1987 420 SEL (M116.9), approx. 200K miles... measured ignition timing at idle (approx. 1200 rpm) is 30 BTDC, with vacuum; factory sticker indicates 12 BTDC at 750 RPM is correct. Static crank, cam and distributor timing are correct and synchronized; timing chain renewal was performed approx. 40K miles ago. Owner states vehicle was being driven at high speeds for a period of time when engine stumble and rapid fluctuation in RPM was noted. Subsequently, vehicle performance has been off a bit and idle speed is now elevated (1200 rpm).

The engine starts readily and revs freely but is rough at idle and stumbles at low speeds. Spark plugs, ignition wires, distributor cap and rotor are new. Vacuum hoses, lines and connectors are intact.

Since the ignition timing is determined by the EZL, in theory it is not possible to set the timing by turning the distributor. Yet, rotating the distributor counterclockwise results in a substantially rougher idle, lower idle speed and reduction in measured timing; things smooth out... and idle speed increases... when the distributor is returned to approx. 30 BTDC.

Any idea as to where the fault lies? EZL control unit?

Many times with the transmission out & replacing the rear crankshaft main oil seal the flywheel ring gear gets re-installed incorrectly. That will either advance the timing 25deg OR retard it that much. Has the trans been out & the rear main seal replaced??

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Some other thoughts. The EZL only picks out one of the stored ignition advance maps on that model. You still idle speed timing with the dist. From your description, I would also search for a major vacuum leak if crank position sensor isn't the problem. I'd hook up a vacuum gauge and see what's going on there. Some shops are finding splits and cracks in the big rubber boot between the plenum and fuel distributor assy. I haven't experienced iit yet. You might try carb cleaner or a smoke mavhine.

Good luck,
Peter

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No prior hx of transmission removal.

Ran diagnostics of ignition-related components: EZL coolant temp and crankshaft position sensors out of spec; no other gross faults noted. Rubber boot for fuel dist. assembly fatigued but no cracks present. Rubber seals joining upper and lower intake manifolds heat-fatigued, brittle and possibly leaking.

Installed EZL control unit of known quality, with no improvement.

Will advise further following renewal of sensors and reinstallation of intake manifold.

update - closure ?
Do you recall what fixed this issue?

.

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