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87 420SEL, starts and only runs a few seconds

18725 Views 41 Replies 9 Participants Last post by  waybomb
87 420SEL, starts and only runs a few seconds - Still need help

Have a 1987 420SEL. 140,000 odometer.

Start the car, rpms come up slow, idles fine, then races and shuts off. If restarted immediately, the process becomes shorter. If a wait of a few minutes, the process may take 45 seconds.

3/4 tank fuel, replaced the fuel filter today, still has same problem. The pumps are running, air filter ok, no wires seem to be off anywhere.

Any ideas on what to check?

Thanks in advance.
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Ignition coil?
Bad ground strap?
Crank position sensor? (green wire going to the Ignition module (EZL) on drivers side fender well under hood.
Here are the fuel pressure specifications for the 420 engine:

System Pressures:

Cold or Warm 6.2 - 6.4 bar (90 - 93 psi.)

Residual Pressure, after 30 Min. 2.8 bar (40.5 psi.)

Lower Chamber Pressures:
Presure specifications given are in bar (psi) below previously measured system pressure.

Normal Operating Temp. 0.4 bar (5.8 psi.)

Idling 68°F (20°C) 0.5 bar (7.3 psi.)

Enrichment Pressures:
Measured at lower chamber port.

Acceleration Enrichment 20°C (68°F) min. 3.8 bar (55 psi.)

Deceleration Pressure:
Measured at lower chamber port.

Decel Pressure Must equal system pressure
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The racing idle just before it stalls could be unmetered air getting into the intake system. Check the following air plenums on top of the engine for leaks or cracks. Especially check the idle control valve hose connections as well as the injector plenums.
This whole system runs off of pressure differential. Very little electrical controls.


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