96 Tahoe - P0300 Random Misfire Code

[nymiskovics]

Looking for some help. Have a 96 tahoe giving me issues. Was running fine saturday morning, left running in driveway, after about 20 minutes, I drove down road, started chugging, and MIL light came on. Shows code p0300. Clear code and it is back within two minutes. Flashes showing sever issue. Backfires at all rpm ranges. Have replaced all four HO2S within the last month in oreder to pass NYS inspection. converters are about 2 1/2 years old. replaced injection spider, plugs, wires. checked EGR, IAC, MAP. Rebuilt engine in July. Has 170lbs compression on all cylinders. re-adjusted all rockers. When I take off the upper intake, the inside is wet from fuel on the rear half of it. Fuel pressure is 60lbs. No fuel leaks. No bent push rods, inspection of valve movement shows no signd of wear on cam. I am stumped. Anyone have similar issue????? I have disconnected all H02S to eliminate computer compensating any fuel, and it runs slightly better, but still cannot drive without making exhaust cherry red.

[rodram]

... you say you have (in addition to many other things) replaced the injection spider.... if I did not know your whole litany of repair/replacements I might have guessed that your injectors were malfunctioning... these Vortecs have the Sequential Central Port Injectors (SCPI)... these can be troublesome injectors that can plug/stick due to deposit buildup.. malfunction can cause random misfire dtc code to show up

 

.. when you replaced the injection spider did you also replace the eight SCPI injectors (each with poppet valve controls on their spray tip) ?

 

.. or was your injector replacement a SCPI to MFI conversion

[[email protected]]

replace the fuel pressure regulator.it is located on the fuel injector assy. that"s where the fuel is coming from

[rodram]

...davidw450....

 

he says that his fuel pressure is 60 psi (just good enough if he is still running the OE

SCPI injection system) spec is 61 to 66 psi

 

fuel pressure regulator location varies depending on if he still has SCPI or if he has the MFI conversion system (if MFI, then the fuel pressure regulator in the new injector housing)

[nymiskovics]

...davidw450....

 

he says that his fuel pressure is 60 psi (just good enough if he is still running the OE

SCPI injection system) spec is 61 to 66 psi

 

fuel pressure regulator location varies depending on if he still has SCPI or if he has the MFI conversion system (if MFI, then the fuel pressure regulator in the new injector housing)

 

 

it is the mfi conversion i believe. There are injectors and poppet valvese at the end of each tube. The original one had just tubes. There is a small gold disk on the side with a tube pointing down. I assume that is the pressure regulator. I know that the fuel pressure will leak down over a couple of days to around 20lbs. I have checked to see if it is the injectors leaking and they are not. However, I do not see any fuel leaking from what I believe is the regulator.

[nymiskovics]

Also, when the truck is running, there is an overpowering fuel smell from the exhaust, telling me it is pouring fuel in

[nymiskovics]

... you say you have (in addition to many other things) replaced the injection spider.... if I did not know your whole litany of repair/replacements I might have guessed that your injectors were malfunctioning... these Vortecs have the Sequential Central Port Injectors (SCPI)... these can be troublesome injectors that can plug/stick due to deposit buildup.. malfunction can cause random misfire dtc code to show up

 

.. when you replaced the injection spider did you also replace the eight SCPI injectors (each with poppet valve controls on their spray tip) ?

 

.. or was your injector replacement a SCPI to MFI conversion

 

 

I replaced the injector in january 2008. It was stillunder warranty so I replaced it again last week

[rodram]

John ... yes it sounds like you are delivering massive amounts of fuel into your engine...

 

.. the original factory injectors on the Vortec will be the Sequential Central Port Injectors... this system requires 60+ psi at the fuel rail, this pressurizes the spider assembly including all of the injector tubes, and then the delivery of fuel for ignition is controlled by the poppet valves at the tip of each injector ... there is a wire that runs to each poppet valve to turn it on and off in sequence .... (problems arise when deposits build up at these injectors causing them to either stick closed, stick open, or just don't spray effectively.... )

 

.. but the MFI conversion is different (was the original factory on the 7.4L engine) ... there are no poppet valves on the end of the injectors... the fuel pressure and the fuel delivery control is all provided in the spider housing )

 

.. so which system do you have...? and if for some reason your injectors are not compatible with the injector spider housing then you won't have proper control of fuel being delivered into your engine

[rodram]

... just so I don't forget to say this later... because of this fuel problem, you may be getting some of that excess gasoline in your oil..

 

.. when you get this all sorted out, I would recommend a new oil and filter change

[kchasse]

another possibility is a bad coolant sensor telling the computer that it is cold outside, let's say -40 when it is actually 50 or 60 so it gives more fuel when it actually doesn't need it. This is just a possibility.

[nymiskovics]

Thanks for all of the help. It turned out to be internal on the distributor. Cam sensor reporting wrong info, causing timing to be way off without MIl REPORTING IT

[rodram]

.. thanks for reporting back on the diagnosis

[baunduz]

thanks for reporting back on the diagnosis