Small, but annoying problem...Lazy Cylinder

[Ulf]

Whenever my 230 has stopped for a few minutes - eg. to fill the tank - it will start up on 5 cylinders only and the sixth will only kick in after appr. half a minute - regardless of whether I drive right away or let it idle until it comes back in the game.
As my previous threads tell, the valves have been adjusted, the linkage has been set, new plugs, new timing chain etc...
Apart from this small problem, the car runs fine except for a little valve clatter (which is within my limits of being tolerable).

Any ideas?

Kind regards

Ulf

[Dave Gallon]

It sounds like the fuel in one of the pipes leading from the injection pump to the corresponding injector is leaking out. This happens when either the injector leaks (allows fuel to drip when the fuel pressure has not reached the opening pressure) or the check valve in the injection pumps allows the fuel to leak back into the pump. The intent is that each of the fuel lines is kept continuously full and under pressure. Thus, when a tiny amount of fuel is injected into the line at one end, the incompressible fluid forces the injector open at the other end and fuel is sprayed into the combustion chamber. As a first step, you may want to have the six fuel injectors tested. They can be tested on a Bosch Diesel injector test stand. The technician looks for a cone shaped spray pattern and no drips when the pressure is just shy of the opening pressure. Unless the injectors are relatively new, the chances are that you will find several injectors that act erratically. After you have a set of six known good injectors, you can drive the car a bit to determine if the problem is resolved or at least improved enough. If not, connect the engine to an ignition oscilloscope and start the engine after it has sat idle for a sufficiently long time to cause the symptom. It will be obvious on the display which cylinder or cylinders are not firing correctly and you will then know which injection pump check valve(s) to change. The injectors are Mercedes 000 078 11 23, suggested US List $244.00!! My price from Mercedes $179.65. AutohausAZ.com is listing them at the very favorable price of $96.22.

[Ulf]

Thanks Dave - have found a Bosch test center very close to my home - would you recommend taking all of the injectors out and bring them to them or to simply bring the car and take them out for testing one by one? Should I cover the holes with something to prevent dirt particles getting in?

Best regards

Ulf (who's Pagoda is pressed into daily service due to broken ignition lock in my BMW...).

[Dave Gallon]

If you feel comfortable removing the injectors then, by all means, remove them and take them to the test center yourself. Yes, be sure to close the resulting holes in the cylinder head and also keep the inlet holes on the injectors closed. I certainly suggest testing all of the injectors. You might as well get a good idea of their condition before proceeding. In the interest of diagnosis, mark the injectors so you know which cylinder each one corresponds to. It probably doesn't make any difference but you may find it interesting to do something like compare the condition of the spark plugs in cylinders with good injectors vs. bad injectors. Of course, this assumes you find a problem. Perhaps (hopefully!) all will be in good order. If that is the case you can skip directly to the check valves. I do hope these tests result in some beneficial results! (I might add that my 280SL suffered from this exact problem many years ago and the cause was a combination of bad injectors and check valves.)

[Ulf]

Thanks once again, Dave - I suppose that this issue could lead to more significant problems later on - running with fuel-contaminated oil, wear to cylinders due to dripping fuel washing away the protective oil layer etc...?

Ulf

[jacovdw]

Ulf, the short answer is yes if you have dribbling injectors...

[Ulf]

Dribbling injector - sounds like a sexually transmitted disease. But how come the car starts great on all cylinders, when cold?

Best regards

Ulf

[Allenh]

The consensus is your problem is fuel delivery related.  But you should rule out any funky type of ignition issue, just so your not, if you will, pissing in the wind.  A timing light can easily verify secondary ignition.

If you can Identify the cylinder by strating up with a plug wire off, you could swap a pair of injectors around and see if the problem remains the same or swaps to the other cylinder. 

Don;t forget that there are two components that trap the fuel and pressure in the line.  The coupling on the injector pump is a check valve as well.  A little leakage here would also cause your problem.

Enjoy an afternoon under the hood.

Allen

[jacovdw]

...But how come the car starts great on all cylinders, when cold?

Due to the extra fuel mist from the cold start valve.

[Dave Gallon]

Correct! The cold start valve provides enough fuel for all cylinders to fire until the fuel pressure in the delivery lines has been restored and then the normal fuel system takes over.