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6.9 K-jetronic rough and high idle cold start

Started by Jan S, 10 June 2021, 05:36 PM

Jan S

Thanks for your tips, goldacre!

The hose you mentions is on my to-do list. But first I need to fix the cooling system and make sure I get the right coolant temperature. Then the Aux Air Valve will fully close.

The rubber connectors under the plenum are indeed out of sight but not out of my mind :) Is also on my to-do list, but later. I haven't been working in those areas before, hence I'm not able to visualize in my head how you tested around cyl. 7 and 8 with gas and that hose. But I believe I understand your aim. I might come back to you on this test, later.
1975-mod W116 450 SE with 6.9 engine

Jan S

Both new and old thermostats work according to spec -- new opens at 87 deg C and old opens at 79 deg C (California car, a previous owner put in a "colder" Thermostat, spec says 87 deg C)

Where do I find the coolant temp sensor? Need to do a bit research  :)

I also want to force-close air through the aux air valve (AAV) to see if rpms remain at 900. No change in rpm I guess the AAV works as it should ......

I suspect the engine has a serious vacuum leak, somewhere. >:(

1975-mod W116 450 SE with 6.9 engine

Jan S

Re-installed the new thermostat tonight and idled the engine for about 30 min.

Idle cold 1100 rpms, then increases to 1400 rpms, followed by a descent to approx 900 rpms warm, all in about 15-20 min.

The temperature gauge shows approx 130 deg F (55 deg C) when warm, see pic (I assume the mark between 100 and 175 deg F is 137,5 deg F?)
1975-mod W116 450 SE with 6.9 engine

revilla

Hi Jan,
As suggested before, your temperature sensor is most likely at fault. They are available at MB or ebay or auto-doc in Europe, not expensive and easy to replace. By experience, cleaning them doesn't solve the problem. They loose their calibration with time/cycles.  A new one will solve your "low" temp issue. Your old thermostat was probably working just fine, but the sensor was sending a wrong value to the gage. I had this happened to me 2 times in W116s.
If you want to test sensor, you can remove it from engine, then re-connect its cable, then ground it through its body to the chassis, then put a heat source to the bulb, see what the gage does.  Ignition in position 1, do not start the engine, coolant will spill all over your garage, unless you plug the empty slot.   Probably a 2 people task.
Good luck
Robert
W116 1977 280SEL & 1979 280SE
[img width=150 height=100]http://revillagodfroy.com/MB%201977%20280%20SEL/album/thumbs/IMG_0371.JPG[/img]

daantjie

#34
Indeed if we assume your gauge is reading low, then a 900 rpm idle is for sure too high for a fully warmed up engine.  You can also use an IR temp gun to read the temp at the thermostat housing.  Granted it will be lower than the actual water temp but it should give you an idea of what's going on.  87C at the hose is almost too hot to touch, depending on how tough you are of course  ;D
Daniel
1977 450 SEL 6.9 - Astralsilber

Jan S

Quote from: revilla on 10 January 2022, 05:27 PM
Hi Jan,
As suggested before, your temperature sensor is most likely at fault. They are available at MB or ebay or auto-doc in Europe, not expensive and easy to replace. By experience, cleaning them doesn't solve the problem. They loose their calibration with time/cycles.  A new one will solve your "low" temp issue. Your old thermostat was probably working just fine, but the sensor was sending a wrong value to the gage. I had this happened to me 2 times in W116s.
If you want to test sensor, you can remove it from engine, then re-connect its cable, then ground it through its body to the chassis, then put a heat source to the bulb, see what the gage does.  Ignition in position 1, do not start the engine, coolant will spill all over your garage, unless you plug the empty slot.   Probably a 2 people task.
Good luck

Thanks, revilla! I will buy a new one. But, can you help me with the location of the sensor? Alternative is to go searching tomorrow  :)
1975-mod W116 450 SE with 6.9 engine

Jan S

Quote from: daantjie on 10 January 2022, 05:38 PM
Indeed if we assume your gauge is reading low, then a 900 rpm idle is for sure too high for a fully warmed up engine.  You can also use an IR temp gun to read the temp at the thermostat housing.  Granted it will be lower than the actual water temp but it should give you an idea of what's going on.  87C at the hose is almost too hot to touch, depending on how tough you are of course  ;D

Thanks, daantjie ..... yes, UTn_boy also suggested IR temp gun. I guess I will buy a new sensor before acquiring a temp gun, to see if that solves the thing.

