Hi All! New member, but I have been watching posts for 4-5 months.

I have a 97 4.3 2X4 with 140K on it. Getting a P0300 code, multiple random misfire. I have done all the easy things, new cap and rotor, plug wires (2 sets to be sure) new plugs (AC) 2 sets to be sure. EGR is clean and tests OK. No detectible vacume leaks. Only mod is a K&N cold air intake (it was cheaper than the replacement OEM part that broke).

Here is what is going on. She runs fine at speed, has good power, and normal gas mileage. At idle it loads up and starts a misfire, if you sit at 2 cylces of a light (common here in So Cal) it will drop a second cylinder. When you gas it, she sputters, sometimes coughs through the intake, and once you hit ~2500 RPM she is firing on all 6 again. I have been a DIY wrench for 30 years, and this one has me baffled .

Please post any possible solutions, I appreciate any help at this point.

Alan

 

Test your fuel pressure. If it "loads up" at idle, you may have a fuel leak internal to the plenum. On the SCFI motor (96+), this would be a bad o-ring between one or more of the electronic injectors and the fuel metering body. A leak down test will let you know if there is a leak.

Another possibility is a bad EGR valve. How did you "test" it? If it is not sealing properly, it can cause excessive exhaust gases to recirculate back into the intake and cause a rich condition by displacing combustion supporting oxygen. You could test this by fabricating a temporary block off plate for the EGR valve. If the problem persists, then it is not the EGR valve.

Have you monitored the short and long term fuel trims at idle and while cruising? If it is either of those two problems, the STFT's should indicate that the PCM is trying to lean out the mixture. Idle long enough and it should start to affect the LTFT's.

 

"Quagmire"
Good word usage for the P0300 situation.

 

Removed the EGR and cleaned all the carbon out. I tested by connecting a 9V battery to the outside leads of the electrical connector. When connected, the valve opened. I also went to AutoZone and they performned the very same test successfully. Reinstalled the tested and cleaned EGR, no improvement.

Runs well first thing in the morning, as soon as she warms up, she starts the miss at idle.

The fuel trim has not been checked. I will look into that, any tips?

Alan

 

You will need a compatible scan tool to read the STFT and LTFT.

Did you actually test the EGR valve for a seal and that it accurately knows where the pintle position is...? Or just that it moves?

 

Just tested that it moved when told to.

 

Changed the ERG valve, no improvement. Found the MAP Mainifold Air Pressure sensor have a degraded grommet, enough to cause a vacume leak. Changed that, runs better but not completely eliminating the miss at idle. Have not checked the fuel trims, no tool. Where else are common vacume leak spots. Checked the resevoir, and lines, all OK.

 

Ok, I finally tracked down the last of the vacume leaks, and reset codes. Got a P0306, pulled #6 plug, wet and fouled. Put in another, still missing on 6, but the code has gone back to P0300. Exhaust smells really rich and moisture at the tailpipe, mostly condensation. Cat is now rattling. Could the bad cat be causing the issue? Do I have a bad injector on #6 that is stuck open? If so, would that cause the random miss by the O2 sensor leaning out the rest due to the rich on #6?

Defined by Webster - Quagmire: 2 : a difficult, precarious, or entrapping position

 

You could run an injector balance test if you had access to a scan tool with bidirectional controls. I would do this with the upper plenum removed and the #6 injector directed into a glass jar so you can watch the spray pattern. If the poppet nozzle is stuck open, it will dribble out rather than spray.

Here is a scenario for you. If the injector is not properly atomizing the mixture for proper combustion, is the exhaust gas from that cylinder actually read as rich? Since the O2 sensors react to the oxygen content in the exhaust gases, the incomplete combustion may result in an actual increase in O2 in the exhaust gases from that cylinder...

 

Thanks Kyle. If I have to tear it down that far, I am going to replace all 6 and be sure I don't have to tear it down again.

Good point on the O2 sensor, since combustion consumes all (almost all) the O2 in the combuistion chamber, you are correct, it would be reading higher than normal O2. That would also be damaging the cat, which is why it is now rattling, correct?