280SE 3.5 Cabriolet 1970

Hello Volker and forum members,

Just signed in for the first time upon the suggestion of a fellow member at Benz World vintage car forum.

For my 1970 model MB 280SE 3.5 cabriolet, I cannot so far find a solution to the following problem:

When I drive the from cold start all is fine until the temperature reaches about 70 degrees C , at which time the car starts losing power and moves hesitatingly. The idle speed at this time is also unsteady.
But, as the car reaches normal temperature a couple of minutes later all works just fine and there is no more trouble.
Could you please propose what action I should take to avoid the car behaving erratically between 70degrees and 80/85 degrees C. For your info the Zusatzluftschieber has been changed to new.

Thanks in advance for any advice,
Ashok

Hi Ashok,

that is a difficult one. Actually there is nothing special in ignition or fuel injection acting specifically at the temperature of 70 to 80°C. It is an analogue electronics with sensors and it has no switch levels at specific temperatures. The values fed in by all sensors are taken as they are and used to "calculate" the right injection pulse width. What is depending on temperature:

Water temperature sensor
Air temperature sensor
Thermo time switch
Auxiliary air slide valve

But they all have no specific 70° characteristics other than the air slide valve. That closes at 70°C. Easy test: Close its air hose at 60°C and watch whether your car still shakes at 70° and runs well at 80°C. Then you can exclude the new AAV as a cause of trouble.

What could indirectly be influenced at 70°C ?

Only one thing comes to my mind: RPM-dependent adjustment of ignition timing. During cold-run AAV increases engine rpm to let it run stable. At 70°C AAV closes and that finally reduces engine rpm to idle-run speed of 750/min. That means that centrifugal governor now comes to a zone it has not been before. Old grease could make it sticky and it could become better when this grease gets hotter. I had such a strange case once. But it started right away from cold-start. How to verify? Check ignition timing when engine reaches 70°C.

Second idea: Do you have an American car? That has a 2 way valve in vacuum line to ignition distributor. It switches ignition timing in idle-run by 15° forward if it is enabled. And there are various gear box oil pressure and temperature switches activating this valve. If the valve closes, ignition timing is pushed backward and reduces rpm by roughly 300. That is due to US emission reduction regulations. Just shortcut this valve by directly linking both vacuum hoses. You find its working principle in "Workshop manual after 1968, chapter 00-79". vdh has an online German version, but I do not know such one in English.

Last idea is a broken vacuum hose or electrical wire that for strange reasons does not work between 70° to 80°C.

Least idea: You should participate in our annual W111 Cabrio Meeting and introduce yourself to the club owner Aron. We meet every year at a different location at ascencion day, tour a bit and enjoy our life.

Hi Ashok,
nice to meet you here again...
(sorry for the delay, internet connection over 'Kabel'=Vodafone is distorted in whole southern lower-saxony...)

You told before your ignition was checked by Bosch Classic Basel (info for Volker).

I remember one posting in another (Opel) forum I can't verify, but the user told there was a faulty temp-sensor, problems at warming up, too.
I can't believe this, but... why should someone tell this without reason.

You could replace the sensor by a fixed-value resistor, 1/4 Watts will do. Value about 80°C = 325Ohm would be fine to check it out, I suggest.

Let the engine warm up a little bit, install the resistor and check if the engine then 'runs over' the critical temperature.

Just an idea to test it out.

Regards
norbert

nordfisch wrote: Hi Ashok,

You told before your ignition was checked by Bosch Classic Basel (info for Volker).

Hi Norbert,

if it is an American car, it has a different ignition distributor and ignition timing than the European cars. 350 engine was normally not used in the US, only for these nice W111 280 SE 3.5 Cabriolets and for W109 300 SE(L) 3.5 . So it is easy for a mechanic to make the wrong assumption that this is like all other 350 engines of Mercedes-Benz. Or one could even have mounted a European ignition distributor with the US switching valve. Correct components are listed in appendix A .

Anyhow when Ashok tells us whether this was an original US or (very few only) European car, we might know whether this could be an issue.

And Norbert's idea to verify both temperature sensors air and water is for sure a good one. Hard to believe though that the temperature sensor is doing a jump in resistance. But with such a strange problem you never know and have to eliminate as many possible reasons for failure as possible.

Hello Volker, Norbert and Members,

First point, the Car is a genuine European model delivered to a Swiss dealer in April 1970 and I am the second owner, since 1975.

Thanks for your help. I'll check out the suggestions this week-end.

Best to all,

Ashok

Hi Ashok,

an original Swiss 280SE 3.5, Cabriolet, that is a seldom one. The trouble is to find which error(s) happen at exactly 70°C when auxiliary air valve has closed. Very difficult via Internet. I wish you luck and please feel free to ask further questions. I know you had your auxiliary air valve AAV replaced. But mayybe you do the quick test anyhow: When engine is warm close air hose to AAV and check whether you have any change in rpm. Another point to tick off on the checklist.