Can you help me with the location of the sensor?
1975-mod W116 450 SE with 6.9 engine

daantjie


[/quote]


Can you help me with the location of the sensor?
[/quote]

OK I assume you mean the sending unit for the gauge?  You will see it kind of hidden at the left far back against the firewall side (as you are standing in front of the car looking at the engine bay).  So it would be at the passenger side (for left hand drive), on the inboard side of the valve cover.  You will see 2x sensors/senders here next to one another.  One is for the cold start injector, and the other one is for the temp gauge.
Daniel
1977 450 SEL 6.9 - Astralsilber

Jan S

1975-mod W116 450 SE with 6.9 engine

goldacre

Hi Jan S, speaking from experience on IR devices I would have a close look at the Raytek brand  8)

In food compliance we need +/- 1oC but for auto use +/-2oC is fine.

Has many uses, one is as suggested to determine naff thermostat OR if pulling a trailer (as I do from time to time) over long distances I use my Raytek to monitor wheel hub temperatures to ensure no bearing failures underway (I have a boat in regional Western Australia so distance is in multiples of 100kl) from experience with wheel bearings is 40 to 48 oC is AOK but higher than that you have a problem  :(

But wait theres more.... can use to monitor your fridge temp as those digital monitors in fridges/freezers have sensors positioned in optimal reading positions so i often find fridges at 7 to 8 oC  >:(

But your in Norway, freezing! had the privilege of visiting there in 2019 and OMG a great country, when this covid bollocks is done and dusted I'm going back there, found an awesome wine bar in Tromso  :-*

G
12/1979 450 SEL 148K on clock (museum piece)
12/1986 Lotus Esprit Turbo 87K on clock 'Darling, look what Q has brought for us, isn't it nice' :)

daantjie

I think I have you guys beat on low temps here in the Great White North.  Though I live in "balmy" Vancouver, places like Edmonton and Winnipeg can easily hit -50C with wind chill :o :o :o
Daniel
1977 450 SEL 6.9 - Astralsilber

Jan S

Although far north, Norway is blessed with the positive effects of the Gulf Stream -- a strong ocean current that brings warm water from the Gulf of Mexico into the Atlantic Ocean and later to the coast of Norway. Enables people to live here  :)

daantjie - Winnipeg and -50 deg C is hard to beat .... requires a coolant mix 100-0 I guess (water being the one digit number)  ;)
1975-mod W116 450 SE with 6.9 engine

goldacre

LOL  ;D

Today where I am it's +45oC  :-\ and we don't get ocean currents in my area, very hot Easterly winds swapping very hot westerly winds in the arvo  :'(

I did record -6oC on my windscreen ice sheet one winter, using my Raytek  :-*

Google Marble Bar, its in the North of Western  Australia,  that's hot.

All AU delivered W116 had the African rated Radiator, huge, I see the Euro W116 have the smaller one with oil cooler, think oil cooler was needed to allow the thinner oils back then to cope, but oil tech now is awesome.

G
12/1979 450 SEL 148K on clock (museum piece)
12/1986 Lotus Esprit Turbo 87K on clock 'Darling, look what Q has brought for us, isn't it nice' :)

Jan S

The coolant temp sensor was shot!

Installed a new today and let the car idle for 30 min. Temp rose to approx. 200 deg F (approx 90-95 deg C). Perfect!

A while back I cleaned, polished and tested the aux air valve (AAV). Worked 100%, hence I assume the AAV is fully closed (during test fully closed at 80 deg C).

Next step is to chase vacuum leaks .... with smoke? Any tips/experiences?
1975-mod W116 450 SE with 6.9 engine

Jan S

Vacuum leaks are on the agenda.

Tested the self-made "smoke machine" today. Got the tip from a guy on youtube.

It worked great until the soldering iron gave up on me, probably due to overheating.

I found a few leaks, see next post.

I need to find a "smoke machine" or buy a better soldering iron. Maybe dry ice .......
1975-mod W116 450 SE with 6.9 engine