1993 Jeep Grand Cherokee 140K miles. I have a repeating dead fuel pressure problem. Sometimes at start I get fuel pressure for starting but it doesn’t last. Other times none at all. Other times, it runs and then fails while driving.
Mechanical work: Replaced the fuel pump (3 times), fuel valve replaced, checked the gas tank/filter for crud (none/clean), replaced engine temp control sensor & thermostat. Been in and out of a good 4WD shop, last time they disassembled the entire system from front to rear could not find a problem. Reassembled and it started. Ran for 2-3 weeks then died again. Towed it home. Tried it 3 weeks ago, it started so I moved it outside to the driveway. Now dead again, no pressure on spray bar. Has also died twice while driving on highway (so not only a starting issue).
Since it does start from time to time, does this mean the crank position sensor is okay? Or since it has failed both while starting and while running could it be the cam position sensor? My model does not have a security system, but it does have an autoshutdown relay. Should I just replace that relay and the fuel pump relay in case they are having intermittant failure modes? Most expensive “not-replaced” piece is the ECM – about $200 for a rebuilt one if I can find one.
HELP I’m stumped and so is my mechanic!!
Have you actually installed a fuel pressure gauge to see what the fuel pressure is? Or are you just looking at the injectors to see if they spray?
When it is dead there is no (zero) fuel pressure. Sometimes during the start cycle there is normal fuel pressure across the spray bar but it dies very soon after start or during the start cycle. My mechanic said that they did not do a leakdown check since when it was dead they could get no pressure at all. It seems to be an “normal or nothing” type of issue.
The fuel pump relay may be causing the trouble. You should be able to prove by checking the voltage at the pump while the trouble is occurring. If you don’t have more than 12 volts at the pump then you need to find what is causing the trouble. The fuel pump relay is a good bet.
Have your mechanic hot-wire the fuel pump through a dash-mounted switch. Label the switch Fuel On, Fuel off. Take power from the “on” post of the ignition switch. You COULD eliminate the switch…Just run a wire from the ignition switch to the pump…
If you have confirmed with a guage that there is no fuel pressure, then forget all of the non related stuff like cam sensors and focus on fuel. That would include of course the pump (was it OEM or after-market), the connections, the harness at the tank, the relay, fuse connection, wires between them, and the computer telling the pump when to run.
Well after nearly a year and half dead in my garage, my Jeep is resurrected thanks to Car Talk! Replaced the crank position sensor, and now have reliable fuel pressure. Still was running very rough, pulled the plugs (two fouled out completely), cap and rotor (both worn), and it seemed to run much better. Then ran into a hard start problem on a cold day – replaced the intake manifold air temperature sensor, which SEEMED to fix that problem. But now am still having a cold start problem. Will transfer this new issue to a new thread…thanks