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Heavy misfire 2000 gmc 3500 sierra 1st 10 miles this am Aok…

Heavy misfire 2000 gmc...

Heavy misfire 2000 gmc 3500 sierra 1st 10 miles this am Aok then misfire and flashing engine light power progressively going down

Mechanic's Assistant: Just to clarify, can you tell me which light it is? And does it blink first, then stay on steady?

Service engine soon constant blink, good morning pearl, chris here

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

Trying to diagnose, have replaced coils & plugs, this has happened before with only slight loss of power, today it can barely get up a slight incline

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Answered in 5 minutes by:
11/21/2017
Pete
Pete, GMC Mechanic
Category: GMC
Satisfied Customers: 32,961
Experience: GM training and dealer tech for 14+ years. Diagnostics
Verified

Hi my name is ***** ***** I will be happy to assist with this today.In some cases I may need more info to assist better.I apologize for any delay.

Have you had the fault codes checked/read out at all to see what codes are set with this issue here?

Let me know so we can continue,thanks Pete

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Customer reply replied 8 months ago
P0300
P0171
Ok you have a lean running code for bank 1 as well as the misfire code.The first thing that needs done is to have the fuel pressure tested to see if there is a low pressure issue.If the fuel filter is plugged or if the fuel pump is weak this will cause this issue here.Your fuel pressure should be about 55 to 60 psi on the gas engines in this model year.
If fuel pressure is ok one very common cause especially on the 7.4 liter and 5.7 liter engines is failing fuel injectors needing cleaned or replaced.Most of the time the faulty injector has to be replaced.
An injector pressure drop test would need o be done to see if any injector is dropping 1.5 psi more or less different then the other injectors when tested.This would indicate the injector is faulty needing replaced.This test is done with a scan tool that can command injector pulses and a fuel pressure test gauge to observe the pressure drop.
One last thing that can cause an issue with misfire and lean readings on bank 1 is a restricted catalytic converter.If you have not had an exhaust back pressure test done this should be done with an exhaust back pressure tested threaded into a pre catalytic converter oxygen sensor port.If the back pressure exceeds 1.5 psi at any time then the catalytic converter needs replaced.It is also a good idea to have the intake checked for any vacuum leaks needing repaired.
Most common, fuel pump or filter or faulty injector.Let me know if you need anything else and feel free to rate my help here.A 5 star rating is appreciated.You can always come back at any time if needed here on this post even after a rating is left.Thanks Pete
Pete
Pete, GMC Mechanic
Category: GMC
Satisfied Customers: 32,961
Experience: GM training and dealer tech for 14+ years. Diagnostics
Verified
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Let me know if you need anything else.

Thanks Pete

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