1973 280SE not holding idle at cold start

[yxi]

Hi, my first post here, just got myself my first w116 280SE(my second MB, have an 84 240d also).

The first problem i have to address is idle after cold starts. Engine starts fine, but it dies out if i release the gas pedal after it is started and running. If i keep my foot slightly on the pedal for some minutes, then release, it is fine. This problem is only on cold starts, when it is warm there are no problems and idle is ok.

Anyone have any idea what the problem might be? Or any suggestions what to check first?

Rgds
Yxi

[daantjie]

Do you have K Jet or D Jet fuel injection?  That will make a difference in the approach to correcting this.  Either way, vacuum leaks can play havoc on both systems, best to start there and check all vacuum lines, by now if they are original to the car, they must all be treated a suspect, as they get brittle with age and break.

[yxi]

It is a d-jet. So you think it can be vacuum problem even though it only appears for the first few minutes after a cold start, and else it is fine?

[daantjie]

Found this little blurp on the Net after some quick searching:

"Cold Starting: When the engine is cold, additional fuel is required for starting due to poor mixing and condensation of the A/F mixture on cold intake, cylinder, and piston surfaces. The ECU senses the cold start with the engine temperature sensor, TS2, and provides additional fuel when cranking. At temperatures below 32 deg. F ("Cold-Cold Start") , a separate thermo-switch and cold-start valve (CSV) in the intake manifold are activated. This injector sprays a fine mist of fuel into the intake system that mixes better with the cold air than the injector spray, and helps make cold starting easier. Some later D-Jetronic systems used a thermo-time switch which limited the duration the CSV would remain open, to prevent flooding and spark plug fouling"

Same as with K Jet, hard cold start seems like could be your Cold Start Valve that is kaput, or Thermo Time Switch, or both.

Welcome to the steep learning curve   You will find that once you have a handle on the system fundamentals, it all becomes a lot easier, and the system itself is quite reliable once sorted out.  Both the cold start injector and thermo time switch are items which can and do go belly up after 30+ years of service.

[daantjie]

Yet another blurp:

"Warm-Up Transition: When the engine is below its normal operating temperature, more fuel is needed to account for condensation and incomplete mixing until the engine is fully warmed up. Engine temperature is sensed by the TS2 sensor. The ECU has a warm-up enrichment circuit that senses the resistance of the TS2 sensor and corrects the mixture for the engine temperature. When the value of TS2 drops below a threshold value (typically 300 ohms), the warm-up circuit in the ECU has a cut-off characteristic, so that no additional decrease in TS2's resistance will affect the mixture (i.e. the engine is fully warmed-up)"

Time for some hoemwork

[daantjie]

http://forum.w116.org/mechanicals/d-jet-aux-air-valve-refurb-(-aav-)/

Not sure if this link will work, but addresses the Auxiliary Air Valve, which can also be a culprit here.  If it is stuck closed, you will not get enough air for the "Idle Up" function during cold start.

[yxi]

Thanks Daniel, then i have something to look into:) I tested starting it today. Temperature outside was between 40-50 F, so not a "cold-cold start" (meaning the thermo switch and CSV isn't activated?). It started without problems on first try, but (as always) I had to help with foot on the pedal for 3 minutes for it not to stall. After 3 minutes i could let go and it idles by itself...

Do you know where to locate the TS2 sensor, and is there a part no? If it isn't expensive, and it is easy to replace, i guess i'll just start with that.

Rgds
Yxi

[daantjie]

Bosch Part#  for Temp sensor II is 0280130014.  They are nice and cheap, let's hope it's that

I would also pull the hoses off the Auxiliary Air Valve to see if the valve itself is not shut closed (cold engine).  The valve must close completely for a hot engine (you can test it by pulling it out, and submerging in very hot, almost boiling water), so that you get a good hot idle and not too lean, but for cold engine you need it wide open to match the rich fuel mixture for idle up.

If it is stuck closed cold, it is toast, and you need a new one.  Some folks try to clean them out but I do not subscribe to that theory, just buy a new one after all these years.  They are pricey new though

[yxi]

So i got to replace the temp sensor II, it didn't make any difference.

Where is the auxiliary air valve located? And what does it look like? I will test it when i know what to look for


What are the other sensors on the engine, i replaced the one to the right in this pic(temp II), what are the two others to the left for?



There is a sensor in the air intake too. Can this one have anything to do with my problem?



As you can see, I haven't got around to cleaning the engine and surroundings yet....sorry for that

[daantjie]

http://forum.w116.org/mechanicals/aav-csv-wur-locations/

Some good pics here

[daantjie]

In your top picture, the middle unit is the thermo time switch, it controls the operation of the cold start injector.  This could also be your issue, you can test the switch itself and cold start injector for function, do a  search on the forum, there is a lot of good recent info on this.

This switch to far left is the sender unit for your temp guage, coolant temp sensor.  This has nothing to do with your problem, it is just a sender unit for temp gauge.

[yxi]

I can confirm it is the AAV that is making the trouble. Disconnected the air line from AAV to intake, start engine and it holds idle very nice right from start. Next step, drain coolant and remove AAV, then try to soak in all sorts of stuff to try to make it work...

[daantjie]

Normally you do not have to drain the coolant to remove the AAV.  Some coolant will spill out, yes, but you can just top it off again once you seal it back up.  Remember to change the gasket at the AAV otherwise you will get a coolant leak after fitting it back sans gasket.

[yxi]

Thanks for the advice. I drained the coolant already, but that's ok because i don't know when it was last replaced anyway. And i have a new AAV gasket already in order

Removing the AAV was no big deal, accessed it from under the car. It's been soaked in diesel over night. Any other good tips how to clean the internals?

[w116john]

hi

just place the aav in the fridge / freezer so its fully open then place in cup of hot water try not to get water in the openings and put it through a few of theses cycles and you should see how well it opens and closes, its said it should be closed at 66degrees but if its closed closer to operating temp its ok

john