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240 83 240 random stalling and near-stalling


Feb 5, 2012
83 240, B23F, M46, stalls randomly. Makes no difference whether it's hot or cold, idling or driving at any speed. Sometimes it stalls instantly, other times it stumbles and continues running. I've noticed that when it dies while driving, and I keep it in gear, the tach continues displaying the engine speed until the engine braking slows the car completely - so it seems the coil continues to fire after the engine dies. The car never fails to start right up again, and otherwise runs great. I can hear either the main or fuel pump relay clicking off when it stalls, and off/on again when it stumbles. Jumpering the relays doesn't make a difference. Haven't rigged up a test light to see which one - in the case of the main relay, I believe it's engaged by the ECU grounding it, making that diagnosis more difficult.

I don't know if anyone can help me with this over the internet, it's painstaking just to read. Here's where I'm at:

Engine rebuilt 20k miles ago
Both fuel pumps and filter replaced in the last 20k miles, wiring checks out, pumps have 12.9-13.1v at hot idle
Seized IAC valve removed, holes capped
New coolant temp sensor 25k miles ago - engine stumbled on startup after plugging it back in (of course it didn't run with it unplugged)
All corroded engine wiring replaced piece by piece, nearly all connections soldered and heat-shrunk, ten-pin gray connector eliminated, most sensor and ECU grounds consolidated and soldered to one large ring terminal bolted to firewall
Replacement coil negative wire is not coaxial, but I doubt those EMF's would affect the LH ECU
From what I remember, the issue is still present with MAF disconnected, in limp-home mode - MAF and connector cleaned, contacts schmoozed for tighter fit
Chrysler ignition unit has clean, tight, greased sleeves on all pins
New cap, rotor, plugs, iPd wires, distributor hall sensor, hall sensor plug pins soldered to ignition unit wires using uninsulated butt connectors
Coil tests fine, new coil didn't help
Hot-wiring the car from ignition switch plug to fuse panel, no change, so ignition switch contacts are ok
Replacing, swapping and bypassing the main and fuel pump relays - no change
New 25 amp fuse and holder, connections at junction box good and tight
Some new fuses, fuse panel connections decently clean, all have been spun around a few times
55 amp Bosch alternator has new slip rings and regulator brushes, new braided ground cable, new cable to starter, charges fine
New injector and coolant temp sensor plugs
Unplugging the O2 sensor made it idle a little better (richer), still stalls
Wiggling any and all connections/wires throughout the car won't cause it to stall
RPM signal goes to/from 4 things - coil, tach, ignition box, LH ECU. Rerouted the four wires and soldered them all together under the dash, eliminating a few connectors.
Car has a new cat and passes smog with flying colors

Time for a new Dave Barton harness, new MAF, other new sensors? It seems the LH ECU misses a power/ground/signal and shuts down. The fact that it still happens in limp-home mode, and coil keeps firing, seems to narrow it down. More testing to do, maybe I'll run through the full gamut of LH system tests in the Bentley manual. That 5 year old coolant sensor is also suspicious.