Help Me Diagnose stuttering issue (Noob Alert)

[Axent]

Hi guys, here are the details. I've got a 99 Tahoe with the 5.7L engine. I just purchased it, it currently has 182,000 miles on it. So far I love the truck, and it only seems to have a few minor issues. I've been tinkering around with it and have got it from 7 codes down to 1.

The dreaded P0300. random misfire.

Here is what I feel while driving.

If I'm at a stop light, and baby it out of the light, the truck will stutter a little bit, when I give it more gas, it will smooth out.

If I'm doing 70 mph on the road, it will stumble. (it's very light but you can feel it) Again, if I jump on the gas, it goes away.

SunlitComet already gave me the advice of replace the spider injector in a different thread, but I'm looking for a way to confirm that this is the issue. I did buy a $50 Actron scanner.

If you've got a step by step instruction that would be greatly appreciated, as I'm a novice. I can read and follow directions, but I'm not a mechanic.

Thanks in advance.

 

[SunlitComet]

Definitely need a fuel pressure gauge on that thing to help confirm. When you have one handy let me know. I will give you some serious step by step.

 

[Axent]

Great. I have both Fuel pressure gauge and Vacuum Gauge ready to go. Here's what I've done so far.

Sprayed carb cleaner around intake and vac hoses. Got no spikes in idle.

Removed and Cleaned Mass Air Flow Sensor

Changed PCV Valve

Cleaned EGR Valve. (Per this forum in Tech info)

New Plugs/Wires/Rotor/Cap (Did not gap the plugs??)

Changed Radiator

Before I started working on it, I paid a mechanic to change the O2 Sensors. (It seems he replaced 2.)..I was under there today, it looks like there's at least 3 from me just quickly glancing around.

So what's next?

 

[Axent]

So I stopped by a neighborhood mechanic today to ask him about the misfire. He didn't check anything out for me, but he spent a few minutes educating me.

I explained the issue, that I've got a random misfire. I told him what I thought the causes might be and what I'd done so far.

Here's what I'd done.

Rotor, cap, plugs and wires, radiator, cleaned egr valve, cleaned Mass Air Flow Sensor (with Mass Air Flow Cleaner).

He said, "Lets go take a look". He took it for a spin. I waited like a new puppy at the curb.

He came back popped the hood and said "it's not a problem with the truck getting fuel, it's not your injectors, pump, or filter."...."If it were me, and I couldn't diagnose it myself, before I paid $300 for a spider injector, I'd replace these 2 things." and he pointed at the Mass Air Flow, and what I believe to be the ignition coil (it has a heat sink on it). I said, "But I already cleaned the MAF"....He said "Cleaning it doesn't make a bad one good again. It makes a dirty one clean."

He said "try this" and he unhooked the MAF cable and told me to drive it home.

It ran significantly better. Like I got another 40% of an engine better. (I didn't think it was running that bad).

Tomorrow I'll replace the ignition coil and see what happens. Maybe it'll cure everything!

 

[LEBROND]

Im having the sameeeee problem my mechanic told me that my computer is supposedly bad but didnt change it yet because i started playing the guessing game;i have changed everything just like you did and its no difference its really become irritating

 

[SunlitComet]

How about not just throwing money at it. Here is the fuel testing to do. Follow it as best as you can.
Testing and Inspection

Circuit Description
When the ignition switch is turned ON, the Vehicle Control Module (VCM) energizes the fuel pump relay which powers the in-tank fuel pump ON. The pump remains ON as long as the engine is cranking or running and the VCM is receiving reference pulses. If there are no reference pulses, the VCM shuts the fuel pump OFF within 2 seconds after the ignition was switched to the ON position or if the engine stops. The VCM will also turn ON the fuel pump for 2 seconds when the ignition is turned to the OFF position.

An Electric Fuel Pump pumps the fuel through an in-line filter to the Central SFI unit. The pump is attached to the fuel level meter assembly inside of the fuel tank. The pump is designed to provide fuel pressure above what is needed by the fuel injectors. The pressure regulator keeps the fuel available to the injectors at a regulated pressure. Unused fuel is returned to the fuel tank by the fuel return pipe.

