Code P0300

2001 CHEVROLET SUBURBAN
200,000 MILES • 5.3L • V8 • 4WD • AUTOMATIC
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DANIEL SANCHEZ
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I cannot find what the problem is.
I got new:
Intake and throttle body gasket
New plugs and wires
New Hoses and EGR gasket
New Map sensor
New MAF sensor
Knock sensors and harness

Freeze frame reads:
P0300
Fuel system 1. OL
Fuel system 2. OL
Load_pct. %7.5
ETC. 99
SHRTFT1. % 0.0
LONGFT1. % -0.8
SHRTFT2. % 0.0
LONGFT2. % -0.8
MAP (inHg) 18.3
RPM (/min) 1557
VSS (MPH) 0
MAF (lbs/min) 2.447
TP. % 10.2
Mar 5, 2019 at 2:33 PM
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ASEMASTER6371
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Good evening,

The first thing I would do is check the fuel pressure. that is the most common reason for the 300 code.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Roy
Mar 5, 2019 at 4:12 PM
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DANIEL SANCHEZ
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So my fuel pressure is fine when the key is clicked forward roughly 55 to 57. But when I try to start the car the fuel pressure drops down to 20 and the car won't start. I did replace my fuel pressure regulator but same issue.
Mar 7, 2019 at 9:00 AM
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ASEMASTER6371
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Got it.

You need a fuel pump.

It is in the fuel tank. The tank must be dropped to gain access.

Roy
Mar 7, 2019 at 9:21 AM
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DANIEL SANCHEZ
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Changed the fuel pump. Same issue!
I did a vacuum test on the intake manifold and it is swinging wildly from 0 to 15 inHg. It will only stay steady is I give it gas at around 20 inHg. So do you know what that may indicate?
Mar 7, 2019 at 2:29 PM
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ASEMASTER6371
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Did you verify the fuel pressure test?

If the vacuum gauge is like that, do a compression test.

Roy
Mar 7, 2019 at 2:31 PM
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DANIEL SANCHEZ
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I did re-verify the fuel pressure it was right on at 59psi. Okay, so a compression test is next. If there is no or low compression where do I go from there?
Mar 7, 2019 at 2:33 PM
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ASEMASTER6371
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Lets see what the results are before we discuss a course of action.

Roy
Mar 7, 2019 at 2:40 PM
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DANIEL SANCHEZ
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Results
Cylinders
1 - 160
2 - 160
3 - 0
4 - 170
5 - 170
6 - 170
7 - 170
8 - 155
I checked cylinder three on three different occasions and all read 0 compression. Would this be the cause of the P0300 code? If so what do I do from here?
Mar 8, 2019 at 8:20 AM
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ASEMASTER6371
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Air up number 3 cylinder and see where it is leaking from. It could be a valve, piston or something else.

The head will have to come off.

That is also the reason for the misfire code.

Roy
Mar 8, 2019 at 8:25 AM