P0101 + P0300 = ?

[89Suburban]

Check the condition of the spark plugs on the misfiring cylinders. It doesn't cost a dime and will rule them out.

 

[tlynch]

I second the fuel pressure remark.
My 08 Yukon was misfiring and acting up but no hard codes. Would come and go, usually when wife was driving. Mine seemed much worse at around 1/4 of a tank for some reason. Never when it was above half, no idea why that was. Truck was pretty new so I did not suspect electrical especially cuz up here we get a lot of rain and all my previous vehicles that had plug and wire issues always showed up more in the rain. This was showing up on beautiful sunny days with less than half a tank and no codes. In “ugly” mode would mis hard, particularly under accel greater than half throttle. Once, on a whim, I filled it up right away when it happened and problem gone. That really convinced me that it had to be fuel related.
Long story short, put in a new fuel pump and problem gone.
Wish I would have diagnosed more but I was not sure how to do that, I kinda figured that if it were something obvious then the ECM would have figured that out and set a code; doesn’t it monitor fuel pressure?
Anyway, I googled and saw that fuel pump issues were not rare so I rolled the dice and got lucky.
I would definitely include a potential fuel pressure problem for your issue, it would be great to hear from anyone who has diagnosed this type of issue and what is best technique.
Good luck with yours.

Thanks for the info.

I am not able to read fuel pressure using Torque, so that aspect is a mystery.

I have new plugs, and I'll report any changes after I install them.
I also have some fuel trim graphs to post, but work and trees breaking under ice have me a little tied up.

 

[tlynch]

Check the condition of the spark plugs on the misfiring cylinders. It doesn't cost a dime and will rule them out.

Thanks for the suggestion.

 

[Fless]

Many auto parts stores have free rentals, so maybe you could pick up a fuel pressure kit. Essentially you put down a deposit on the tool, and that's refunded when you return it in good condition. Unless you take too long to return it, then you've bought it. Check around.

 

[tlynch]

Many auto parts stores have free rentals, so maybe you could pick up a fuel pressure kit. Essentially you put down a deposit on the too, and that's refunded when you return it in good condition. Unless you take too long to return it, then you've bought it. Check around.

Will do!

 

[tlynch]

Update 2/26/21:

Sorry for the gap in reporting. What with a week of freezing rain, then a transmission failure, it has taken me awhile to get back to my misfire hunt.

Up to now, the misfire has been a problem on cylinder #4, with a couple other misfires here and there. Today, I didn't bother retest misfires. I just swapped the #2 and #4 coils, and replaced the #4 plug. Once everything was together, and the battery reconnected, I now have a misfire on #6, but everything else looked OK.

Next, I swapped the #4 (was #2) coil and the #6 coil, and replaced the #6 plug. Still have misfires on #6.

It seems odd to me that the misfire moved from #4 to #6, but at least I know the issue isn't plug or coil. I'll try to swap wires tomorrow to see if that makes a difference.

 

[tlynch]

Update 5/29/21:

I finished replacing all of the spark plugs, and thinks worked pretty much the same. I had a local indie shop take a look while doing an oil change. I filled them in on the whole history.

Basically, they found nothing. Their experience is that intake manifold gaskets are more prone to leaking in cold weather. Shrinkage I guess. In April in Texas, they found no leak. They did recommend buying an AC Delco MAF if I saw more MAF errors.

Eventually I saw another MAF error, so I made sure the new one was AC Delco. I have not had any problem since then. We will see if cold weather next winter brings more woe.

I definitely want to thank everyone who chimed in with helpful ideas. I appreciate it more than you know.

 

[Rocket Man]

Update 5/29/21:

I finished replacing all of the spark plugs, and thinks worked pretty much the same. I had a local indie shop take a look while doing an oil change. I filled them in on the whole history.

Basically, they found nothing. Their experience is that intake manifold gaskets are more prone to leaking in cold weather. Shrinkage I guess. In April in Texas, they found no leak. They did recommend buying an AC Delco MAF if I saw more MAF errors.

Eventually I saw another MAF error, so I made sure the new one was AC Delco. I have not had any problem since then. We will see if cold weather next winter brings more woe.

I definitely want to thank everyone who chimed in with helpful ideas. I appreciate it more than you know.

That was good advice on the MAF. Ive heard of problems with the cheap aftermarket ones.

 

[tlynch]

That was good advice on the MAF. Ive heard of problems with the cheap aftermarket ones.

I don't drive it much, but it has been trouble-free since the new one went in. That is a welcome change.

I am happy not to spend time trying to rack misfires, swap coils, check for air leaks, and visit with he mosquitos on the driveway.

 

[tlynch]

Well, the saga continues.

The MAF code returned, so I took it to the shop again. They detected an intake manifold leak this time, as well as knock sensor errors. Both repairs were done, but I still get the MAF error. They are puzzled.

I guess whatever is the ultimate cause is indirectly tied to the intake process. Maybe the computer has a hiccup that makes it freak out about air intake values. Maybe something tied to fuel.

The truck runs OK. I'd be happier without the CEL on.