Diagnostic Aids

Tools Required

• J 34730-1A Fuel Pressure Gauge with J 34730-250 Fuel Pressure Adapter Kit.
• J 42873-1 and the J 42873-2.

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

1. Wrap a shop towel around the fuel pressure connection to absorb any small amount of fuel leakage that may occur when installing the fuel pressure gauge. Turn ON the ignition leaving the engine OFF, the fuel pressure should be 415-455 kPa (60-66 psi) when the pump is running. This pressure is controlled by a spring pressure within the regulator assembly.

1. The fuel pressure that continues to fall is caused by one of the following items:

• • The in-tank modular fuel sender is not holding pressure.
  • The fuel pressure regulator valve is leaking.
  • A Central SFI injector is leaking.
  • Fuel line is leaking.

1. If the fuel pressure is less than 379 kPa (55 psi) at idle, it fails into the following 3 areas:

• • The pressure is regulated but less than 379 kPa (55 psi).The amount of fuel reaching the injector is sufficient, but the pressure is too low. The system will run lean, hard starting cold, no start, overall poor performance, and may set a DTC. Refer to Engine Cranks but Does Not Run. See: Powertrain Management\Computers and Control Systems\Testing and Inspection\Symptom Related Diagnostic Procedures
  • A restricted flow causing pressure drop - Normally, a vehicle with a fuel pressure of less than 300 kPa (44 psi) at idle is inoperable. However, if the pressure drop occurs only while driving, the engine will normally surge then stop running as the pressure begins to drop rapidly. This is most likely caused by a restricted fuel line or plugged filter.
  • A leaking or contaminated pressure regulator valve or seat interface may not allow the regulated pressure to be achieved.

Notice: Do not allow the fuel pressure to exceed 517 kPa (75 psi). Excessive pressure may damage the fuel pressure regulator.

1. Restricting the fuel return line allows the fuel pressure to build above regulated pressure. With the fuel pump commanded ON, the fuel pressure should rise above 455 kPa (66 psi), as the valve in the return line is partially closed.

1. When the engine is idling, the manifold pressure is low (high vacuum) and is applied to the fuel pressure regulator diaphragm. This will offset the spring and result in a lower fuel pressure. This idle pressure will vary somewhat depending on the barometric pressure; however, the pressure at idle should be less indicating the pressure regulator control.

1. If the injector(s) is not grounded, physically remove suspect injector(s) and inspect for a fuel leak while system is pressurized. If the fuel injector(s) are not leaking and fuel pressure is not holding, the fuel pressure regulator should be inspected for a slow diaphragm leak.

---------- Post added at 08:45 PM ---------- Previous post was at 08:43 PM ----------

read numbers below the charts 1-1-1-1-1-1 as 2-4-15-21-26-29. it is a glitch in page formatting.

 

[Axent]

I don't know what the pictures are. What is the picture with the red arrow? What am I supposed to do with it? What is the picture with the white arrow? What do I do with it.

Also, the bottom 2 pics that you sent (under the pic with the white arrow) don't show up. The link is broken.

 

[SunlitComet]

You got to the post to late where the pictures change. will redo it now so print it out. It is a glitch I have not found a fix for yet.

 

[Axent]

So here's an update. I took the day off today so I have some time to mess around with the truck. I decided to a new ignition control module/ignition coil. I swapped the ICM, tested it with a scan. My P0300 has now changed to ONLY a P0303. In a way, I'm oddly excited about knowing what cylinder is giving me the issues, but now it's a matter of fixing it.

I swapped the coil and rescanned, nothing changed. Still have the P0303 so I'll take the coil back to the store.

 

[LEBROND]

So here's an update. I took the day off today so I have some time to mess around with the truck. I decided to a new ignition control module/ignition coil. I swapped the ICM, tested it with a scan. My P0300 has now changed to ONLY a P0303. In a way, I'm oddly excited about knowing what cylinder is giving me the issues, but now it's a matter of fixing it.

I swapped the coil and rescanned, nothing changed. Still have the P0303 so I'll take the coil back to the store.

yep the same exact cylinder as mine..ive given up i been running seafoam alot and it seems to have been lightning up and its really just bad upon the first startup or when its cold