P0300 code of death

[6.0 agony]

2001 HD2500 with 6.0 is giving me an almost constant flashing SES and the P0300 misfire code. Had a buddy hook it up to a Tech II and all cylinders except 2 were showing misfires and two were showing a large number(I can't remember which ones).

 

I had already changed the plugs before the scan. I was going to do the wires next until I saw almost all cylinders were showing a misfire. So, I had them do a crank relearn, no change. Then I changed the exhaust manifold as they were leaking, no change. Then I tracked down a vacuum leak in the EGR tube to the intake which I was sure was the issue, no change except smoother idle.

 

The odd part is the truck runs fine. Except a slightly rough idle before the vacuum leak repair I can't feel any misfires. I have a cheap scanner and I am getting + % on the long term fuel trims. About 4-5% through town and a steady 9.38% at 65 mph. Also, the before cat 02 sensors, and one after cat 02 sensor is reading 400-800mvs. The bank 2 after cat 02 sensor is reading 40-120mvs. To me, that says the cat is doing it's job as there is more oxygen and less fuel, but I have no idea if that is right logic. Plus, if there was a problem with bank 1 02 sensor or cat, wouldn't bank 1 fuel trim only be affected?

 

I also made sure there was no fuel in the regulator line and the truck hold fuel pressure for several minutes and has 54 lbs at idle.

 

Stumped.

[txab]

Did you check intake manifold gaskets? Quite common to leak. Also check your fuel pressure. Key on engine off

[6.0 agony]

Did you check intake manifold gaskets? Quite common to leak. Also check your fuel pressure. Key on engine off

Thanks for the reply.

 

I didn't change the intake gaskets as I didn't get a change in idle when I sprayed it with carb cleaner, but it was brought to my attention that the inside portion could be leaking so that may be next.

 

When I turn the key I get 52 lbs of pressure and it holds for several minutes after turning the key off.

[efritz]

Intake manifold gaskets are sooooooo common and cause the 0300 code. theyre cheap too...

[Doug_Scott]

Something I was told way back when all this fuel injection stuff became mainstream was to do the basics first. By basics I mean look at it like it had points, distributor, carb and normal camshaft timing. Leave the computer parts till it passes the basic checks.

 

Not sure if you understand that high oxygen numbers in exhaust is not a good thing. It generally means not enough fuel otherwise known as a lean mixture.

 

Have you checked compression on all cylinders? Have you had one of those fuel injection service done? When you get misfires on all cylinders, you need to check the common parts involved. Intake vacuum leak is one of those things.

 

A misfire as reported by PCM(or ECM) is not like someone pulled the plug wire off. It can mean a poor fuel burn. You may not feel it, since it is not a dead spot in the running of the engine. If you are getting misfires on all cylinders, you will not feel it at all, since the feeling you get from one cylinder misfiring and 7 cylinders not misfiring is easy to feel the difference. If all eight are misfiring, you have nothing to compare the feeling to. It is kind of hard to describe what I am trying to say here.

[6.0 agony]

Something I was told way back when all this fuel injection stuff became mainstream was to do the basics first. By basics I mean look at it like it had points, distributor, carb and normal camshaft timing. Leave the computer parts till it passes the basic checks.

 

Not sure if you understand that high oxygen numbers in exhaust is not a good thing. It generally means not enough fuel otherwise known as a lean mixture.

 

Have you checked compression on all cylinders? Have you had one of those fuel injection service done? When you get misfires on all cylinders, you need to check the common parts involved. Intake vacuum leak is one of those things.

 

A misfire as reported by PCM(or ECM) is not like someone pulled the plug wire off. It can mean a poor fuel burn. You may not feel it, since it is not a dead spot in the running of the engine. If you are getting misfires on all cylinders, you will not feel it at all, since the feeling you get from one cylinder misfiring and 7 cylinders not misfiring is easy to feel the difference. If all eight are misfiring, you have nothing to compare the feeling to. It is kind of hard to describe what I am trying to say here.

Thanks for the reply.

 

My logic was the 02 sensor AFTER the cat should show a leaner mixture.

 

I agree to do the basics first, which is what I was trying with fuel pressure, checking vacuum leaks, etc. But it is frustrating that these engines have all these sensors and we still have to throw parts at it to fix them.

[6.0 agony]

Intake manifold gaskets are sooooooo common and cause the 0300 code. theyre cheap too...

Thanks. Look like this will be my next attempt.

[6.0 agony]

Update,

 

I swapped out intake gaskets and no change. I had it scanned again and cylinders 6 and 1 were misfiring the most. Swapped out 4 and 6 injectors and 1 and 3 coil packs. Still no change. Just for the hell of it I did another crank relearn and so far no light!

 

I talked to a tech and he's thinking that I needed the crank relearn after fixing the egr tube leak.