1999 5.7 350 vortec P0304 cylinder 4 misfire

[socal k1500]

Due to your fuel trim readings being around 15 percent I'm leaning towards your intake manifold gasket. I'd check fuel pressure and smoke test before though.

to bring an update to you guys for future people who may have this issue i bought the upgraded injectors and intake manifold gasket and had a shop install them, I notice a difference but sadly the truck still misfires on cylinder #4 going up hills around the same RPM range.

The shop also performed a compression test on #4 and its good. They also checked fuel pressure.

I replaced the o2 sensors they were fried probably OEM.




My next step today at the advice of a mechanic buddy is to install a new crankshaft position sensor. Shot in the dark at this point im starting to lose hope.

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i pulled spark plug and checked wire they both look good.

 

[socal k1500]

Crank shaft sensor was pretty worn looking. But no scratches or knicks

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[HotWheelsBurban]

to bring an update to you guys for future people who may have this issue i bought the upgraded injectors and intake manifold gasket and had a shop install them, I notice a difference but sadly the truck still misfires on cylinder #4 going up hills around the same RPM range.

The shop also performed a compression test on #4 and its good. They also checked fuel pressure.

I replaced the o2 sensors they were fried probably OEM.




My next step today at the advice of a mechanic buddy is to install a new crankshaft position sensor. Shot in the dark at this point im starting to lose hope.

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i pulled spark plug and checked wire they both look good.

That plug wire is not touching anything it shouldn't? Usually the #5 and #7 are the worst for this, but the other cylinders can do it too.
Also have you checked the distributor cap and rotor for cracks or carbon tracking? These can also cause a misfire.

 

[socal k1500]

That plug wire is not touching anything it shouldn't? Usually the #5 and #7 are the worst for this, but the other cylinders can do it too.
Also have you checked the distributor cap and rotor for cracks or carbon tracking? These can also cause a misfire.

I replaced the distributor with an aftermarket and made sure cam retard was in spec after i did it. The rotor that came with it was bad and failed on the freeway so the shop installed one.

just for the hell of it after doing some reading apparently some people have had issues with ACDELCO wires that im using so i swapped all the plugs and wires. They all looked normal

i replaced the PCV valve it was pretty worn. Cleaned the EGR valve and made sure it was functioning. Cleaned the IAC valve and made sure it was functioning.

Next step this weekend is a proper vacuum test with a smoke machine a buddy has. But it just baffles me how something like that could cause a misfire in the same cylinder every time at the exact same rpm range. Im also borrowing a more expensive ob2 scanner from a co worker so ill post another freeze frame from the latest.

i found this bulletin on another chevy forum which sounds exactly like my problem.

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[socal k1500]

That plug wire is not touching anything it shouldn't? Usually the #5 and #7 are the worst for this, but the other cylinders can do it too.
Also have you checked the distributor cap and rotor for cracks or carbon tracking? These can also cause a misfire.

Welp got it all back together, drove it the nearest hill, replicated the conditions as last time and got misfires

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[socal k1500]

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[SableSlayer]

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Your long-term fuel trims are still a little high. Which intake gasket did they replace? Upper or lower? When mine was doing this I chased it forever just like you have. I ended up finding that the lower intake gasket was collapsed causing an internal vacuum leak

 

[socal k1500]

Your long-term fuel trims are still a little high. Which intake gasket did they replace? Upper or lower? When mine was doing this I chased it forever just like you have. I ended up finding that the lower intake gasket was collapsed causing an internal vacuum leak

They just did the plenum gasket when they did the spider upgrade. I mis spoke earlier

 

[socal k1500]

Your long-term fuel trims are still a little high. Which intake gasket did they replace? Upper or lower? When mine was doing this I chased it forever just like you have. I ended up finding that the lower intake gasket was collapsed causing an internal vacuum leak

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The people I bought the truck from let the truck sit outside on the street for long stretches, I’ve heard that can make the lower intake gasket go bad. Will a vaccum gauge hooked up to a vaccum line show this ? Or can I visually check ?

 

[SableSlayer]

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The people I bought the truck from let the truck sit outside on the street for long stretches, I’ve heard that can make the lower intake gasket go bad. Will a vaccum gauge hooked up to a vaccum line show this ? Or can I visually check ?

Okay if you've done the smoke test and didn't find any leaks then I highly recommend you replace the lower intake gasket