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2004 impala 3.8 supervharged. Wiil crank over but not run.

Customer Question
2004 impala 3.8 supervharged...

2004 impala 3.8 supervharged. Wiil crank over but not run. If you hold it part throttle while cranking it will start but acts like it flooding over & misfiring, let off the throttle & dies. Every once in a great while, it will crank & idle perfectly but as soon as you rev it it goes to running bad & dies. No security light & no codes on scanner.

Mechanic's Assistant: Does the light flash, blink or stay solid? Have you tried to reset the security system?

With the ignition in the on position, the light is solid, when the car does run, it is not on at all, I have done the relearn procedure also.

Mechanic's Assistant: Are you fixing your Impala yourself? What have you tried so far?

Yes, removed & replaced the fuel filter, I can hear the fuel pump come on. Replaced the battery.

Mechanic's Assistant: Anything else you want the mechanic to know before I connect you?

I think thats it.

Submitted: 25 days ago.Category: Chevy
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Answered in 29 minutes by:
5/28/2018
Chevy Mechanic: Ivan Alvarez,
 replied 25 days ago
Ivan Alvarez
Category: Chevy
Satisfied Customers: 174
Experience: lead technician at Performance Tire and Automotive
Verified

Welcome to just answer. Does your scanner show live data?

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Chevy Mechanic: Ivan Alvarez,
 replied 25 days ago

What are the values for the mass air flow sensor (maf) when you get it to run?

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Chevy Mechanic: Ivan Alvarez,
 replied 25 days ago

A​lso look at fuel trim values and let me know where they are at. on both the short and long term trims.

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Customer reply replied 25 days ago
12.4 inHg
0.8 lb/ min
Customer reply replied 25 days ago
Not sure what you mean by fuel trim, I don’t see that in my data
Customer reply replied 25 days ago
STFT=-25.0
LTFT=-14.8
Customer reply replied 25 days ago
Sorry MAF=0.8 lb/min
Chevy Mechanic: Ivan Alvarez,
 replied 25 days ago

W​hat you sent is the fuel trim data. It looks like its trying to take fuel away. Have you tried disconnecting the maf sensor to see how it runs?

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Customer reply replied 25 days ago
it has no change
Chevy Mechanic: Ivan Alvarez,
 replied 25 days ago

Have you checked fuel pressure with a gauge?

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Customer reply replied 25 days ago
Not yet.
Chevy Mechanic: Ivan Alvarez,
 replied 25 days ago

G​et fuel pressure readings when you can. With car running and when off. See how fast pressure drops when off. its possible you have one or multiple leaking injectors.

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Customer reply replied 25 days ago
Why is it that every once in awhile it will idle perfectly and then suddenly start running bad and die. Leaking injectors either leak or they don’t right? If fuel pump is bad, will it make pressure at times and not have pressure at times?
Customer reply replied 25 days ago
Idling has 48 psi.
Customer reply replied 25 days ago
Now idling MAF shows 0.6 lb/min
Customer reply replied 25 days ago
Fuel pressure does not fall off when it dies
Chevy Mechanic: Ivan Alvarez,
 replied 25 days ago

Fuel injectors can intermittently stay stuck open. Just have to rule things out. For the fuel trims to be so negative you are right that its flooding out. Its running too rich. What does the coolant temp sensor show when running?

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Customer reply replied 25 days ago
196
Chevy Mechanic: Ivan Alvarez,
 replied 25 days ago

Does your scanner give you misfire data? If so what cylinders show misfires?

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Customer reply replied 25 days ago
It does show me that
Chevy Mechanic: Ivan Alvarez,
 replied 25 days ago

Check to see what cylinders show a misfire.

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Customer reply replied 25 days ago
Sorry, meant to type it does not show me that
Chevy Mechanic: Ivan Alvarez,
 replied 25 days ago

Oh ok. Another possibility is the ignition system. If its not getting good spark to plugs it wont burn the fuel and that would cause negative fuel trims like you have. May be coils or ignition module. So since your scanner doesn't show that data you can someone keep the car running while you pull off wires from the coils one by one to see if there is good spark and at the same time you should notice a difference on how the engine runs. The spark should jump at least an inch. Hold the wire close to the coil tower and it should jump to wire from coil. Dont hold it so close to the boot because it can shock you.

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Customer reply replied 25 days ago
Seems to have good fire.
Customer reply replied 25 days ago
Any other ideas?
Chevy Mechanic: Ivan Alvarez,
 replied 25 days ago

Create a large vacuum leak like the brake booster hose. Control the vacuum with your finger and see if you can get the car to run better.

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Customer reply replied 25 days ago
Ok. I’ll give it a try tomorrow and let you know. Thanks
Chevy Mechanic: Ivan Alvarez,
 replied 25 days ago

Ok.

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Customer reply replied 24 days ago
Creating a vacuum did not help.
Customer reply replied 24 days ago
Update...was unplugging the wrong sensor, was unplugging the IAT, i'm a dummy, unplugged the MAF & it ran, plugged it back up while it was running & it died & would not restart until I unplugged the MAF & it started, Will purchase one tomorrow & see if that fixes it.
Chevy Mechanic: Ivan Alvarez,
 replied 24 days ago
Ok. Sounds good.
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Chevy Mechanic: Ivan Alvarez,
 replied 22 days ago

Any new info?

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