1998 yukon Mystery p0300 and rough idle still

[OilBurner2003]

Hello again everyone, As most people know I have around 14 plus posts about this damn code, and I'm running out of ideas and options.

I have replaced, that I KNEW were bad:
Spark plugs
spark plug wires
distributor cap
Rotor
Ignition Coil
fuel pump
Fuel pressure regulator
Fuel injectors, upgraded to updated version
Fuel filter
upper Intake gasket

and much more. Mechanic of mine cleaned the MAF sensor and changed the PCV valve, stuff like that and it was doing well until I took it for a test drive. The only thing I could possibly think of is maybe the MAF sensor is going bad? I have no clue. At this point, It could be a valve, a rocker, a piston ring, or something else I have no clue about. Currently on a 30+ day wait time so I have a little while to keep troubleshooting. If you guys did all this work and were still having issues, What would your next step be? Any advice is appreciated. Keep in mind I have not done a compression test yet, I think I might get a few comments about that. Also, I have not done a smoke test for vacuum leaks. Let me know!

 

[OR VietVet]

Since I am a "basics first" kind of guy, I seem to remember on another thread telling you to do a compression test and do a wet test if needed. Next would be a cylinder leak down test if needed. No doubt some of the replaced components were needed but likely not all. When the plugs are ALL OUT, that is the time for doing the tests I have recommended. Checked for any vacuum leaks? I forget, was the cam sensor replaced?

 

[OilBurner2003]

Since I am a "basics first" kind of guy, I seem to remember on another thread telling you to do a compression test and do a wet test if needed. Next would be a cylinder leak down test if needed. No doubt some of the replaced components were needed but likely not all. When the plugs are ALL OUT, that is the time for doing the tests I have recommended. Checked for any vacuum leaks? I forget, was the cam sensor replaced?

I’m trying to find a leaks using the smoke method, cam sensor has not been replaced

 

[OR VietVet]

I am still gonna say that you need to remove the compression as a contributing factor. Post all results here. Watch you tube videos for pointers.

 

[OilBurner2003]

I am still gonna say that you need to remove the compression as a contributing factor. Post all results here. Watch you tube videos for pointers.

Looks like I’m renting a compression tester tomorrow! I should look into the leak down test, someone was showing me that it could be a non properly seating intake valve.

 

[OR VietVet]

Usually a P0300 code indicates more than one cylinder misfire because typically one cylinder will set a cylinder specific misfire code but it sometimes does not happen that way. The compression test will give you a chance to check all cylinders current efficiency. Also a chance to see if in fact is more than one cylinder and able to compare. Valve train problems can be found with low compression that does not come back up during a wet test. That typically means valve problems but the leak down text can pinpoint which valve.

I cannot say this enough. Watch a few videos if you have not done before and write it all down and do pics of what those new plugs look like and keep them in order.

 

[OilBurner2003]

Usually a P0300 code indicates more than one cylinder misfire because typically one cylinder will set a cylinder specific misfire code but it sometimes does not happen that way. The compression test will give you a chance to check all cylinders current efficiency. Also a chance to see if in fact is more than one cylinder and able to compare. Valve train problems can be found with low compression that does not come back up during a wet test. That typically means valve problems but the leak down text can pinpoint which valve.

I cannot say this enough. Watch a few videos if you have not done before and write it all down and do pics of what those new plugs look like and keep them in order.

Yessir, I’ll update you when I’m starting

 

[exp500]

Your scanner, even a cheap bluetooth will help here. For instance- torque pro on live data Pick a few sensors and monitor them on a test drive. CPS, Cam sensor, map sensor,temp sensor ....longer list.

 

[OilBurner2003]

Your scanner, even a cheap bluetooth will help here. For instance- torque pro on live data Pick a few sensors and monitor them on a test drive. CPS, Cam sensor, map sensor,temp sensor ....longer list.

Funny you mention it, I have a SCAN GUAGE and I’ve never seen the MAP sensor give me a reading.

 

[east302]

Does it give misfire counts by cylinder number?

I don’t recall for sure, but it may not throw a misfire DTC for a specific cylinder unless the counts pass a certain threshold.