Runs rough once it is warmed up

Welcome to 2CarPros.

If you have to press the throttle pedal to the floor to start it, to me that indicates the engine is getting too much fuel. On a fuel injected vehicle, pressing the throttle when starting shuts the injectors down. So, my first suspect is a faulty fuel pressure regulator or the engine coolant temperature sensor is bad.

Here is a link that explains about the cause of a backfire:

https://www.2carpros.com/articles/engine-backfires-while-running

Here is a link that explains in general how to test fuel pressure and the fuel pressure regulator:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Here are the manufacturer's specs for pressure:

Fuel Pressure

Central SFI .................... 60-66 psi
SFI .................... 56-62 psi

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If fuel pressure is good, then suspect the ECT (engine coolant temperature) sensor. Based on the reading from this sensor, the vehicle's computer determines the engine's air / fuel mixture ratio. Now, lets say the engine is warm (approximately 200°F) and the ECT tells the computer the temperature is -40°F, the computer will compensate by making the mixture very rich thinking it is needed because of the cold temperatures. The result is what you are experiencing and no check engine light is set because the computer only knows it is getting a signal (right or wrong).

To check the ECT signal requires a live data scanner. Once the engine warms up and starts giving you trouble, then you need to scan the sensor to see exactly what temperature it thinks the coolant is. If it's off, replace the sensor.

Here are the directions for replacing the sensor if you find it's bad. The two attached pictures correlate with the directions.

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ENGINE COOLANT TEMPERATURE (ECT) SENSOR
Engine Coolant Temperature (ECT) Sensor Electrical Connector

pic 1

Engine Coolant Temperature (ECT) Sensor

pic 2

CAUTION: Use care when handling the coolant sensor. Damage to the coolant sensor will affect the operation of the fuel control system.

REMOVAL PROCEDURE
1. Relieve the coolant pressure.
2. Disconnect the negative battery cable.
3. Drain the cooling system below the level of the sensor.
4. Disconnect the electrical connector releasing locking tab.
5. Remove the coolant sensor from engine.

INSTALLATION PROCEDURE
1. Coat the threads (only) with sealer P/N 9985253 or equivalent.

Tighten
Tighten the sensor to 13 N.m (10 lb. ft.).

2. Install the coolant sensor in the engine.
3. Connect the electrical connector.
4. Refill the cooling system.
5. Connect the negative battery cable.

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If you look at pic 3, I circled the sensor and location. Don't get this confused with the sender.

Let me know if any of this helps or if you have other questions. Honestly, my gut is telling me to check the sensor first, but that's your call.

Take care,
joe