I recently bought a '89 300se 175k mile with a perfect Black Pearl body (start with the good) near no service records and a one breath away from failure timing chain/valve train noise. It was running and driving when I bought it, noise from engine not withstanding the only issue was a hard start. Not to mention the "normal" age related issues I expected. bought the car for the right price and started my rebuild.

One of the two service records which came with the car indicated low compression so I decided to replace the engine with a JY 107k mile unit from a W124 (later '89 M103 engine with the improved hardened camshaft). Replaced all seals and gaskets prior to installing motor (Head, Sump, Main, Cam, etc) and timing chain and all guides. Used old engine CIS unit with new brass Bosch Fuel Injectors, replaced all rubber CIS parts and CS hoses. Rebuild Power Steering Pump, replaced Transmission TC Seal and Dipstick Tube Seal, motor mounts, trans mount, complete tune up (Rotor, Cap, Wires, Bosch copper plugs) transmission cooler lines, belts hoses, water pump, drive belt tensioner, air filter. Exhaust center and rear mufflers replaced and one cracked weld on a pre cat was re welded.

I addressed the obvious suspension needs- HD Bilstein shocks front and rear, Front Guide Rod bushings, rear sway bar links, idler arm bushings, center link and outer tie rod assemblies. Lower Ball joint boots are starting to degrade but have not yet failed so I will leave that for full front end rebuild at a later date.

Replaced the fuel pump with a Genuine MB pump and new filter (preventative as old pump was still working).

Interior- Dash was cracked so I replaced that with a good JY dash pad, replaced LF PW motor, LR door lock actuator, LF seat spring, and removed aftermarket radio and replaced with a Becker 1492 head, amplifiers, and complete wire harness from a 1994 e420 MB Audio (complete interior removal almost), replaced Power antenna unit with factory Hirschman.

Every thing is back together and I am having a start but no run issue-

1. Turn Ignition key I can hear fuel pump run 1-2 seconds
2. Car will start and run for 2-4 seconds, starves for fuel and dies
3. Tach does not work now (did work but I removed cluster to repaint needles not sure if this is related or not)

I have replaced the OVP Relay, symptoms continue and dissection of old relay revealed no obvious flaws (cracked solder joints), but was original relay and needed replaced anyway. Tried starting without OVP same conditions but ABS lamp lit up.

All vacuum connections are in good order.

Swapped CPS with no change in run conditions.

All engine sensors were swapped from my car onto new engine.

Checked all electrical connections to various sensors.

Fuses all replaced with brass/copper, ice cube relays are all original except for headlight wiper being replaced to updated "green" one.

Jumped FPR and ran engine at normal operating temp for 30 minutes to seal new head gasket. Engine ran great but maintained a high idle (guessing 1200-1500 RPM range). No CEL were lit.

Swapped Ignition EXL with no change in conditions.

Swapped FPR with no change in condition (was a used JY Relay however)

I am not sure the tachometer issue is related or not (clock still works so it is not a gauge grounding issue)

It goes into start mode but not into run mode any ideas on likely causation here?