S-Class (W126) 1979-1991: 300 SE, 300 SEL, 380 SE, 380 SEL, 420 SEL, 500 SEL, 560 SEL, 360 SEC, 500 SEC, 580 SEC, 300 SD TURBODIESEL, 300 SDL TURBO, 350 SD TURBO, 350 SDL TURBO

1986 420 running rich, rough idle, carbon fouled cross post from Benzworld

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Old Jan 27, 2021 | 09:37 AM
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420 sel 1986
Angry 1986 420 running rich, rough idle, carbon fouled cross post from Benzworld

08/27/2020

Pastor brings 1986 420SEL to me in running order, smooth idle, great power.

- New plugs ( incorrect Bosch Resistors)

- Valve Cover Gasket replaced

No more valve cover leaks, but after a few miles the engine shakes violently at idle, no loss of power or hard start yet….

09/15/2020

- Pastor brings 420 to back, rough idle, engine shakes, neighbor installed cheap rotor and distributor cap

- I replace spark plugs again (unknowing that the 420 has resistor wires from factory and needs resistor less plugs) rough idle consists, no change

- Oil Light and check engine light on…. Oil level and pressure is great, I assume faulty sensor but negligible considering performance and no leak, still running strong both in city and highway.

10/15/2020

- 420 becomes increasingly worse following one can of BGK44, Rough idle is worse, loss of power, hard start now, Engine finally stops running Will start but dies within seconds, very very carbon fouled on all plugs, strong smell of fuel from exhaust, running rich!!

- I get the 420 back, this time we do full system back, both pumps, filter, accumulator

- This time we do the correct BP5ES Resistor less plugs, verify good spark, still no start

12/30/2020

- Begin diagnosing fuel leak, pressure is good to the Distributor, EHA valve leaking, lots of fuel in the AFM housing and all my brand new spark plugs are Carbon Fouled, likely from cranking for so long?

- Distributor rebuilt, internals were toast…. New EHA valve, injector seals and housing, injectors soaked and ultrasonically cleaned.

- 420 starts again, rough idle and have to floor it to keep it alive but it runs, once warmed up it will drive around the block but still smells extremely rich and low power under acceleration, once warmed up, excellent power and take off.

- Clean and check function of Coldstart valve, works well, suddenly temp gauge doesn’t work? No broken wires, connections are good… great more problems

Current date

- Vacuum check last night, my economy gauge pins left during operation, but why the hell not, identify big leak on AFM housing, but I don’t have high idle and I’m running rich? No adjustments to A/F on my behalf but tamper ball is missing.

- *Rotor arm has left the building…* cheap Italian part looks like, ordered MB OEM arm and Ignition coil ($20 why the hell not)





My next steps…..

- Replace Distributor rotor

- Replace ignition coil

- Get my good spark back again

- Clogged cat or O2?

- Get it running rough again, verify Duty Cycle and make adjustments to A/F ratio with new and rebuilt parts?

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