po 300 random misfiring

[gchc1ff]

I have a 2000 Pontiac Bonneville SLE with 180,000 + miles. With re occuring po300. replaced timming chain and gears. upper intake gasket, throttle body gasket, cam and crank sensor, ignition module, plugs and spark plug wires. Any thoughts?

[crash93ssei]

Have you checked your coils?

[gchc1ff]

Yes have a new coil and swapped in all poistions no change.

[crash93ssei]

Have you checked your fuel pressure, and is that the only code you ever get?
Does the engine feel like it has normal power across the RPM range?
What spark plugs did you use?

[gchc1ff]

No I haven't checked the fuel psi yet. Yes po300 is the only code I get. The engine is fine up to 2000 rpm the craps out till 2500 rpms then at 3000 rpms is maxs. I used AC Delco plugs.

[crash93ssei]

Have you checked out your catalytic converter yet? It could possibly cause a misfire if it is plugged bad enough, and I have had them plug to where the engine won't rev past 3,000 RPM.

Other then that, I would check your fuel pressure, and pull the plugs to see what they look like.... super white, black, full of soot, etc..

[gchc1ff]

Disconnected cat. no change. Pulled plugs, they looked good, not fouled or super white.

[crash93ssei]

How does it run at idle, and does the engine have any power up to when it starts misfiring?

[gchc1ff]

Also replaced fuel filter, was clean.

[crash93ssei]

Well, my last idea is to check for a big vacuum leak and check the fuel pressure. Any chance the brake booster hose is not hooked up?

[gchc1ff]

The brake booster is hoooked up. I am going to inspect the lower intake gaskets for vacuum leak and or coolant in the intake. Note this car has had no trouble since 50,000 miles, until now. Just replaced the factory water pump, since it was off for timming chain and gear replacement.

[agrazela]

I am going to inspect the lower intake gaskets for vacuum leak and or coolant in the intake.

Random misfire is sometimes a symptop of leaking LIM gaskets.

[gchc1ff]

I replaced the lower intake gasket. Also replaced upper intake. Replaced fuel pump and regulator. Now it won't run unless the gas pedal is floored. Any new ideas?

[golfyeti]

your symptoms sound similar to when I had a bad fuel injector. I too replaced many ignition parts before having someone find the bad injector as the culprit (found while the LIM gasket was being replaced).

[gchc1ff]

Still have po 300. Have replaced cranck sensor, cam sensor, timming chain and gears, cam interupter magnet, upper intake manifold, lower intake gaskets, igniton module, fuel pump and fuel psi regulator, plugs and wires, fuel filter. Anyone have any ideas?

[Mechanical Mike]

See if it runs better with the MAF sensor unplugged.

[gchc1ff]

I forgot to say the mass air flow sensor was replaced and map sensor. With the mass air flow sensor unpluged it runs worse.

[myfirstbonnie]

I replaced the lower intake gasket. Also replaced upper intake. Replaced fuel pump and regulator. Now it won't run unless the gas pedal is floored. Any new ideas?

What has been done since this?

Why did you do all the work in the original post? Was it because of the P0300 or other reasons?

Verify the spark plugs are connected to the coils correctly. Do not trust the markings on the coils, chase each cylinder back to the coils.

Get a can of carb cleaner and spray around all the vacuum hoses, LIM, and UIM to look for vacuum leaks.

Next, check all of the spark plugs for good spark, pull each plug wire and use an old spark plug to see if each cylinder has good spark.

Have you reset the codes and verified no more have come up?

[gchc1ff]

The reason all the work was done is the po 300. I have checked the plug wiring and coils. The spark is good, used a spark tester for dis ign. Checked for vacuum leaks with carb cleaner. Have reset the check engine light and it returns.

[gchc1ff]

I found out today that cyl. 2 and cyl. 4 where miss firing. Pulled plugs and were carbon fouled. The injectors were pluged. Ordered new injectors and will replace pluged cat. Will let you know if it is fixed.