Ok so first off thank you to anyone taking the time to read this.

I am a new member, first post, and have owned a handful of chevy's with the 3.8l engine. Currently I have a 2000 Monte Carlo and a 2003 Impala both with the 3.8l.

Here is my problem with no details spared lol

My 2000 monte is my daily driver. Random one day started misfiring, pulled code P0300. I have my impala so swapped ignition module and fixed the problem. I got a junk yard ignition module to put in the impala, I start it up and still P0300. Swapped out the ignition module at the junk yard and yet again still P0300. Perplexed, I swapped ignition module back off the monte and reinstalled original ignition module in the impala, still P0300. Meanwhile I installed junk yard ignition module in the monte and have been driving it fine daily.

Since this happened about a month ago in my free time ive been swapping parts and trying to chase this problem. So far I have:

-Swapped all coil packs
-Swapped IAC
-Swapped MAF sensors
-Checked spark and have good spark to the wires
-Made sure all wires and plugs are good
-Did a battery disconnect to reset computers

None of these did a thing each time P0300 comes on at idle in park and then once in gear the car shudders badly, when driving it actually feels fine at higher speeds and can hardly tell its misfiring, but back down at idle it gets very rough again. I havent driven it long enough to get it hot but its not stalling just misfiring. I dont hear or see any vacuum leaks.

Problem now is my family needs this car as a daily driver again and its really got me stumped because i simply swapped ign modules and now developed this problem, one would think its something in the area i touched. But any help or ideas would be greatly appreciated. My next part thoughts would be cam sensor maybe ...... i honestly dont know.

Thanks again.

--Joe--

 

P0300 I believe is a random mis-fire could, so tracking down things that would affect multiple cylinders (the ICM) are good options. You may want to inspect your vac lines and also remove the vac line from the fuel pressure regulator. If you have wet fuel in that line, the regulator is bad and should be replaced ASAP.

You can also have the auto parts store bench test the ICMs.

 

PO300
PO403
PO405
PO446 ✴︎ ⬅︎ ⬅︎ ⬅︎

All these Codes were triggered by a faulty FUEL TANK PRESSURE SENSOR!

Wait for the Low Fuel Warning before attempting to DIY.
Start engine.
Pull FUEL PUMP Relay ... Let Engine stall to relieve pressure in system.
Remove GROUND Wire from Battery.
With car Raised and SAFETY STANDS securely in place.

Support Tank with Jack.

(I made a "Jig" to attach to the Jack. Sawed the square bottom off from Milk Crate. Drilled holes through Hockey Puck. Mounted Puck to center of Milk Crate with Heavy Duty Zip Ties. Affixed carpet to other side of Milk Crate bottom for padding.)

Lower tank enough to access Sensor atop Fuel Pump.
Unfasten Pigtail from Sensor.
Unclip and Remove Sensor ••• Straight-up.
DO NOT BREAK THE END OF SENSOR OFF INSIDE PUMP!!!

If you need a Charcoal Canister do it when you replace this Sensor.

Removing Fuel Pump Relay to Relieve Fuel System Pressure throws a Code which may require a MIL Light reset.
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