SL500 R129 poor engine running

[NMSL500]

Hi all, Advice on the following appreciated.
Just come into work in the SL and it seems to have developed a misfire which wasnt there on saturday when I drove it home in the pi**ing rain
Car starts and runs fine until you try to apply some throttle. The miss can be felt through the car at all revs over 1000 up to 4500
The rev limiter seems to have reduced, i.e. she now wont go over 4500 RPM and flutters as it would if you were holding the throttle to max under normal circumstances.
Any ideas?
Have pulled of the filter housing and am about to get the plugs out for a look but something tells me this is probably ecu related
Car is a 96 model
Thanks in advance.

 

[Doodle]

The symptoms sound similar to the intermittent problem on our 95 SL, but it would also make the tickover lumpy...and sadly the fix for ours wasn't cheap.

Problem was traced to a failed AFM, which had also damaged the throttle body when it failed...about £900 worth of parts

It could always be water in the electronics, but the SL's bits are fairly well sealed from the elements.

 

[Tan]

Check the mass airflow sensor (MAS)

 

[stats007]

The fact that the rev limit has lowered would indicate that it's an ECU issue as opposed to the distributors which cause problems. Sounds like it will be the air sensor or lambda sensors - though you should have a check engine light up for this.

Edit:

I think on the '96 model you have one of these?

Engine 119.982

 

[NMSL500]

Thanks for all your input guys

Have just removed all the coils (No dizzy on this 96 motor) and low and behold 3 of the spark plug tubes are full of water!! How the hell it has got in there god only knows. The engine has never had the pressure washer treatment and never will while in my charge.
The rain on saturday was heavy and I did end up "fording" a couple deep puddles on the drive through my village but I cant for the life of me see how it got that high up the engine.
Waiting for a set of plugs to arrive and will blow out the water and install and see what happens
Will let you know.........

 

[Doodle]

Yup, that'd do it!

How the hell it made it past the covers, and then the seals on the plug caps god only knows, that's quite some going!

 

[stats007]

New water injection system?

'96 was the cross over year for distributors and coil packs I take it?

 

[Vlad]

The 500's are also very prone to getting damp under the distributor caps....its a well known problem, so you may want to clean them out if the new plugs do not cure the mis-fire.

Once you have removed the clip on plastic cover, which covers each cap, it just a case of undoing three bolts....two on either side and one right underneath.

Good luck.

ps; is the undertray still on your car....I'm just trying to figure out how the water got into the spark plug chambers

 

[stats007]

He doesn't have distributor caps on this model as he mentioned, but you're right in that they're problematic.

 

[Vlad]

Ooops I just assumed that it was the same M119 5.0 V8 as fitted to my 1993 W124 500E, which whilst it was distributor-less, still had caps on each bank...is this not the case on this 1996 model?

 

[stats007]

Yours should still have had distributors - I presume they were removed retrospectively?

 

[NMSL500]

Thanks for all your help on this one guys
the water in the tubes was definately the problem and the old girl has now returned to her fomer powerful glory.
Cheap fix in the end but my wallet was shaking with terror for a while!!