W126 420SEL 1991 - Running very lumpily and cutting out at low revs

[Bunriots]

Hello all,

This is my first post on this great forum. I have a 420 SEL 1991 which we are in the process of recommissioning. It was laid up for over 10 years in a barn. It runs very lumpily, misfiring slightly, and very frequently cuts out at low revs. It starts first time every time, even after cutting out but after some time running it starts to cut out, dying slowly and not responding to the accelerator. It will then start first time and accelerate fine until the next time it gets to low revs. In heavy traffic where speeds are 20-30 MPH it will also try to cut out, which I counteract by slipping it into neutral, revving it a bit, and then putting it back in Drive at the slightly higher revs (with the resultant jerk as it reengages Drive). I'm sure this is not good for the gearbox. We have changed all the fuel pipes and one of the pumps around the fuel tank and the fuel filter as we thought this might be the issue, but this made no difference whatsoever. We have also changed all the plugs and HT leads - two of the plugs were a bit oily, and a third one had a white tip, possibly indicating a lean mixture. We changed the distributor cap (as it was cracked) and the rotor arm. We are at a loss now. What could it be? Is it still a fuel issue? Is it a timing issue? Is it the injectors? Is it a controller? Any suggestions would be very welcome.

Thanks!
Dermot

 

[00slk]

When my 450 SE starting running in a similar fashion, it was the fuel pump starting on its way to total failure. Okay when it was cold, when it started to warm up it would loose some power, misfire and eventually stop. Though it would start up straightaway again.
You have changed the pump and filter. Is it a genuine item or aftermarket one? There could be dirt in the system from inside the fuel tank, which has found its way through to the injection system?
Wouldn't rule out a vacuum leak or something simple like hot and cold sensors.
But I would be changing the crank position sensor first.

 

[ajlsl600]

Rotten vac pipe? Crank sensor not impossible but under outlined circumstances it wud surprise me.

 

[Bunriots]

Rotten vac pipe? Crank sensor not impossible but under outlined circumstances it wud surprise me.

When my 450 SE starting running in a similar fashion, it was the fuel pump starting on its way to total failure. Okay when it was cold, when it started to warm up it would loose some power, misfire and eventually stop. Though it would start up straightaway again.
You have changed the pump and filter. Is it a genuine item or aftermarket one? There could be dirt in the system from inside the fuel tank, which has found its way through to the injection system?
Wouldn't rule out a vacuum leak or something simple like hot and cold sensors.
But I would be changing the crank position sensor first.

Thanks @00slk. The pump is aftermarket. We will have a look at the filter in the tank as we missed this. Where should we look for a vacuum leak?

 

[Bunriots]

Rotten vac pipe? Crank sensor not impossible but under outlined circumstances it wud surprise me.

We will have a look at the vac pipe @ajls600. The ones under the rocker covers, right?

 

[ajlsl600]

Inlet manifold area, not inside r covers . (I doubt), around distributor. Around fuel system get a schematic if unsure .

 

[Bunriots]

Inlet manifold area, not inside r covers . (I doubt), around distributor. Around fuel system get a schematic if unsure .

Will do, thanks!

 

[horatio]

K-jet doesn’t like sitting around. I would have a watch through videos like Mercedes-source on YouTube to get a good understanding of how it works.

 

[mattkh]

Check to see if it has a fuel accumulator.

 

[Bunriots]

Check to see if it has a fuel accumulator.

It does

 

[Bunriots]

K-jet doesn’t like sitting around. I would have a watch through videos like Mercedes-source on YouTube to get a good understanding of how it works.

Thank you! These are great!