500SEL misfirres intermittently and lacks power

[spectre]

I've had 2 500se's and 2 300se's in the 126 series, but I've never had one which has been such a PIG! I've only had this one 3 weeks and knew it needed a service when I bought it, but I need some advice?

On startup, the 500SEL misfires for about 3 seconds then comes alive. For the first 30 seconds (or so) of driving its the way a 500 should be; full of grunt and power. But then the gearchanges become weak and the car lacks power for the majority of time (nothing there when you floor the pedal and no noticeable 'kickdown'). Every now and then, there's a bit of power when flooring it, but nothing like it should be. sometimes while running it misfires, (not really noticable when driving but very obvious on idle).

What I've already done to try to fix it:
Replaced with new: Distributor cap, arm, HT leads, plugs and air-filter
New waterpump and thermostat.
Replaced the kickdown relay with a used one .
Checked the gearbox fluid; was low so i topped it up with about 1/2 litre.
Fixed a blowing downpipe.

Nothing has solved the problem! ???

I have not changed the fuel filter but the car revs up quite happily when its not misfiring (in neutral). Nor have I changed the gearbox filter or the ignition coil.

If you've had a similar problem then you know just how frustrating it is, so please help.

I know the gearbox was recently replaced by the previous owner because the dredded reverse packed up, but the garage did a poor job when they fitted a used-replacement box and have managed to break an engine mount-rubber, they didn't connect the kickdown cable either. The gearchange improved a bit when I connected the cable to the throtle linkage but it's still not right. Could they have left a vacum pipe off or missed something else? Could this be a fuel distributor problem? Could this be the ignition module mounted on the front of the inner wing just behind the headlight? How do I check? Any tips would REALLY be appreciated.

Dave

 

[spectre]

Update;
Yesterday, while driving home, I had NO BRAKES when i hit the pedal. It went all the way to the floor with very little braking effect! No ABS warning light.

I thought this was the mastercylinder, but after I pumped it it seemed ok.. air in the system? NO!

Anyway, when i next started the car (this morning) the brakes were fine. (only tested it on the drive) THEN BAM! just like that, it all fell into place when I checked the ABS.
The car skidded; NO ABS! but no light>> I've only had the car a few weeks, so I never checked the ABS light on startup. It wasn't working!

WHATS THIS GOT TO DO WITH THE INTERMITTENT MISFIRE??? EVERYTHING! For some reason this was causing the problem the whole time! I took out the relays from the ABS unit, started the car and for the first time since I had it, it was purrrrfect!
Drove it around the block, confident that I wouldn't have the floppy brake-pedal that I'd experienced the day before, and DAMN it felt good! No wonder This thread had no replies, It wasn't the clearest problem to diagnose. Now all I have to do is change the ABS warning light on the dash and try replacing the relays I took out to see if that solves the ABS fault. Failing that, its a trip to the main-stealer to get it on the diagnostic

 

[markmerc]

Good luck mate I wish you luck.These old mercs can be a right pain.