Vehicle: 1993 Grand Laredo 4.0, 4wd, Automatic, Air 170,000 miles
Engine codes: 12, 55 (using check engine light blinking)
Note: The engine is never started during the following tests except where indicated:
Normally when I switch the ignition on, my fuel pump will operate for about 1.5 seconds and stop. This charges the fuel rail with about 40 psi.
Recently, I noticed that the vehicle has become harder/longer to start, but only when cold.
I've noticed that the fuel pump does not run immediately when I switch on the ignition, but there is a delay up to 30-40 sec. when the engine is cold. I've also noticed that the "check engine" light is also delayed by the same amount as the initial 1.5 sec. fuel pump cycle when cold.
I've check the fuel rail pressure with a gauge, and once the fuel pump runs (after the delay), the pressure is good (40 psi).
I removed the plastic cover from the fuel pump relay and the fuel pump will run each and every time I manually activate the relay regardless of temperature. I've replaced this relay with no change in symptoms.
After the initial 20-30 sec. period, the fuel pump will operated for
1.5 sec. each time the ignition switch is cycled from off to on. However, if the engine is cold and I wait 10 - 15 sec. between each off-on cycle of the ignition switch, the fuel pump delay begins to surface again (2-4 sec delay). The longer I wait between off-on cycles of the switch, the greater the delay for the fuel pump.Now if I start the engine and let it run for 120 sec., there is no fuel pump delay. However, if I allow the engine to remain off/cool for 120 sec, the delay begins to re-appear but with a shorter delay of only
6-10 sec. The longer the engine is allow to cool, the longer the delay.I suspect that I have a sensor that is failing and that it is temperature sensitive, and this sensor is affecting the operation of the PCM with respect to the initial operation of the fuel pump.
My O2 sensor is a 4 wire device, which I believe indicates it has an internal heater. It's the original sensor. I haven't changed it due to the replacement cost, but I am beginning to suspect that it may be contributing to the fuel pump delay symptoms. I don't know if an O2 sensor can produces the above symptoms.
Your comments, advice, etc. are appreciated.