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Pete
Pete, Auto Service Technician
Category: Chevy
Satisfied Customers: 25287
Experience:  14 years GM dealer Technician.
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96 chevy k1500 5.7 runs good untill 3/4 throttle or more

Customer Question

96 chevy k1500 5.7 runs good untill 3/4 throttle or more justb
96 chevy k1500 5.7 runs fine untill 3/4 throttle or more then wont gain rpms just bogs down has 60 psi fuel pressure key on engine off has anywere between 48 to 55 psi while driving replaced fuel pump,filter ,and pressure regulator. blocked return line has 65 psi at idle still drops down to 50 while driving
Submitted: 1 year ago.
Category: Chevy
Expert:  Pete replied 1 year ago.

Hi my name is ***** ***** I will be happy to help with this today.I need a little more info to assist better.

Can you tell me the fault code numbers that are setting for the oxygen sensors?

Is the engine rattling at all when the lack of power occurs?

Let me know so we can continue,thanks Pete.

Customer: replied 1 year ago.
trouble codes p0131,p0137,p0151,p0157. no rattling just sounds muffled
Expert:  Pete replied 1 year ago.

Were the catalytic converters removed and is the sensor wiring still in place but no sensors?

When you accelerate or rev up the engine does the fuel pressure drop off briefly then come back up or does it continue to drop at all?

Let me know so we can continue,thanks Pete.

Customer: replied 1 year ago.
it had a new exhaust system put on from manifolds back no converters but the 02 sensors are installed. yes it will come back when you let off but if you hold it to the floor it will stay between 50to53psi. tried power braking it so i could spray it to make sure its a fuel problem but it runs fine power braking it will just sit there and burn the tires
Expert:  Pete replied 1 year ago.

O.k.,with the key off after the pressure builds to the 60 psi range does the pressure start to drop off on the fuel pressure test gauge rapidly or within a few minutes time?

Customer: replied 1 year ago.
drove it about ten minutes ago and its still at 58 right now
Customer: replied 1 year ago.
if i hold it to the floor while driving it will just keep slowing down every few seconds it will get a little power then just falls on its face again
Expert:  Pete replied 1 year ago.

O.k.,does your scan tool show the oxygen sensor data signals and if so are the pre cat oxygen sensors (bank 1 and bank 2 sensors 1) showing below .5 volt or below 500 millivolts and dropping as the issue occurs?

Customer: replied 1 year ago.
they will both go down to 4 millivolts if held on the floor as soon as i let off switches to rich around 800 millivolts
Expert:  Pete replied 1 year ago.

O.k.,the engine is running lean then and this is what these codes are indicating.I suspect this may be a mass air flow sensor fault with all that has been checked and done for this already.I have seen the mass air flow sensor act up and not set a code,this can be common.In some cases you can remove it then remove the screws that hold it together and remove the screen from it and use mass air flow spray cleaner from a local parts store and spray off the thin sensor wires on the inside of the sensor to correct this here.In other cases I have had to replace it.In some cases you can tell it is this sensor if with your key off you unplug it then start the engine and operate it very briefly to see if the issue still occurs.If it doesn't then the mass air flow sensor needs replaced.

If more help is needed, use the reply tab to continue our conversation. If no further assistance is needed, kindly rate my service.You can rate at any time and we can continue to work on your question as this will not close out your question.Keep in mind in some cases it can be difficult to fully diagnose or help repair your vehicle without seeing it and I work on delivering the best possible answer from what I see to work with.Thanks Pete.

Customer: replied 1 year ago.
i understand its difficult not having the vehicle there. i unpluged the mass air flow and drove it yesterday it seemed a little better i have that spray i will try it quick
Expert:  Pete replied 1 year ago.

O.k.,it is best to take it apart if you can when you spray it to clean it so you can clean it off really good.Let me know if this corrects the fault here.

Thanks Pete

Customer: replied 1 year ago.
ok cleaned it still acts the same and getting the same 02 readings im wondering if the mass air flow sensor is just bad cause it did run better with it unpluged ? i would think if the injector was bad it would have missfires
Expert:  Pete replied 1 year ago.

Yeah,if the injector or any of the injectors were an issue you would have a misfire.The engine is definitely running lean,it is possible all injectors are slightly restricted causing a lean condition if there was some kind of contamination in the fuel at some point or even now.You can try adding cleaner to the tank and see if it clears up.

Other then this I would think with the pump,regulator and filter being replaced already this leads me to believe the mass air flow sensor is at fault,there is a vacuum leak or possibly a timing issue.

Thanks Pete

Customer: replied 1 year ago.
i sprayed all over looking for a vacumn leak came up with nothing my cam timing is at -1 it is weird thou when i go into troubleshooter adjustments for cam timing it tells me na but on live data it says its -1
Expert:  Pete replied 1 year ago.

When you rev up the engine I believe around 1500 to 2500 rpm stopping at each 500 rpm interval does it go over -20 at all?

Customer: replied 1 year ago.
today it says its at zero and going from 1500 to 2500 it stays at zero
Expert:  Pete replied 1 year ago.

O.k.,timing is o.k. then.

To me from here this does sound like a fault with the mass air flow sensor if there are no vacuum leaks and the fuel in the tank is good.

Thanks Pete

Customer: replied 1 year ago.
hey pete thanks alot for your help
Expert:  Pete replied 1 year ago.

Your welcome.Let me know if you need anything else and feel free to rate my help if you don't mind.

Thanks Pete

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