

I supposed the gauge did not show correct temp - if I open the heat, it will blow hot air after 5-10 minutes of running. If coolant was cold, this should be impossible...

When I rotated the fan by hand, with engine cold and stopped, it rotated "somehow sticky", i.e. not fully free, but neither rock-hard as if clutch was fully engaged.

My coolant was highly diluted during summer - put in distilled water.

Any ideas?

~Nautilus

[QUOTE=Nautilus]Are you sure?

http://www.peachparts.com/shopforum/tech-help/4189-1987-260e-overheating.html?highlight=air+pocket

Put in a new Behr T-stat and flushed the system with distilled water. Drove the car out to get some beer and could not get the gauge to go over 60. Everything is exactly the same as before changing the T-stat. The good news is that I flushed out the "green" coolant but the bad news is that car remains problematic after $$ spent.

Between the tip of that single pole sensor and its base, I got 245 ohm. Should I just change the sensor?

In another forum, someone showed me a four pin senor for coolant temperature. Is there more than one sensor for engine temperature?

Look at what Benzmac had to say in this thread:


http://www.peachparts.com/shopforum/tech-help/7560-1986-300e-coolant-temp-sensor.html?highlight=sensors

If you got 245 ohms , you are on the aux. fan switch, not the gauge sensor.

This is the fan sw., the other I listed is the gauge sensor

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1A70KFNW8&year=1991&make=MB&model=560-SEL-001&category=G&part=Auxiliary+Fan+Switch

<<Is there more than one sensor for engine temperature?
>>

Yes
You will know if you have the gauge sensor cuz the gauge will stop working when you have it unplugged...
....do this when warm so you know there is some gauge reading

hello Arthur, I am pretty sure I got the 245 ohm from the gauge sending sensor. It looks exactly like the P4036-12281. When gauge was reading 60 C, I unplug the cable from this unit and the gauge goes directly to 0. I believe I got another two poles sensor which should be the fan switch. Now I need to find the sensor for EZL and CIS-E.

Please help.

OK

If the gauge went to zero, that is it and you have a bad sensor...[ providing you got the reading from THAT sensor and not the aux fan 2 pole]
get a new one ..I see they are only $15.

The other sensor [ 2 wire] is your aux fan switch.. Fast Lane show this to also be a single wire one, but they are mistaken.. I looked up your car on the fan schematic and it takes the 2 wire thermistor one like all the '91s do...


The cis/ezl sensor is the 4 prong one .. nothing to do with the temp gauge...

I found the 4 prong one for CIS/EZL right under the air filter housing. Got about 850 ohm reading across which says I am in the ball park (840 ohm @ 50 C). I will do another reading tomorrow morning when the car is cold and I should get about 3K ohm.

I felt kind bad after all this trouble, all I will accomplished for $70 is changing to the right coolant and able to read the temp right. It has nothing to do with how the car is running. Man, I had hope I could get rid of the high idle problem by achiving right temp.

Thanks for all your help.

Yeah, It cost money to go to School..
On the four prong , the sensor is actually 2 seperate sensors in one case ..... [ across from one another ] , so a little test is to make sure they both read out the same .....

Yes, they do. I had hope they don't but they do. Good night!!!

somewhat happy to report
 

that the gauge is showing the right Temp again. Ordered a temperature sender ($14 + $6 shipping) and got about 500 ohm at RT. The bad one gave me ~1500 ohm at RT. Upon idling for ~5 minutes, I got 82 C reading.

Anyway, flushed the system with distill water. Replaced a perfectly good T-stat, replaced a bad sender, fill it up with Zerex G-05 coolant and distill water. Total cost of $70 but learnt everthing about the cooling system.

After my tragi-comical adventure with the alternator and the additional removal of radiator, I had to complete the coolant - poured in about 2 liters of pure antifreeze(a local, poor-quality brand). Now it heats up better: 75-80C during driving, 80C or over in stop and go traffic. Guess the "diluted" coolant it previously had(mostly distilled water) made it run colder in the hotter months of the year ....

~Nautilus