Jeep YJ Wrangler 2.5L MPI Engine Will Not Start or Deliver Fuel When Cold

[ratoja]

My Jeep YJ Wrangler 2.5L (MPI not TBI) engine has no fuel pressure when it cold.  Installed a new fuel pump and pressure regulator, swapped relays.  When the Jeep is parked inside it runs and has fuel pressure.  When we put the gauge on to check fuel pressure at the rail, inside we get around 39 psi.  When we let the Jeep sit outside overnight and check pressure we have none.  Any ideas?  Thanks.

[Moses Ludel]

ratoja...Welcome to the forums!  The 1991-95 YJ Wrangler 2.5L MPI fuel injection system is much like the 4.0L Chrysler MPI design.  Let's walk through possible issues...

The drop in fuel pressure overnight is not uncommon.  Bleeding off at the fuel pump or regulator on the rail is not unusual.  I always "prime" our MPI 4.0L before cranking by turning on the key for a moment before attempting to start the engine.  I listen for the fuel pump running, knowing that this charges the rail.  A new pump and regulator should reduce the likelihood of bleed-off, though it can still occur.  The new pump came with a fresh sock filter, right?

So, when the vehicle is parked outside and cold, if you turn on the ignition key to just the ON position (no cranking), do you hear the fuel pump run?  This check may require a helper to stand near the fuel tank and listen for the pump, though usually you can hear it from the driver's seat.  If you do not hear the pump running, there is obviously no current to the pump.

To save a good deal of time and guessing, here is factory coverage of the Auto Shut Down function and how the fuel pump is activated.  Note that relationship between the ASD and the fuel pump and also the fuel pump and the ignition switch.  Read the ASD RELAY—PCM OUTPUT and also the FUEL PUMP RELAY—PCM OUTPUT details carefully:

Jeep MPI ASD Relay and Fuel Pump Function.pdf

Since your problem occurs when parked cold outside, I would target a wiring issue or plug connector (open circuit) .  If the pump is not running, turn off the key and remove the fuel pump relay.  Check the available fuel pump relay current with the key OFF and ON.  If the key ON shows no current with the vehicle cold, check for continuity and ohms resistance from the relay hot wire to the ignition switch.  In all checks, turn the key on only momentarily when you test for voltage.  Turn the key OFF and give it a rest between checks.  If no current, perform a voltage check at the ASD relay.

If you do have current at the fuel pump relay's hot socket terminal when you first turn the key ON, then check for continuity and ohms-resistance between the relay terminal socket and the fuel pump's hot lead terminal at the fuel pump module.  (This will take a length of wire.)

Note that both the ASD and fuel pump relay rely upon the PCM to complete the ground.  Check the PCM plug connector for corrosion and oxidation.  Check for ground continuity and resistance from the PCM to chassis.  Check continuity and resistance between the PCM plug's fuel pump relay contact to the fuel pump relay grounding socket at the Power Distribution Center (the PDC or relays box).  

The Jeep YJ Wrangler 2.5L models have a history of poor body-to-engine grounds.  Also important is the grounding to the chassis at the fuel pump module.  Cold temperatures cause contraction and can cause opens or excessive wiring resistance.  There could be an open in a hot lead or a ground—or a defective ignition switch.  Grounds and hot leads are equally important on a 12VDC system.  

If you live in a climate with corrosive roads or humidity, there could be weak connections, resistance at plug junctions or poor grounds.  Check the body to engine grounding on your Jeep YJ Wrangler, a common trouble spot.

Let's work through this...Share your findings.

Moses