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Ask Jared D Your Own Question
Jared D
Jared D, ASE Certified Technician
Category: Chevy
Satisfied Customers: 1355
Experience:  Shop Supervisor at County Fleet
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Reduced engine power, 127,000 automatic, z71, 2003 tahoe, i

Customer Question

reduced engine power
JA: How many miles are on the car? Is it an automatic or a manual transmission?
Customer: 127,000 automatic, z71
JA: What is the model/year of your Chevy?
Customer: 2003 tahoe
JA: Are you fixing your Tahoe yourself? What have you tried so far?
Customer: i have replaced the throttle body, the harness at the throttle body, the pedal assembly, the fuel filter and the fuel pump
JA: Anything else you want the mechanic to know before I connect you?
Customer: no
Submitted: 2 months ago.
Category: Chevy
Expert:  Jared D replied 2 months ago.

Good morning,

When it goes into limp mode, do you know which trouble codes are setting?

Customer: replied 2 months ago.
They all had to
Do with the throttle body and the pedal
Expert:  Jared D replied 2 months ago.

Do you know which they were specifically? They could be circuit malfunction codes, position codes, correlation codes, etc. It sounds like you have replaced just about everything in an attempt to fix it, if I have the specific codes I might be able to help you pinpoint it rather than just throw parts at it.

Customer: replied 2 months ago.
I don't have the specific codes. I just put the new fuel pump in it and I can't get it to start
Expert:  Jared D replied 2 months ago.

Ok. When you turn the key to the run position can you hear the fuel pump buzz for a few seconds? It should make an audible buzzing sound for about 3 seconds each time the key is turned to run.

Customer: replied 2 months ago.
No sound
Customer: replied 2 months ago.
I double checked the wiring on the new harness
Expert:  Jared D replied 2 months ago.

Are you getting power to the fuel pump relay?

Customer: replied 2 months ago.
How do I check that
Expert:  Jared D replied 2 months ago.

The PCM B fuse powers the relay, check to make sure you are getting 12 volts to the PCM B fuse.

Next, check side of the fuel pump relay for a picture that looks like a map. It should like like this picture.

Expert:  Jared D replied 2 months ago.

It will have the pins labeled and the pins that correspond to the parts I circled in red should have power. The one on the right, that leads to the PCM B fuse will have power all the time. The one on the left will have power with the key in run (but only for 3 seconds after the key is turned).

Customer: replied 2 months ago.
I'm getting 12v at the pcmb fuse. The picture on the relay is nothing like the picture. 85 87
86 30
Customer: replied 2 months ago.
Here
Expert:  Jared D replied 2 months ago.

So pin 30 should have 12 volts constant, pin 86 should have 12 volts when key in run for 3 seconds. Pin 85 will be ground, and pin 87 should feed the fuel pump. So if you unplug the relay and put a jumper wire (or paperclip) across pin 30 to 87 it should make the fuel pump run- this would test whether the fuel pump is working. It will pump for as long as you have the jumper in since this would be bypassing the relay circuit and feed voltage right to the pump- assuming you have 12 volts on pin 30 like you should.

Expert:  Jared D replied 2 months ago.

I'm leaving the shop for the day, I'll check back in this evening to see if you need additional info.

Customer: replied 2 months ago.
0 volts at the 30
Expert:  Jared D replied 2 months ago.

Weird. Do you have 12 volts on any of the pins?

Customer: replied 2 months ago.
I only checked that one. When I tried the jumper it blew the fuse
Expert:  Jared D replied 2 months ago.

Also, 86 might have ground and 85 voltage with key in the run- my diagram doesn't label the pin numbers and I can't remember off memory.

Expert:  Jared D replied 2 months ago.

If you jumped 87 to 30 and the fuse blew that means the gray wire from the relay to the fuel pump is shorted to ground somewhere or the fuel pump is defective.

Expert:  Jared D replied 2 months ago.

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Customer: replied 2 months ago.
Pump was wired backwards. I hear the pump run now.
Customer: replied 2 months ago.
Still won't start. Almost a full tank when I turn the key I show over 3/4. When I turn it to start it goes down to empty the battery gauge also goes all the way left.
Expert:  Jared D replied 2 months ago.

Does the engine crank but not start or does it not crank at all?

Customer: replied 2 months ago.
It cranks but won't statt
Expert:  Jared D replied 2 months ago.

If you spray starting fluid or fuel in the intake duct will it start for a few seconds?

If it does then you may not be getting injector pulse which is usually triggered by the crank position sensor. If you have a scan tool you can use it to look for an RPM signal while cranking, if you don't have an RPM signal when cranking then the crank position sensor is dead.

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