Misfire code P0305

Good morning.

Wow, lots of parts with no results.

What was the compression?

Did you check the number 5 injector with a noid light to be sure the PCM is commanding the injector to fire?

Another possible issue is mechanical. It could be a valve not moving enough from a worn camshaft or lifter.

I attached a flowchart to follow for the misfires.

Roy

I replaced the EGR valve today. Still running rough. Different now. Rough even when idling. Cam sensor then it is going into the shop so computer can be reset. Any other suggestions? I have read many questions and replies about Tahoes.

Why did you replace the EGR valve?

Resetting the computer does nothing. It will just clear the monitors and require a drive cycle to set them for emissions.

Cam sensor will not cause rough running.

You need to start with basics such as plugs, checking the fuel pressure and compression test.

Roy

Sorry. Thought you could see reply before you. Have done all that you suggested.

Okay, and the results? Iif all that is good, you missed something.

Roy

Not resolved yet. We have read many times cam sensor was a problem.

The injectors were more of an issue than the cam sensors.. If you have good spark to that cylinder but it misfires I would go to the injectors
Reference the GM bulletin below

Bulletin No.: 00-06-04-003

File In Section: 06 - Engine/Propulsion System

Date: January, 2000

Subject: Rough Idle After Start, and/or a Service Engine Soon (SES) Light
(Unstick and Clean Central Sequential Fuel Injection (CSFI) Poppet Valves)

Models:
1999-2000 Cadillac Escalade
1996-2000 Chevrolet and GMC C/K, S/T, MIL, G, P Models
1996-2000 Oldsmobile Bravada
with 4.3 L, 5.0 L, 5.7 L Engine (VINs W, X, M, R RPOs L35, LF6, L30, L31)

This bulletin is being revised to add new information to the Correction section. Please discard Corporate Bulletin Number 87-65-07A (Section 06 - Engine).

Condition

Some customers may comment on rough idle after start-up, especially if the vehicle has sat overnight. These symptoms may be intermittent. The Service Engine Soon (SES) light may also be illuminated. Current misfire, or history misfire codes may be detected with the Tech 2 scan tool.

Cause

A deposit build-up on the CSFI poppet valve ball and/or seat may cause the poppet ball to stick open or closed. In either case, the specific cylinder will be mis-fueled, resulting in a cylinder mis-fire condition.

Correction

A new injector unsticking and cleaning process has proven to be effective in restoring poppet valves to an "as new" condition. Injector replacement should NOT be considered as a correction for this customer concern.

The first step in this process is to make sure that the poppets are operational. The cleaning procedure will be ineffective on a poppet nozzle that is stuck closed. The J 41413 Evap Pressure/Purge Station, in conjunction with the J 44466-10 pressure regulator/ hose assembly, provide the required high pressure (150 psi) source to perform the unsticking procedure.

The J 39021 fuel injector tester and accessories provide the means to "energize" the injector. Some later model-year vehicles may utilize the Tech 2 to "energize" the injectors.

The second step of this process is to run the engine on a solution of 5% Top Engine Cleaner and 95% gasoline. This will effectively clean any deposits from the ball and seats of the poppets. It is very important that the engine fuel system is separated from the vehicle fuel system. Top Engine Cleaner may have detrimental effects on the fuel pump.

Parts Information

Part Number Description 12345382 Threadlock 1050002 Top Engine Cleaner (U.S.) 992872 Top Engine Cleaner (Canada) 12345104 Port Fuel Injector Cleaner (U.S.) 10953467 Port Fuel Injector Cleaner (Canada)

Parts are currently available from GMSPO.

Warranty Information

For vehicles repaired under warranty, use:

Labor Operation Description Labor Time J5645 Injector, Fuel Clean 1.3 hrs S/T J5645 Injector, Fuel Clean 1.0 hr G, M/L, P J5645 Injector, Fuel - Clean 01.1 hrs C/K

Diagram/instructions for replacement are attached below

I have a misfire first thing in the morning and as it warms up the misfires goes away. I have replaced all the plugs, wires, cap, rotor, entire fuel injectior system with pressure regulator and performed a compression check on the left bank, between 160-165 psi, I noticed their is oil residue on plug #5, can that be the problem, if so why is the compression ok on that cylinder, could oil be leaking through the lower intake manifold gaskets or could it be a valve guide issue?

it could be, did you check for any vacuum leaks. this can cause misfires also

misfires on cyl 3 and 7. we have already replaced entire distributor and all spark plugs and wires. what next?

check compression. if ok then its a fuel injector or a vacuum leak at the intake manifold gasket. both are common on this engine. to determine you will need to do a fuel injector balance test.

My 2002 chevy tahoe was running fine yesterday, but today will not hold an idle then shuts off. It gives a Po300 multiple misfire code. Could this be caused by bad fuel? We had just fueled up.

Mutiple/Random misfire are usually caused by vacuum leaks and bad fuel.

Intermittent p0300. It is a flex fuel. I can drive it for a couple of days with no problem. Next time I drive it, flashing check engine light and is missing. Only after it gets hot and at highway speeds seventy plus mph around 2000 rpm. I changed plugs and wires, all Delco. Checked fuel pressure, good 58-60 psi. Checked intake for vacuum leaks, none found. Checked and cleaned all engine and body grounds. Checked coil and injector harnesses for shorts. Cleaned throttle body and did an upper air induction cleaning. Removed PCM plugs to check for corrosion, all clean. All coils and injectors are firing. Also, both upstream O2 sensors have been replaced. I have been out-of the business for a few years now and do not have the luxury of using a nice scan tool. Could a crank sensor relearn or possibly replacing crank sensor do the trick?

Hello Brian,

This sounds like a classic MAF problem. You might be able to clean it and have some luck but if not you must replace it.

The problem is that the sensor is going out but not enough to trigger a check engine light.

Here are a could of guides that should help you get the problem fixed.

https://www.2carpros.com/articles/mass-air-flow-service

and

https://www.2carpros.com/articles/how-to-replace-a-mass-air-flow-sensor-maf

Please let us know what happens.

Best, Ken

I have already changed out plugs, wires, 2 of the o2 sensors, and fuel filter. I am running out of ideas could you please help me, also the number 5 spark plug was the most corroded if that means anything.

A code P0300 may mean that one or more of the following has happened:

•Faulty spark plugs or wires
•Faulty coil (pack)
•Faulty oxygen sensor(s)
•Faulty fuel injector(s)
•Burned exhaust valve
•Faulty catalytic converter(s)
•Stuck/blocked EGR valve / passages
•Faulty camshaft position sensor
•Defective computer

If you see more corrosion on cylender 5 check to make sure the compression is good. ALso, make sure to check the things listed above.

Let me know what you find.

Joe