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Ask AMEDEE Your Own Question
Category: Chevy
Satisfied Customers: 26414
Experience:  ASE certified tech ADVANCED LEVEL SPECIALIST. Wisconsin certified emissions state inspector
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Customer Question

My 04 Impala 3.4L hesitates and sometimes stalls when taking off from a stop. It only does it when it reaches norm operating temp. I have noticed that when the a/c is on, it will not hesitate or stall. It runs perfect. But last night, while driving on the highway (a/c off), the engine stopped running. I had to pull over and the check engine came on also. I checked it with the scanner and it came up as P0440. I know this has to do with an EVAP, but would this make my car turn off? Also, what can I do to solve this problem?
Info from the scanner:
1. DTC - P0440
2. Fuel System 1 Status - Open Loop
3. Fuel System 2 Status - Not Supported
4. Calculated LOAD Value - 3.53 %
5. Engine Coolant Temp - 176.00 °F
6. Short Term Fuel Trim - Bank 1 - 0.00 %
7. Long Term Fuel Trim - Bank 1 - -3.91 %
8. Intake Manifold Absolute Pressure - 9.18 Inhg
9. Engine RPM - 0.00 Rpm
10. Vehicle Speed Sensor - 61.52 Mph
11. Air Flow Rate Mass Air Flow Sensor - 0.49 Lb/Min
12.Absolute Throttle Position - 0.00 %
Submitted: 7 years ago.
Category: Chevy
Expert:  AMEDEE replied 7 years ago.



How does the engine idle when warm?


DO you have plenty of acceleration when warm?



Customer: replied 7 years ago.
When the engine is warm the idle is not bad but it feels a just a little rough. I have put a video on youtube of my engine. Hopefully this helps.
Expert:  AMEDEE replied 7 years ago.

THat is quite interesting!


IT sounds like you might have an air or a vacuum leak.


Just be sure the fuel pressure is up to par and that the fuel pressure holds steady when the engine is turned off. THis will verify that there is no gas leaking into the engine that is not accounted for.


ONe common cause for stalling is common cause would be the Idle Air Control motor. It may be bad causing your condition. This is very common on older cars. The IAC motor gets lazy and cant keep up with the fast idle changes. Also when the IAC motor is out, I rec to check the passages for carbon build up. If they are plugged they need to be cleaned out.

Customer: replied 7 years ago.
What would be the best way to check for an air or vacuum leak?
Also, I changed the IAC about 2 months ago. I was told by a friend that maybe the CPS might be the problem. Is that possible? Sorry to ask again but would the EVAP code be related to the car stalling?
Expert:  AMEDEE replied 7 years ago.

THe evape code will most likley not have anything to do with the car stalling. Just be sure the purge valve is not stuck open.


To check for vacuum leaks, open the hood and listen for a hissing sound when the engine is running.

Unmetered air can enter the engine through a vacuum leak, a dirty airflow sensor that is not reading airflow accurately, an EGR valve is not closing and is leaking exhaust into the intake manifold, an EGR valve that is allowing too much flow.
If it is hard to pinpoint take some brake cleaner or starting fluid around the intake manifold and vacuum lines and see if the engine stumbles or if the idle is affected. Be extremely careful when doing this!

Customer: replied 7 years ago.
Thank you I will try that. Do you mind if I ask if the numbers that my scanner popped look okay? Could a Crankshaft Position Sensor cause the stalling? I will get back to you once I finish checking for leaks
Expert:  AMEDEE replied 7 years ago.

If the engine was turned off (key on engine off) when these readings were taken, then yes... I think they look OK.


The crankshaft position sensor could very well do this. THis might not be a bad thing to check. However, more common then that would be the crankshaft position sensor wires that run from the sensor up to the ignition module. They sometimes rub against the exhaust manifold or the exhaust heat shield and cause a stalling condition. CHeck these wires when you are checking the crankshaft position sensor.


Expert:  AMEDEE replied 7 years ago.



Did you have any further questions?

Customer: replied 7 years ago.
I am going to check for the vacuum leaks today then I will get back with you. Thank you very much!
Expert:  AMEDEE replied 7 years ago.

SOunds good!


Please let me know what you find!


I have more things we can check if needed!

AMEDEE and 5 other Chevy Specialists are ready to help you
Customer: replied 7 years ago.
Sorry for the delay, but I couldn't find any vacuum leaks. But I did get some advice about driving with the gas cap off and it seemed to run better. I was told that maybe the evap canister vent might be blocked. Have you ever heard of this happening??
Expert:  AMEDEE replied 7 years ago.

In rare cases, yes. However, there is usually something more to this problem then just an evap canister.


You could have a bad mass air flow sensor. Or the snorkel from the mass air flow sensor to the throttle body could be ripped. Both are very common and can cause you vehicle to loose power along with many other drive ability issues. First look for cracks or rips or anywhere air can enter the engine that is not accounted for by the mass air flow sensor. Next comes the Mass Air Flow (MAF) sensor. This is a very important sensor input to the computer. It uses a hot wire sensing element to measure the amount of air entering the engine. The MAF sensor then outputs an analog voltage signal to the PCM proportional to the intake air mass. The PCM calculates the required fuel injector pulse width in order to provide the desired air/fuel ratio. If the sensor is bad than this air/fuel ratio will be off causing severe performance issues. In extreme cases, I have seen these two problems cause a no start condition. This input can also be used in determining transmission Electronic Pressure Control, shift and torque converter clutch scheduling. The check engine light may even pop on if the sensor is bad, the snorkel is ripped or a tube has fallen off. This will result in a lean condition.


Expert:  AMEDEE replied 7 years ago.

Diagnostic Codes: P0440


1. Activate the Evaporative Emission Control (EVAP) system vent and purge solenoids.

2. Check for the EVAP system to hold vacuum with either a gauge on the service port or by monitoring the Fuel Tank Pressure Sensor (FTPS) voltage.

3. Deactivate the EVAP purge solenoid, vacuum should hold and FTPS voltage should be higher than 1.5 volts.


Potential Causes:

Evaporative Hoses
Evaporative Purge Solenoid
Evaporative Vent Solenoid
Gas Cap
Vapor Canister


Tips: FTPS voltage should be about 1.3 to 1.7 volts with the gas cap off. With vacuum on the system it should go up and hold. Inspect for leaks at the fuel cap, filler neck, fuel sender o-ring, evaporative canister, evaporative vent solenoid, evaporative purge solenoid, and evaporative hoses.