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Fuel Pump Priming ? /Starting


NAXJA Forum User
LA California
Through out the years i have always noticed my fuel pump prime when the key is in the ignition (step before crank). THis usually is a slight hum, which i know to be perfectly normal. Over the past few months its becoming harder to start . When it does start i usually have to lay off the gas for a min or when i do press on it it sounds as if its gonna bog down, and other times it starts at an extremly low RPM and it sounds as if the jeep revs itself to keep itself alive.

THo recently instead of the hum that i usually hear, i hear a screetch. It sounds like nails on a black board. Does this mean its gonna die ??

Once the jeep is hot, it drives fine.

jeepsrock said:
THo recently instead of the hum that i usually hear, i hear a screetch. It sounds like nails on a black board. Does this mean its gonna die ??

Yeah, screech isn't good from the fuel pump. May or may not be your driveability issue but I'd suggest a new pump, filter, test your regulator and go from there.
How does one test the regulator ? Over the last past few days i have noticed this noise (screetch ,nails on black board ) is coming from the front of the jeep during the pumps priming ? What has to do with the fuel in the front that can do this ??

Erm - what year? Really important...

The fuel pump isn't so much actually "priming" as it is developing system pressure. It is normal for the fuel pump to run, say, 3-10 seconds on pretty much any FI vehicle in order to develop system pressure - if you were to turn on the key and not start the vehicle, you'd hear the pump run for a few seconds and then stop.

If the screech is coming from the fuel pump, it will want replacing. Most (electric) fuel pumps are either "vane" type or "gerotor" type - but both screech when failing. A "vane" pump uses an vane wheel with springloaded "wiper" blades, and the constant volume change between the vanes is what develops both suction (fuel pickup) and pressure (fuel discharge.) Screeching in a vane pump usually means that one of the vanes has gotten hard, or has worn away to where one or more springs is contacting the inside of the champer.

A gerotor pump uses a set of nested gears, with one having one tooth less than the other. This results in another sort of "constant volume change" - and gerotor pumps usually fail due to advanced wear.

If you have a RENIX XJ (1987-1990 w/4.0,) the long start times are partly caused by the ECM itself - it's not that it's failing, it's just that the RENIX ECM does not start sending out ignition or fuel pulses until it A) reaches 300RPM and B) recieves a valid "sync" signal from the distributor (the RENIX will run without the Sync signal, but it prefers not to.) The fix for that, I have found, is to replace the start with a high-torque unit (which develops engine RPM faster) and make sure the alternator can keep the battery topped off. I have a local shop that knows precisely what I want when I come in, and makes sure I get it (I get the rewinds from him - I install them myself.)

Oh - make sure the screech isn't coming from the belt or the power steering pump - as they get old, Saginaw pumps can get noisy on cold starts - that's why I want you to make sure where the noise is coming from...

Well its 94 cherokee 4.0

THe noise happens at the same time the pump primes ( when the key is inserted and turned b4 engine crank), but it sounds the screetch is from the front. THe screetch only happens when the jeep has been parked overnight ? ?? If i go to the store and come back to start it there is no noise , just the normal soft pump prime noise from the back.