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

[Bunriots]

Hello all,

This is my first post on this great site. 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, the distributor cap (as it was cracked) and rotor arm. We are at a loss now. What could it be? Is it a timing issue? Is it the injectors? Is it a controller? Any suggestions would be very welcome.

Thanks!
Dermot

 

[Bellow]

changed all the plugs

What did the old ones look like? Black/sooted?

 

[alabbasi]

1) Did you drain the tank and was there good flow? Tanks can rust and there's a mesh strainer at the bottom which needs to be removed and cleaned
2) Did you smoke test for vacuum leaks? There are two large vacuum hoses on the inside side of the cam covers that can crack and leak, as well as many other vacuum lines
3) You might have a damaged fuel distributor, they can gum up and so can the injectors. The central membrane can also break which can cause this type of running issue.

 

[Stratman]

Those distributors are known for moisture ingress. It comes up from the sump past the seals and into the cap. Having said that, from your description it does sound like a fuel flow and/or quality issue.

 

[Bunriots]

Thank you guys. We will investigate and report back.

 

[Bunriots]

What did the old ones look like? Black/sooted?

Two of them were oily and a third one had a white tip.

 

[Bunriots]

1) Did you drain the tank and was there good flow? Tanks can rust and there's a mesh strainer at the bottom which needs to be removed and cleaned
2) Did you smoke test for vacuum leaks? There are two large vacuum hoses on the inside side of the cam covers that can crack and leak, as well as many other vacuum lines
3) You might have a damaged fuel distributor, they can gum up and so can the injectors. The central membrane can also break which can cause this type of running issue.

Thanks @alabbasi. All great suggestions. We will check these out. Re. #2 and #3, do you or does anyone here have diagrams of the vacuum and fuel distribution systems to share?

 

[alabbasi]

Thanks @alabbasi. All great suggestions. We will check these out. Re. #2 and #3, do you or does anyone here have diagrams of the vacuum and fuel distribution systems to share?

Fuel should be pretty straight forward as there's one line into the fuel distributor and one line for the return. The rest are to the injectors and the EHA controls the fuel trim so to speak.

Vacuum is a different story but before you get into it, smoke test it to look for leaks. A rubber glove wrapped over the air flow meter should keep the smoke from escaping, Often the intake gaskets start to fail and create running problems. IIRC correctly, the bottom of the air flow plate is also made out of plastic and can crack.

 

[Bunriots]

Fuel should be pretty straight forward as there's one line into the fuel distributor and one line for the return. The rest are to the injectors and the EHA controls the fuel trim so to speak.

Vacuum is a different story but before you get into it, smoke test it to look for leaks. A rubber glove wrapped over the air flow meter should keep the smoke from escaping, Often the intake gaskets start to fail and create running problems. IIRC correctly, the bottom of the air flow plate is also made out of plastic and can crack.

Thanks again. I'm on the case!