mercedes w126 420sec cold start problem

[Firepants78]

Hi there,
for some time I have a problem with morning firings (especially when it's cold and wet)
any idea when start checking.
whether it can be battery fault? lately I exchanged 10 year old Bosch 75Ah/720 cca on Varta 74Ah 680 cca.

 

[Stratman]

Welcome to the forums

A new distributor cap and rotor arm could go a long way to improving things. I have vague recollections of a seal at the base of the distributor itself that can let in moisture from the crankcase, and it's moisture that can be a real problem on these engines.

 

[Firepants78]

Thanks, for quick answer

A new distributor cap and rotor arm was changed about year ago (unfortunately I don't know what make), new leads as well (Beru),
I check that seal today.

 

[ivan1983]

Are you running the correct rotor, leads and plugs. They all have different resistance values which could affect cold start performance.

Also what are the symptoms of your cold start problems.

Does it need to be cranked for a long time, does it run rough when first started.

Could be the cold start injector not firing....

 

[grober]

You want to check if the cold start valve [ the extra injector] is squirting out fuel into the manifold when the engine is cold . It its not -it might be the fuel pump relay part of which might power it /------/or a thermo time switch which provides power for the cold start valve for a limited period about 8 -15 seconds/------ depends on the age of the car

 

[Firepants78]

Yes it need to be cranked for a long time, and run rough when first started.

Maybe a silly question, but where is the thermo switch?

If cold start valve working only when engine is cold, what voltage should be on them when is cold 12V ?

 

[grober]

If you disconnect the start valve and direct it into a large receptacle [ glass bottle] it should spray for several seconds when the engine is cold. Remember that's flammable petrol you are dealing with!
[YOUTUBE]WPX8fveQdts[/YOUTUBE]

 

[Firepants78]

Today I measured the voltage on the cold start valve, and I have some weird jumping readings in mV

And please be patience - I do not know much about electrics, and the last visit to a MB specialist cost me £ 2900 and still is not good.

 

[grober]

Cold start valve supply voltage should be a steady 12v when present. If you don't get 12v I would suspect part of the fuel pump relay assembly.

 

[Firepants78]

How to recognize is a thermo time switch fault or fuel pump relay??

Both of these parts are not cheap...

Cheers.

 

[grober]

The fuel pump relay can often be repaired by resoldering the connections on the relay internal circuit board. You may or may not have a thermo time switch depends on the age of the car. Without seeing the car its difficult to tell. Have you tested whether the valve is spraying petrol or not? I can understand if you are reluctant to do this and if this is the case you will have to take the car to a garage familiar with these Bosch injection systems- doesn't necessarily have to be MERCEDES SPECIALIST. A competent Bosch service centre might do.

 

[Firepants78]

I disconnected cold start valve, when I try start car, absolutely nothing, no fuel through the valve, valve by connecting to the battery, doing beautiful mist.

If the valve is mount on my engine (4.2 V8 1989), where should I look for it.

Thank you from advance

Is any chance to bridge that relay (just for test) ??

 

[grober]

This is the nearest diagram I could find. As you can see the fuel pump relay N16/4 is really a multiple [x3] relay that performs many functions. It feeds the 2 fuel pumps M3/1 and M3/2 and the lambda sensor heater G3/2 for example You can't just bridge it. The cold start valve is Y8 fed from relay pin 87V into socket 4 of the FUEL PUMP relay socket

If this makes little sense to you best take the car to someone can understand it. usually the best thing is either repair the relay by resoldering it or replace it. Have you any friends who are electronic enthusiasts who could try to repair it for you?

 

[E270 Owner]

Hi

Might make more sense/help for him if the legend is posted too

Would help me too, cant find it in WIS Asra.......

 

[grober]

There was no full legend accompanying the diagram sorry-- just this

 

[E270 Owner]

There was no full legend accompanying the diagram sorry-- just this

Brilliant thats all I need

 

[Firepants78]

I'll try resoldering it, if I damage them, the parts (brand new cost £200) from ebay.
Repair (replacing) it will solve the problem? or can I still expect some surprises.

 

[grober]

I'll try resoldering it, if I damage them, the parts (brand new cost £200) from ebay.
Repair (replacing) it will solve the problem? or can I still expect some surprises.

If your starting problem is due a malfunction of the cold start valve.
If the cold start valve problem is due to a lack of a 12V supply to it.
If the fuel pump RELAY is faulty then possibly yes. But as I said without the car in front of me ------- impossible to say as its an old car and could have multiple problems. sorry.
You might be able to pick up a replacement FPR from a breakers yard??

 

[Firepants78]

My fuel pump relay is No. 003 545 25 05, that numbers must match or can i fit some replacement from w126 8 cylinders?

And is there a way to determine malfunction fuel pump relay?

Regards Maciej

 

[Firepants78]

On Friday I'll order a new fuel pump relay,fingers crossed this will solve the issue