W124 cold start woes

[Sp!ke]

My W124CE has started to misbehave recently in terms of cold starting.

Once warm it appears to be mostly fine but there is a small amount of hesitation just above tickover.

Eventually after much turning over, it will fire initially on two or three but quickly catch and then it'll be fine. The revs stay high for maybe 20 seconds or so and then drop to normal (if that provides any clues).

Looking at the manual there appears to be a number of items that could be the cause and I'm not sure exactly where to start and the last thing I want to be doing is randomly replace parts.

I found an article which suggested the following potential causes.

1)No fuel pressure;
Test the fuel pump and pre-pump and do a fuel pressure and volume test.Fuel pressure is fine I believe as the car runs fine once running

2)Jammed and/or sticking airflow sensor;
Remove the boot that goes to the intake and loosen the center line on fuel distributor.This will relieve the control pressure.Press down on the center bolt of the plate and you should feel no resistance or binding.Also check that the rest height and centering are correct.

3)Auxillary valve is sticking;
How does one test this?

4)Defective cold start injector;
If the injector does not function,the air-fuel ratio will be lean,preventing easy start up.
How does one test this?

5)Shorted or defective thermo time switch;
The cold start injector will not work if the thermo time switch is defective.Check this in the event that your cold start injector does not function before replacing.
How does one test this?

6)Control plunger sticking;
Remove the fuel distributor from the airflow sensor and check to see if it moves freely.


Before I begin to tackle this, is this a common problem and is therefore something I should check first and foremost?

 

[nick mercedes]

My W124CE has started to misbehave recently in terms of cold starting.

Once warm it appears to be mostly fine but there is a small amount of hesitation just above tickover.

Eventually after much turning over, it will fire initially on two or three but quickly catch and then it'll be fine. The revs stay high for maybe 20 seconds or so and then drop to normal (if that provides any clues).

Looking at the manual there appears to be a number of items that could be the cause and I'm not sure exactly where to start and the last thing I want to be doing is randomly replace parts.

I found an article which suggested the following potential causes.

1)No fuel pressure;
Test the fuel pump and pre-pump and do a fuel pressure and volume test.Fuel pressure is fine I believe as the car runs fine once running

2)Jammed and/or sticking airflow sensor;
Remove the boot that goes to the intake and loosen the center line on fuel distributor.This will relieve the control pressure.Press down on the center bolt of the plate and you should feel no resistance or binding.Also check that the rest height and centering are correct.

3)Auxillary valve is sticking;

4)Defective cold start injector;
If the injector does not function,the air-fuel ratio will be lean,preventing easy start up.

5)Shorted or defective thermo time switch;
The cold start injector will not work if the thermo
time switch is defective.Check this in the event that your cold start injector does not function before replacing.

6)Control plunger sticking;
Remove the fuel distributor from the airflow sensor and check to see if it moves freely.

7)Restricted injectors;
For this to be the cause,they would have to be severely restricted,which is why I listed it last.Do
an injector flow test and ensure that the delivery is
near equal.

Before I begin to tackle this, is this a common problem and therefore something I should check first and foremost?

If it's a 300CE, it might be worth checking the dizzy cap...

 

[Sp!ke]

Its a 230 and no, its definately the cold start mechanism.... starting when warm is fine.

Start the car cold is made much easier by manually choking the air intake....

I changed the distributor cap/arm along with the HT leads as that was my first thoughts. No joy though

 

[Will]

As I understand it, the thermo time switch is basically a temperature sensor which lets the car decide whether it is warm or not and therefore whether mixture enrichment is needed.

Should be a resistance value for when it is cold and another for when it is warm. This will impact whether the cold start injector is utilised or not so they are definately related in that sense.

There are more complicated methods to ensure that the KE Jetronic (fuel injection system fitted on most 80s MBs such as yours) is running in closed loop mode but that'll take a bit more depth.

Does your car have ABS at all?

Will

 

[nick mercedes]

Its a 230 and no, its definately the cold start mechanism.... starting when warm is fine.

Start the car cold is made much easier by manually choking the air intake....

I changed the distributor cap/arm along with the HT leads as that was my first thoughts. No joy though

OVP relay?

 

[Will]

One of my thoughts too Nick - worth checking hence ABS question

Will

 

[Howard]

Sp!ke , if you can get over to mine tomorrow morning, you can try my OVP in it if you want ..... as it's the same car, it should fit....

I know mines OK as i replaced it about 2 months ago .....

I also have one that may or may not be good , i can't remember .....

 

[Will]

And if you need a spare, I might have one

Will

 

[Sp!ke]

I do have ABS, and the ABS light has recently stuck on too - can the two be linked?

 

[Will]

It's your OVP relay spike.

Lift the plastic cover behind the battery, and you should see the OVP relay. it's aluminium cased, about an inch square and two or three inches tall.

There's a clear plastic cover over it with a red 10A blade fuse inside.

Firstly check that this isn't blown. If it isn't, and you want to try another, PM me and I'll let you try one (or you can try Howards)

Good luck

Will

 

[Dieselman]

The thermoswitch is exactly that, open circuit when hot, closed circuit when cold. This in turn passes power to the cold start injector, so if there is no 12v to the injector it won't squirt.
To check the switch either use a meter or short it out, to test eh injector unscrew it from the mainfold and squirt it into a jar whilst turning the engine over.

The problem does sound possibly spark related, you need to check the actual spark voltage. There are cheap testers available for dong this.
Alternatively it could be low fuel pressure caused by drain back to the tank when standing. there is a one way valve in the pump that can fail. A replacement in line valve would be the answer, however this normally gives hot starting issues due to boiling fuel.

It could also be an air leak into the manifold causing weak mixture at closed throttle. check vac lines and spray plus gas or butane around manifold joints.

It should be pretty easy to sus as your injection is mechanical with only a few electrical mechanisms added for coldstart.

 

[Sp!ke]

I have been probing in that area today having heard that the OVP was an achilles heel. Fuse looked Ok, mebbe try Howards since he works on my doorstep

About time he and I met up for a drink....

How much are replacements anyway?

 

[Will]

How much are replacements anyway?

I have a spare you can have if it's suitable

Will

 

[Dieselman]

I have been probing in that area today having heard that the OVP was an achilles heel. Fuse looked Ok, mebbe try Howards since he works on my doorstep

About time he and I met up for a drink....

How much are replacements anyway?

Just short out the relay and see if it fixes it. The faulty side of the OVP is the relay control so shorting it will eliminate it.

 

[Howard]

Sp!ke i'll be at work tomorrow afternoon after 2 pm ....

But, i don't normally bring the car round , i leave it at dads in Teddington and walk over the footbridge.

If you want i'll bring the car round tomorrow to the lock ...

Let me know before 12 tomorrow .... :bannana:

 

[Sp!ke]

> Howard, dont worry, sounds like I can perform the same test by shorting it. Your work times and mine seemrather tricky, besides, starting up and warming the car before the test will defeat the purpose somewhat.

>Will, Assuming I need one, how much do you want for it?

>Dieselman, Any idea which pins I short circuit?

 

[Will]

Spike, you can have it if you need it.

PM sent

Will

 

[Howard]

I'm there till 10pm ...

If you want to come and get it after work and let the car cool off and try it (before bringing it back so i can go home ) .... you are more than welcome !

 

[Dieselman]

No, 'cause I haven't got one but it will be the thickest cables. You could check with a meter for which one goes live after it trips in.

 

[Will]

How did you get on with this in the end?

Will