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Originally Posted by Bad_Wing
Hi all,
Recently I have been having problems with my 93 Jeep Grand Cherokee.
Specs:
6 cyl. 4.0
4x4
A/C
MFI
It progressively needed more cranking time for startup in the mornings. So I reacted with a simple tuneup, consisting of plugs, wires, cap, rotor, air filter, and a fuel filter. Although this clearly improved performance while running, the startup issue stayed the same, if not, got worse.
I did the "key switch to run 3 times" fault code check. No error codes.
I noticed the Jeep did not seem to have proper fuel pressure in the mornings, so I figured it is the anti-drainback valve in the pump assembly. I decided to put in a new fuel pressure regulator and a fuel pump (thorougly cleaned the tank using an approved antibacterial soap before reassembly). No change in startup performance.
I checked all connectors and ground locations and saw nothing visibly wrong.
I figure it must be a sensor that the control module is looking for before startup. I replaced the crankshaft sensor, making sure it is mounted as close to the flywheel as possible. I also tested the camshaft sensor, it seems to be performing as expected. No change in startup performance.
I downloaded a schematic from Autozone.com and reviewed it. The problem is neither my schematic nor Hayne's repair manual clearly defines the logic to the startup process.
During some of my wrenching, I did notice two things:
First, if I leave the key in the "run" position for a varying amount of time, the fuel pump will suddenly cycle and deliver pressure to the fuel system. After this occurs, the jeep fires up like it was brand new.
I can force this condition, by turning the key to the "run" position and disconnecting the negative (-) battery cable and reconnecting it. Every time I do this, the jeep will cycle the fuel pump and start perfectly.
My theory for why this works is that if I disconnect the negative (-) cable and reconnect it, the control module doesn't have time to go through it's software and is tricked to latch the fuel pump relay, allowing the jeep to start...neat huh? I dunno?!?!
My question is, does anyone know precisely what the control module looks for before startup?
Help me guys, don't let the dealers win...keep me out of there!
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