SOLVED**350SL cold start problems

Hello everyone

first of all, many greetings for all the best in the New Year.
Last week I tried to start my SL350 but without success.
Air temperature was about 10°C ( not so cold ) but the car attempted to start with a very low idle and then dies.
Even if I tried to push on the pedal, the engine stops.
In these days, I made the following checks :

- Wiring checks--> all was ok except for the following value that are slightly different from the normal:
MAP sensor resistor primary winding --> 76,7 OHM ( Normal 80-120 )
Injectors resistance --> 1,7 OHM instead of 1,2 OHM

- Cold start valve ---> I tested the spray come, not the duration

-Power signal to ECU --> ok

- Fuel pump pressure --> 2 BAR during crancking

- Trigger contacts signal --> ok

- Fuel lines have been all renewed this year

- Wiring harness connectors and rubber sleeve are new

- I also did a visual inspection of the rotor cap and distributor that shown some signs of carbon deposit but not so bad

After these checks I was able to start the engine but the cold idle remains low; anyway I was able to drive the car outside of the garage.
At normal operating temperature the car runs great and the idle speed is about 750 rpm

Today I tried to restart it but without success, the problem remains the same ( low idle and engine stalls )

Now I've the AAV in my hands and I checked the function by heating the probe with hot air; it seems to work fine.
The piston goes up when hot and return down when cold.

At this point I don't know what to do

I hope that someone can help me

Regards

Alessandro

Welcome, Alessandro to the forum!

The measurements you made are not spectatual.

Does the AAV open when cold or is it clogged?

Low RPMs means there is not enough air...

Regards
Norbert

Hi Alessandro,

I would normally also suggest an auxiliary air valve AAV failure. But I am hesitant as you describe that pushing the pedal does not let the car run as well when it is cold.An AAV opens a bypass to throttle and is nothing else than opening throttle a bit. So if it was AAV, opening throttle by foot should solve the problem.

I once had a case where ignition caused such a problem. When distributor was cold, 350 SLC would hardly run 40 km/h. But when it was warm, it would run like normally. That was hanging centrifugal weights in distributor. I only saw it when I left the car in my cold garage and directly hooked it up to my ignition oscilloscope next mornung. I swapped distributor and it was gone. Later I disassembled that one and cleaned and retuned it son my ignition distributor test stand and it worked again.

Another issue could be a broken engine temp sensor. Engine needs a lot more fuel while running cold.

Hello everyone

Thanks for you reply
I checked the AAV and I found that blowing air from the inlet side ( at room ambient temperature ) it flows normally from the outside so I can assume that it is ok
When the temperature rise, the piston moves upward and closed the air flow ( in realty not at 100 % since a little amount of air still flows through )
The only thing I found strange on the bottom of the valve is that the ring that retains the bulb in position appears like to be partially missing ( maybe corroded )
My idea is to try to start the engine without connecting the AAV hose and see if something changes.
About the engine temperature sensor, does exist a procedure to check it ?

Regards

Alessandro

Hi Alessandro,

I have seen few AAvs which had such a corrosion. The danger is that coolant could then pass through AAV into manifold. And residuals of coolant will finally make the piston in AAV stuck. Afterwards it is very difficult to take AAV apart without destroying it.

For coolant temperature sensor readmy my Checklist .

You should ignore my guess about distributor. It is my hottest guess.

Hello
after some test finally I dismantled the injector valves and tested the spray cone and injected fuel quantity.
The spray come seems to be ok but I found the following discrepancies on the injected quantity :

#1 : 2,0 cc
#2 : 2,2 cc
#3 : 2,6 cc
#4 : 2,4 cc
#5 : 2,6 cc
#6 : 2,6 cc
#7 : 2,4 cc
#8 : 2,4 cc

Now my idea is that the difficult cold start could be caused by a partially blocked injector so I'll proceed with an ultrasonic cleaning.
After dismantling the injector I also found that the injector sleeve are damaged so I think that another problem could be an air leak from them.
Unfortunately the item A1160780083 seems to be unavailable from MB
Do you have any suggestion ?

Thanks and best regards

Alessandro