w126 - misfire after start up

[john rh]

Hi,

My 500 misfires for about 2-3 seconds after start up. It always does it, hot or cold, unless the car has only been switched off for 30 minutes or less, in whcih case it starts fine on all 8.

I am almost positive that the miss is fuel related because it also happens whilst driving along when I switch onto petrol after running on LPG for some time.

Are they meant to do this... is it a "feature" of the engine? (The owners handbook does say something about not engaing Drive until the engine is running smoothly).

If not, any ideas as to the cause? The fuel filter looks old and I'll replace that when I can.

Thanks

John

 

[john rh]

Does anybody else's w126 do this?

 

[ianrandom]

I think mine does a bit, just on start up. I just ignore it, doesn't last long. I had put it down to the cold start injector giving it extra fuel. I wonder though, does the cold start injector have a temperature sensor?

 

[mhcrichton]

Hi John. I came across this post just now. I have exactly the same problem in my 1991 R129 500SL. Like yours, my car is fine if it's just been driven but otherwise it's rough for 2 seconds upon startup.

I've spent a considerable amount of money trying to get rid of this annoying problem but to no avail. Did you ever discover what the problem was, and if you fixed it?

Any information would be gratefully received.

Regards, Michael

 

[mhcrichton]

Or Ian, did you do anything about this same issue you had?

Cheers

Michael

 

[stwat]

MB recommend replacing the injectors every 70.000 miles on the KE-Jet fuel injection system.

 

[grober]

There is a thermo time switch associated with the cold start injector. Its function is to regulate the period of time the cold start injector operates in relation to the engine coolant temperature. If it or the cold start injector were faulty this might give rise to your symptoms. basic circuit here

 

[mhcrichton]

Interesting, I might investigate as suggested. Thanks... !

 

[mhcrichton]

Thinking about this, it doesn't have any trouble starting up. It's just that when it does, it sounds horrible and shuddery and like it's running off 4 cylinders for about three seconds. If the cold start valve or thermo switch was faulty, wouldnt the car be difficult to start up, but when it does it should run smooth from the beginning? It feels like a distributor issue but I've just had all that replaced.

 

[grober]

Thinking about this, it doesn't have any trouble starting up. It's just that when it does, it sounds horrible and shuddery and like it's running off 4 cylinders for about three seconds. If the cold start valve or thermo switch was faulty, wouldnt the car be difficult to start up, but when it does it should run smooth from the beginning? It feels like a distributor issue but I've just had all that replaced.

It would have to be a fault which supplied too much fuel rather than not enough. This might be due to the start injector running for longer than intended or when the engine would be too warm. When the engine is cold the petrol/air mix condenses on the walls of the inlet starving the engine of fuel. The function of the start valve is to wet the walls/surfaces to allow the mixture to reach the cylinder. It a pretty extreme degree of enrichment and only functions for a short time e.g. 7.5 seconds at -20C! So you can see any extension of that period might lead to problems.

 

[stwat]

I'd go for it being worn leaky injectors weeping slightly.

 

[mhcrichton]

stwat - yeah one mechanic suggested it could be dripping injectors. I'll investigate the injectors and the cold start valve/thermo switch.

 

[mejermy]

Have you checked the plug end of the ignition leads? Mine is run only a few times now between summers, when it comes out to do 1000 miles or so per year, and I found when running it up before Christmas that my leads had 'verdigris' on them. This seemed to inhibit good conduction when cold. Once scraped off to bright metal the problem went away. I had looked before this at all the usual suspects - dizzy cap, plugs themselves, leads sparking, cold start valve etc.

 

[mhcrichton]

mejermy - I have just replaced the ignition leads now as well, unfortunately it didn't help.

 

[mhcrichton]

Injectors out for testing and if necessary replacement.

 

[roastbeef]

Sounds like leaking injectors to me as well got 2.8 Capri does the same misfires after a drive and been left for over half hour you can unplug the cold start injector just to rule it out car will still start fine with out it connected unplug electric plug leave fuel pipe connected or before you start car just turn ignition on 3 to 4 times with out cranking engine this should prime fuel system up to re pressure the fuel system may help

 

[mhcrichton]

Hi All - thanks for your input, replaced injectors and fixed the problem straightaway. Awesome!