Tahoe started to run weird!

[hertahoe]

OEM or equivalent! Just be sure they are genuine. Plugs are a hot item to have been counterfeited. Basically, if they're really cheap, they're likely knock-offs. Best case, they just wear out quickly. Worst case, they fall apart and cause major engine damage.

I'm not totally sure, but I think Gen 3 and Gen 4 engines take the same plugs. I put NGK Iridiums (TR5IX) in mine when I refreshed/upgraded the engine in mid-2020. Should be good for 100K miles at least.

Plug one is a SOB to get to...It had some caked on residue on the ground electrode. Not much, but it shouldn't be there. No oil residue on the outside of block to speak of. The engine bay is very clean. I had the NGK's as well. Maybe I will get the ACDelco 41-962 or just stick withNGK

 

[hertahoe]

Taking a little longer to fix the Tahoe since both my kids came down with Covid...11 and 15 and they have really sore throats, headaches and coughs. I changed out #1 with new NGK 7397 and it threw P0300 (multiple misfires) So I put the rest in thinking the others were bad, which they were in need of change. Cleared codes and started it up. Idles slightly rough. So I take it for a drive around the block and yellow check engine light is flashing and exhibiting same signs as before. Get back and scan says no DTC's. So I unplug scanner (Innova 3011), scan again, and no codes. I was going to switch out coil from #1 to #3, but I think that might be a waste of time now. Why would it throw P0300 after changing out one plug? Wouldn't it throw a code if the check engine light is flashing?

 

[MassHoe04]

P0300 can often be the result of fuel issue, rather than spark. You may have a clogged fuel injector.

Here is a good explanation on possible causes.

https://www.carparts.com/blog/p0300-code-random-or-multiple-cylinder-misfire-detected/

Let's hope it is something easy like a clogged injector.

Try running some good injector cleaner through the next few tanks.
Chevron Techron Complete Fuel System Cleaner or other brand that contains polyetheramine (PEA) , might be worth trying.
Not that Seafoam is a bad product... You just need something containing PEA for injector cleaning.
It is cheap and worth a try!

 

[Fless]

Smelling gas... get a scanner on it and document the long- and short-term fuel trims.

 

[hertahoe]

P0300 can often be the result of fuel issue, rather than spark. You may have a clogged fuel injector.

Here is a good explanation on possible causes.

https://www.carparts.com/blog/p0300-code-random-or-multiple-cylinder-misfire-detected/

Let's hope it is something easy like a clogged injector.

Try running some good injector cleaner through the next few tanks.
Chevron Techron Complete Fuel System Cleaner or other brand that contains polyetheramine (PEA) , might be worth trying.
Not that Seafoam is a bad product... You just need something containing PEA for injector cleaning.
It is cheap and worth a try!

I doubt I could go through a tank of gas with how it drives currently. Still wondering why my reader doesn't pick up any codes after I cleared p0300?

 

[MassHoe04]

My Zurich scanner from Harbor Freight had a sub-menu, that I didn't know was in there until just recently.
On mine, it had a menu labeled "GM OEM Enhanced".
I didn't see it before and didn't know what it was, so I clicked on it.

Looked like it had every code ever triggered.

I was lucky to stumble on that sub-menu!
I had intermittent ABS light with chimes pinging at me every time I got up to 55+ mph.
No codes. No CEL. Just the ABS light and the chimes coming on; then shutting off when I slowed down.
The GM OEM Enhanced sub-menu was where I actually saw a fault code indicating a problem with the left front ABS sensor.

That was the only place there was any information on faults recorded, but not tripping the CEL or main codes picked up at the top level of the scanner.
I replaced the one bad sensor and my problems were finally resolved.

Maybe your scanner has the same sub-menu to let you dig deeper into faults recorded by the computer, but not tripping main codes when you do the basic top-level scan.
Start poking around the menus and buttons on your scanner to see if you do.
Worth a shot...

 

[rockola1971]

A flashing check engine light while engine is running means you have active spark knock (AKA Preignition/Misfire). With 136k on the clock I would suspect bad spark plug wires. Start the engine up at night and pop the hood and see if you can see any spark hopping to metal from the spark plug wires. Likely its time for a replacement set of wires and plugs couldnt hurt. A ignition coil either cracked and jumping spark directly to ground (metal) or is getting ready to die (they both yield the same result which is misfires). The unburnt smell of fuel is the dead giveaway. Spark is not getting to 1 or more plugs late or not all intermittently.

 

[Rocket Man]

Have you had the cats checked to see if they’re restricted?

 

[hertahoe]

Have you had the cats checked to see if they’re restricted?

I just drove it around the neighborhood and it threw P0300 and its smelled like cats backed up. So should I try the lacquer thinner fix?

 

[hertahoe]

A flashing check engine light while engine is running means you have active spark knock (AKA Preignition/Misfire). With 136k on the clock I would suspect bad spark plug wires. Start the engine up at night and pop the hood and see if you can see any spark hopping to metal from the spark plug wires. Likely its time for a replacement set of wires and plugs couldnt hurt. A ignition coil either cracked and jumping spark directly to ground (metal) or is getting ready to die (they both yield the same result which is misfires). The unburnt smell of fuel is the dead giveaway. Spark is not getting to 1 or more plugs late or not all intermittently.

The wires looked good when I removed plugs. They are MSD red and didn't see any issues. I will try your suggestion later tonight. I just replied to OP and I got P0300 and smelled liked cats do when backed up.

 

[Rocket Man]

I just drove it around the neighborhood and it threw P0300 and its smelled like cats backed up. So should I try the lacquer thinner fix?

I don’t know what that is but if they’re bad then they need to be replaced. A shop can do a flow test or there are other ways to check. And if your CEL is flashing, you’ll do engine damage if you keep driving it.

 

[MassHoe04]

Once you start approaching the 150k+ mile range, the days are numbered for converters.

This was the cat on my 05 Jeep 4.0L... The chunks you see in there were ceramic material from the mini-cat up front on the collector pipe. We don't have those on the GM trucks, but look inside the main converter in my hand.

Behind the chunks from the mini-cat, most of the main ceramic honey comb was clogged. I don't think lacquer thinner or Cataclean would have helped much!

 

[gooffeyguy]

That's when you take a piece of rebar and start gutting that cat, lol

 

[Rocket Man]

That's when you take a piece of rebar and start gutting that cat, lol

Not unless you can tune out those downstream O2 sensors.

 

[gooffeyguy]

Not unless you can tune out those downstream O2 sensors.

Or plug in some MIL Eliminator (basically a resistor on the O2 plug).

 

[iamdub]

Not unless you can tune out those downstream O2 sensors.

Or plug in some MIL Eliminator (basically a resistor on the O2 plug).

Or the spark plug non-foulers. I've seen some purpose-made variants that use steel wool in them to better filter the exhaust from the sensors.

 

[hertahoe]

So I swapped coil #1 to #3. Idled a little rough and drove it to a street that I could accelerate and it threw P0303. So that means I have a bad coil! Going to store for one to see if it solves the issue.

Edit - It worked. Thanks for all the assistance.