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Marty
Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 53168
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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HEY MARTY, THIS IS JIM I HAVE A 420 SEL, REPLACED PLUGS WIRES,

Customer Question

HEY MARTY, THIS IS JIM I HAVE A 420 SEL, REPLACED PLUGS WIRES, CAP ROTOR, SPARK PLUGS , FUEL FILTER, I HAVE WHAT SEEMS TO BE A DEAD CYLINDER ON NUMBER 5, ALL CYLINDERS ARE AT 150 ON COMPRESSION, I HAVE SWAPPED LINES ON FUEL DISTRIBUTOR TO SEE IF THE ISSUE TRAVELED AND IT DID NOT, WERE CAN I GO FROM HERE, I ALSO REPLACED THE INJECTOR ON NUMBER 5, JUST SEEMS AS IF CAR IS RUNNING VERY LITTLE FIEL IS COMING OUT OF THE INJECTORS, CAN IT BE IGNITION MODULE?
Submitted: 1 year ago.
Category: Mercedes
Expert:  Marty replied 1 year ago.
Hi Jim, No this cannot be an ignition module problem. It sounds like you might have a bad cam lobe on #5 cylinder or possibly an intake leak at the manifold. When you pull the plug wire on #5 with the engine running does it have little or no affect on the engine running? When you swapped the fuel lines at the fuel distributor you only swapped them at the distributor and not at the motor, is that correct?
Customer: replied 1 year ago.
THIS IS A 87 420 SEL, CAR HAS ALSO SAT SINCE 2012, CORRECT NO CHANGE WHEN RUNNING WHEN I PULL THE SPARK PLUG WIRE OFF NUMBER 5, WE SWITCHED THE LINES AT THE FUEL DISTRIBUTOR.
Customer: replied 1 year ago.
ALSO HOW CAN I CHECK THE FUEL PRESSURE? IS IT POSSIBLE I HAVE LOW FUEL PRESSURE ISSUE/
Customer: replied 1 year ago.
ALSO IF CAM LOBE WAS WORN, WOULD I STILL HAVE 150 PSI COMPRESSION ON NUMBER 5?
Expert:  Marty replied 1 year ago.
By switching the lines at the fuel distributor end only you have ruled out a bad fuel distributor. By switching the injector you have ruled out a bad injector. The only things left are a vacuum leak at the manifold or a worn cam or a bad spark plug, plug wire or distributor cap. I'm sure you've ruled out the ignition already. If you have, I'd spray around the intake at #5 cylinder with some carb spray to check for a vacuum leak. Next I'd pull the valve cover and inspect the cam and the rockers. Yes you can still have compression with a bad cam although I'd expect it to be lower.
Let me know what you find out.
Marty
Customer: replied 1 year ago.
Ok Marty i will he did say number 1 cylinder had 125 psi comp, but when he removed the plug wire the car did change rpms but number 5 cyl didnt, ok i will pull it tommorrow and see, can i visually tell by looking at the lobe to tell if its bad? Or what would be a tell tale to address.
Expert:  Marty replied 1 year ago.
Are you sure the firing order is correct? You can look at the tips of the lobes and compare them to each other. They all should be nice and rounded at the top (no square edges).
Customer: replied 1 year ago.
Hi Marty still going around with this 420sel, i put a another fuel distributor on the car and the problem traveled from no fuel coming out of the distributor on cyl 5 , now its cyl 7??? i spray fuel into the throttle body and it does idle up but only for a few seconds, wierd thing i lossen the "" plug"" on the base of the fuel distributor and gas leaks out but the car smoothes out and runs good but fuel is leaking when i tighten it back up , it starts to miss again.
Customer: replied 1 year ago.
Can it possible be the two fuel pumps are losing pressure? in drive it will miss then smooth out only for 2 to 3 seconds and then fall apart again, this car sat since 2012 and the fuel coming out of the gas tank is almost dark brown, i drained the tank , new filter and fuel but no change.
Customer: replied 1 year ago.
the car does have a dead miss up until 1800 rpms in park
Expert:  Marty replied 1 year ago.
Hi Jim, this really sounds like you are getting bad fuel distributors. I've gone round and round with use fuel distributors in the past. I've had to buy three or four used fuel distributors before I finally got a good one. If your miss is moving when you change the fuel distributors I think you are just getting bad fuel distributors. It won't be a fuel pump problem if it's only affecting one cylinder.