Head Scratcher.

racedvl · January 27, 2008

Hi folks.

I have a 2000 Silverado 4x4 Z71 extended cab short bed, 5.3L vin T. 127K on the clock.

A while back I got a cylinder 8 misfire that tripped the check engine lamp. Plug is whiteish, so lean. Decided to check the injectors with my trusty Ohm meter, and the looked good, but how would I know if it was clogged, so I went ahead and changed the #8 injector with a new Delphi injector. No dice. Moved the coils around. Again no dice. Now at this time I am getting a P0300 ramdom misfire code.

Also I might add, the misfire is only at idle.

I changed the intake manifold gaskets, they did not look good, but that was not my problem.

Hooked the truck up to a smoke machine, to check for a warped manifold, no smoke. No vacuum leaks.

Scanned it again, and cylinder 5 is missing a SMALL amount compared to #8.

Did a compression test on #8 (140 psi)

Did a cylinder leak down test on #8 (passed).

Also pulled the rocker arms on cylinder #8 when we did the leak down test and check the valve springs. And the looked OK. Couldn't press down on them by hand, and did not see any cracks.

So we put a scanner back on it and looked at fuel trim levels (good), and things of that nature.

While watching the active misfires, we pulled the the evap solinoid and covered the hole with a thumb. Here is where this gets fun, cylinder 8 was misfiring appx. 20 time per 100 cycles, when we thumbed the hole and started the truck, #'s 8 & 5, dropped to no more than 2 misfires per cycle.

Replaced the evap, and the misses went back up just like before. #8 around 20 misses, and #5 around 5 misses. Then we tapped the evap sol. and get this..... # 8 almost stopped missing altogether, but #5 shot up to around 20 misses per cycle!!!!!

This is at idle engine either hot or cold. One other side note.... the spark advance does jump around a little bit, but not too terrible.

Any ideas???

bobcat5544 · January 27, 2008

You have either a vacuum leak or fuel pressure is a little low.

Did you check your fuel pressure?

Fuel System Diagnosis

Circuit Description

When you turn ON the ignition switch, the powertrain control module (PCM) turns ON the in-tank fuel pump. The in-tank fuel pump remains ON as long as the engine is cranking or running and the PCM receives reference pulses. If there are no reference pulses, the PCM turns the in-tank fuel pump OFF 2 seconds after the ignition switch is turned ON or 2 seconds after the engine stops running.

The electric fuel pump attaches to the fuel sender assembly inside the fuel tank . The in-tank fuel pump supplies fuel through an in-pipe fuel filter to the fuel rail assembly. The fuel pump provides fuel at a pressure above the pressure needed by the fuel injectors. A fuel pressure regulator, attached to the fuel rail, keeps the fuel available to the fuel injectors at a regulated pressure. Unused fuel returns to the fuel tank by a separate fuel return pipe.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

When the ignition switch is ON and the fuel pump is running, the fuel pressure indicated by the fuel pressure gauge should read 379-427 kPa (55-62 psi). The spring pressure inside the fuel pressure regulator controls the fuel pressure.

A fuel system that drops more than 34 kPa (5 psi) in 10 minutes has a leak in one or more of the following areas:

- The fuel pump check valve

- The fuel pump flex pipe

- The valve or valve seat within the fuel pressure regulator

- The fuel injectors

A fuel system that drops more than 14 kPa (2 psi) in 10 minutes after being relieved to 69 kPa (10 psi) indicates a leaking fuel pump check valve.

Fuel pressure that drops-off during acceleration, cruise, or hard cornering may cause a lean condition. A lean condition can cause a loss of power, surging, or misfire. You can diagnose a lean condition using a scan tool. If an extremely lean condition occurs, the heated oxygen sensorss will stop toggling. The heated oxygen sensor output voltages will drop below 300 mV. The fuel injector pulse width will increase.

Important: Make sure the fuel system is not operating in the Fuel Cut-Off Mode. This can cause false indications by the scan tool.

When the engine is at idle, the manifold pressure is low. This low pressure is applied to the fuel pressure regulator diaphragm. The low pressure will offset the pressure being applied to the fuel pressure regulator diaphragm by the spring inside the fuel pressure regulator. When this happens, the result is lower fuel pressure. The fuel pressure at idle will vary slightly as the barometric pressure changes, but the fuel pressure at idle should always be less than the fuel pressure noted in step 2 with the engine OFF.

A rich condition may result from the fuel pressure being above 427 kPa (62 psi). A rich condition may cause DTC P0132, DTC P0152, DTC P0172 or DTC P0175 to set. Driveability conditions associated with rich conditions can include hard starting followed by black smoke and a strong sulfur smell in the exhaust.

This test determines if the high fuel pressure is due to a restricted fuel return pipe, or if the high fuel pressure is due to a faulty fuel pressure regulator.

