97 Tahoe 5.7 hard starting high idle

[east302]

Hard hot starting sort of points to a coolant temperature sensor issue. Holding the gas down is “clear flood mode” which stops injector flow while cranking.

A scanner would tell you what temperature the sensor is reporting. It should be roughly ambient when engine is cold and around the thermostat setting when hot.

A fuel pressure test wouldn’t hurt, either. Harbor Freight sells a reasonable test kit so you should be able to spot pressure decay with engine off. Parts stores may rent them as well.


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[Chewy11B]

I would be checking your tps voltage sweep and iac positioning. If your moving the pedal and slowly releasing there could need to be an issue with the sweep of the tps. Also iac runs on a series of gears with the plunger sometimes being slow or not moving at all due to carbon build up. Give her a nice throttle body cleaning too make sure there's no issues with air making through even with blades closed at idle.

 

[RPMerf]

I would start with a Bluetooth/OBD2 or WiFi/OBD2 device to watch the sensors. Connects to your phone. Use an app like Torque to read the OBD2 data. Run through the sensors to make sure the values are correct. They are cheap on Amazon. You can get a cheap one for like $10 or a little better one for like $20.

Check your coolant temperature sensor with the engine hot and cold.
Watch your throttle position sensor, idle control motor, MAP sensor as you open / close the throttle

Might be able to see "correction" values also. These are how far your fuel requirements vary from the base tune. Higher values point to an issue

 

[Ryan cin]

I would start with a Bluetooth/OBD2 or WiFi/OBD2 device to watch the sensors. Connects to your phone. Use an app like Torque to read the OBD2 data. Run through the sensors to make sure the values are correct. They are cheap on Amazon. You can get a cheap one for like $10 or a little better one for like $20.

Check your coolant temperature sensor with the engine hot and cold.
Watch your throttle position sensor, idle control motor, MAP sensor as you open / close the throttle

Might be able to see "correction" values also. These are how far your fuel requirements vary from the base tune. Higher values point to an issue

I have a scanner with a data stream but I’m not sure what the codes are supposed to read or what any of the values mean. When my foot is off the accelerator the tps reads 0. Does that mean it works?

 

[Ryan cin]

I would be checking your tps voltage sweep and iac positioning. If your moving the pedal and slowly releasing there could need to be an issue with the sweep of the tps. Also iac runs on a series of gears with the plunger sometimes being slow or not moving at all due to carbon build up. Give her a nice throttle body cleaning too make sure there's no issues with air making through even with blades closed at idle.

Can I check these with a scanner? I cleaned my iac and it didn’t seem to change anything. I also u plugged it and started the truck and everything was the same.

 

[Ryan cin]

Hard hot starting sort of points to a coolant temperature sensor issue. Holding the gas down is “clear flood mode” which stops injector flow while cranking.

A scanner would tell you what temperature the sensor is reporting. It should be roughly ambient when engine is cold and around the thermostat setting when hot.

A fuel pressure test wouldn’t hurt, either. Harbor Freight sells a reasonable test kit so you should be able to spot pressure decay with engine off. Parts stores may rent them as well.


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Would the coolant temp sensor reading wrong cause too much/too little fuel to go to the engine? It seems like it’s lean when it’s running but way too rich when I have to start hot, which is why I have to hold the pedal down

 

[east302]

I have a scanner with a data stream but I’m not sure what the codes are supposed to read or what any of the values mean. When my foot is off the accelerator the tps reads 0. Does that mean it works?

Watch how the reading changes from closed to full open throttle. If your readout is in terms of voltage, it should be less than 1.25V (normally 0.45-0.85V) with ignition on (engine off) and increase smoothly as the throttle is opened. No erratic jumps, in other words. Wide open throttle would be around 5V.


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[east302]

Would the coolant temp sensor reading wrong cause too much/too little fuel to go to the engine? It seems like it’s lean when it’s running but way too rich when I have to start hot, which is why I have to hold the pedal down

The computer can call for too much fuel if it’s getting an erroneously cold reading from the sensor. Your scanner likely has this as a reading (PID). Look for ECT sensor maybe. If it’s reading way off (think sub-zero degrees or something) then that sensor is definitely suspect.


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[Ryan cin]

The computer can call for too much fuel if it’s getting an erroneously cold reading from the sensor. Your scanner likely has this as a reading (PID). Look for ECT sensor maybe. If it’s reading way off (think sub-zero degrees or something) then that sensor is definitely suspect.


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That’s it! Replaced the ECT today and so far so good. Idle is back down, started fine hot and cold. My old one was reading -40°F with key on but not running and while running. Replaced it now I get a reading and seems to be running good. On another note, one of my monitors for inspection is also ready now, and it wasn’t before. Thanks!

For anyone else with similar issues that happens to stumble across this thread:

-Hard starting
-High idle in park and gear (~1100 rpm)
-Having to flood clear to start hot

All fixed by replacing the ECT sensor.


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[east302]

Good deal, glad you got it sorted out. Thanks for the update.


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