SL500 119 engine misfire

[Sp!ke]

I've just been getting my hands dirty at work giving the once-over of a colleagues 119 engined SL500 that seems to be suffering from a misfire.

He's been away for a couple of weeks, meanwhile the car has been sat outside and his drive in to work this morning was 'somewhat tricky' according to him.

My initial thoughts were distributor caps/leads and damp. But a quick poke around and the leads all look correctly seated and pretty dry and clean (I didnt actually take the distributor caps off as I dont have the appropriate sized allen key with me)

Leads, caps and rotor arms were replaced 6 moths ago to cure similar symptoms so I was leaning away from it being these. However I did notice that on both distributor caps where the screw holds them onto the engine it looks like they may have been overtightened as there is a little crack near the upper screw holes on both. (not sure if this is by design but it looks like a crack which could allow moisture in)

I can also hear whistling which made me think of a vacuum leak but even with the air filter housing removed I cant pinpoint the source of the whistle.

So, two questions....

Do these Beru distributor caps have a slit in them by design near the upper screw threads or have these indeed cracked, possibly through being overtightened?

Any suggestions on which potentially whistling vacuum pipe is most likely to cause this lumpy running/misfire?

 

[BlackC55]

The first thing I would do is remove the caps. They are the usual culprit.

 

[jaymanek]

I think you will find that the cracks are not actually cracks...

I would check and change the plugs, I think the M119 should not be using resisitor plugs and most places will seel you resistor plugs, only MB keep the correct ones. (Look for plus with no R in the model number) e.g. WR7DC and W7DC.

Then change the rotors, caps and maybe even leads.

Its always a good start.

Of course there is always the possibilty of wiring loom issues depending on its age.

 

[Vlad]

Spike I had exactly the same problem on a W124 500E...turned out to be the caps.

If my memory serves me correctly earlier 119 engines can have modified caps fitted with better sealing or something along those lines.....i seem to recall my Indi mentioning something about this.

 

[Sp!ke]

Plugs, leads, distributer caps and rotors were all changed 6 months ago or less for original Mercedes items so other than a bit of damp maybe, it would be unlikley that they'd need changing so soon wouldnt you think?

 

[BlackC55]

Nope.

Not using the car is it's worst enemy. Moisture builds up in the caps and corrodes the rotor and the brass "ends" inside the cap.

It goes all green a furry!

 

[jaymanek]

If its damp thats getting in, it will be this time of year that the problems will start.

You will start to see all the "my 300 SE/SE/SL/E is spluttering" threads appearing soon as they also had this issue.

 

[ray_hennig]

Clean 'em up ...

Plugs, leads, distributer caps and rotors were all changed 6 months ago or less for original Mercedes items so other than a bit of damp maybe, it would be unlikley that they'd need changing so soon wouldnt you think?

I had issues with the original 17-18 year old cap and rotor arm so I simply emery-papered any corrosion and the misfire disappeared.

At the same time I installed new non-resistor plugs (Bosch, Beru, NGK) after MB supplied me with an exotic and expensive resistor plug.

I also recall cracks around the screw holes - these cracks were in a heavy plastic shield over the cap, not in the cap's base material.

Incidentally, a recent warm misfire was cured by a new fuel filter and mixture re-adjustment.

Try all those things first as they are cheap and let us know.

All the best from the deepest, dampest Dordogne.

RayH

PS: The 300CE lives outside ALL the time, heat, cold, damp. With the electrics in good nick, it ALWAYS starts first time.