A lean condition may result from the fuel pressure being below 379 kPa (55 psi). A lean condition may cause DTC P0131, DTC P0151, DTC P0171, or DTC P0174 to set. Driveability conditions associated with lean conditions can include hard starting, hesitation, poor driveability, lack of power, surging, and misfiring.

Restricting the fuel return pipe with the fuel pipe shut-off adapter causes the fuel pressure to rise above the regulated fuel pressure. Using a scan tool to pressurize the fuel system, the fuel pressure should rise above 427 kPa (62 psi) as the valve on the fuel pipe shut-off adapter connected to the fuel return pipe becomes partially closed.

Check the spark plug associated with a particular fuel injector for fouling or saturation in order to determine if that particular fuel injector is leaking. If checking the spark plug associated with a particular fuel injector for fouling or saturation does not determine that a particular fuel injector is leaking, use the following procedure:

Remove the fuel rail. Refer to Fuel Rail Assembly Replacement .

Reinstall the crossover pipe to the right fuel rail. Refer to Fuel Rail Assembly Replacement .

Connect the fuel feed pipe and the fuel return pipe to the fuel rail. Refer to Fuel Rail Assembly Replacement .

Lift the fuel rail just enough to leave the fuel injector nozzles in the fuel injector ports.

Pressurize the fuel system by using the scan tool fuel pump enable.

Visually and physically inspect the fuel injector nozzles for leaks.

racedvl · January 27, 2008

I was gonna test vacuum and fuel pressure today. Still might, just spent the morning plowing a changing oil in our delivery fleet. Also, that's alot of good info, thanks.

racedvl · January 27, 2008

Ok, I did a fuel pressure test and at idle I am only coming up with 47-48 LBS of pressure. And if I pull the fuel pressure regulator, it only gets to about 54 LBS of pressure.

So because I have white on the plugs, but no lean code, could low fuel pressure be my problem for the misfire?

Or do I have a low pressure problem + a misfire problem?

I am gonna disconect my fuel pressure regulator and plug the vacuum line, and see if she runs better this week.

minibike132 · January 28, 2008

You said fuel trim was "good". If its hovering at or around 0 I don't think you'll find this is the only issue.

racedvl · January 28, 2008

You said fuel trim was "good". If its hovering at or around 0 I don't think you'll find this is the only issue.

It went from +2 to -2 if I rembember correctly. Back and forth.

I did disconnect the vacuum line from the regulator and the misses seem "weaker" and less often.

Here's my little side delima, If I can't get this thing figured out, I want to just trade it in. I really don't wanna toss $$$ and parts at it, esp. if I decide to trade it. But I have only had te thing for a year, so I don't wanna end up "upsidedown" on it. So fixing it would be great. . . . . .

racedvl · January 28, 2008

Ya know what, how much can the spark advance move around? What is a safe amount? I'm wondering if I need a fuel pump and crank sensor. . . . .

bobcat5544 · January 28, 2008

Ok, I did a fuel pressure test and at idle I am only coming up with 47-48 LBS of pressure. And if I pull the fuel pressure regulator, it only gets to about 54 LBS of pressure.

So because I have white on the plugs, but no lean code, could low fuel pressure be my problem for the misfire?

Or do I have a low pressure problem + a misfire problem?

I am gonna disconect my fuel pressure regulator and plug the vacuum line, and see if she runs better this week.

Your main problem is fuel related.

Look at the fuel pressure regulator vacuum line does it look wet or smell of fuel. The 5.3L has a fuel pressure regulator problem and can cause the problem you are having.

racedvl · January 28, 2008

The regulator is not leaking at this time. With it unplugged the truck still misses, but not as bad, or often.

bobcat5544 · January 28, 2008

The nexted step is to block off the return line from the pressure regulator. If pressure goes to normal then the regulator is at fault. If it still does the same thing then it is the check ball in the fuel pump, which the fuel pump will need to be replaced.

racedvl · January 28, 2008

Ok, I will check into that, but do you really feel that could be the only problem for this misfire? or do you think there is more to the story?

bobcat5544 · January 28, 2008

If I had your truck in front of me these are the steps I would be doing on it. From what you have stated about your fuel pressure it is low and it will cause all of the problems you are haveing. And Yes there could be something else, but we must find out why you have low fuel pressure 1st and go from there.

racedvl · January 28, 2008

Awesome, thanks! I just get nervous just tossing parts at it, to see what sticks. I have done that in the past way too many times. But I guess I won't know if that is the only problem till I get that one fixed. I really aprecate the input. Thanks!

racedvl · February 2, 2008

Update.... After getting a better fuel sample, I found that I have alot of those little black "pellets" & water in my fuel. I cleaned out the rails and after a little bit it started to miss again. Took another sample & more of the same. So I think I'm gonna have to drop the tank & clean it and the rails again. Hopefully that should take care of it. I think that I'm gonna have to do that evap vent relocation kit as well.