Spark Plugs keep fouling & P0172 code

[Jerome Nelson]

Brief History:​

This has been going on for years now. I made a post in 2021 and that helped solve some of the problems I was having! But now I've narrowed down a specific, reoccurring issue: fouling plugs

Reoccurring Issue:​

About twice a month I'll get a P0172 — System Too Rich Bank 1.
Rarely I'll also get a P0175 — System Too Rich Bank 2
The truck runs fine and I've replaced everything related to the code, so I clear the code.
After about 2 months, the truck doesn't accelerate properly. I also get mild vibration. No "missing" codes.
I check the plugs and the threads are about 50% "oily"/dirty. Once I clean them all the truck runs great again!

​

Tried Remedies:​

Items replaced/cleaned so far over the last 2.5 years:
• Replaced upstream 02 sensors (twice)
• Replaced MAP
• Replaced MAF (twice)
• Replaced engine coolant temperature sensor
• Replaced fuel pressure regulator
• Replaced Spark Plugs (2 years ago, Double Platinum Spark Plugs, instead of Iridium)
• Replaced Spark Plug Wires (2 years ago)
• Replaced Coils (all 8, 2 years ago)
• Replaced Fuel Injectors (1 year ago)
• Replaced Valve Cover Gaskets
• Cleaned Throttle Body (twice)
• Replaced Air Filter
• Techron Complete Fuel System Cleaner (x3 - once every 9 months)
• Seafoam (x4 - once every six months)
• Seafoam fuel additive (x2)
• Taking it to a Master Chevy/GMC mechanic and have him run diag and tests. (No conclusion.)
• Only use non-Ethanol fuel
• Added a catch can
• Reset the PCM. (disconnect battery, connect battery leads (sans battery), let sit for 1 hour)

NOTES:
It has an electronic Idle, so I couldn't clean the Idle Air Control Valve.
It also doesn't seem like it has any vacuum leaks.
We only drive it 2-3x week. ~10K miles a year
Central Florida

The only thing (that I know of) that I haven't tried is replacing or updating the PCM. (I don't believe it's ever been updated (2000).

Specs:​

2000 Chevy Tahoe
5.3
2WD
205K miles
4L60E 4-X Speed (rebuilt last year)

Questions:​

1. Any idea what is causing the fouling spark plugs?!
2. Any idea why I keep getting a P0172 code?
3. I used Double Platinum Spark Plugs, instead of Iridium. Is that causing a problem?!

I appreciate any feedback/guidance you can give!

 

[Big Mama]

Welcome to the forum from Virginia. Sorry you’ve been dealing with this so long. There are some great folks on here that can help more than I can. Do you notice any smoke from the exhaust at start up or when driving?

 

[Mudsport96]

You are only getting the rich codes? Nothing else?
Have you done/had a compression check done?
Where are you gapping the plugs?
Is it all the plugs getting fouled or just certain ones?
It could be possible after 20 years that the injectors are sticking and over fueling, but that is something a shop would have to check.

 

[15burban]

Is everything you've replaced so far oem parts? A lot of aftermarket sensors don't work very well or work for long.

 

[strutaeng]

That's a lot of new sensors. I'll ask again if they were GM Genuine or high quality replacements, especially the injectors. Correct flowrate injectors?

Have you checked fuel pressure? Maybe (for some unknown reason) you fuel pressure at the rails is higher? Sounds like you have rich on Bank 1 but sometimes also Bank 2. You need to monitor the fuel trims at various engine RPM and watch the other sensor data to try to figure out what's going on. Mechanic shop with decent diagnostics should be able to do this.

I swapped an L59 engine (from a 2004 Yukon XL) into my 99 Silverado V6. My ECU was sourced from a 2000 Silverado. My base tune was the standard LM7. The truck ran around -11% LTFT on both banks, at any given RPM range. I knew what the problem was: flex fuel injectors are like 33 lb/hr vs lm7 are like 24 lb/hr iirc. Truck ran fine, except for the fuel trims. Occasionally it would throw an O2S voltage code, which was legit. I had to change injector flow rate with HP Tuners and now it's good.

 

[Jerome Nelson]

Do you notice any smoke from the exhaust at start up or when driving?

Only some basic condensation on cold morning startups. Never while driving.

 

[Donal]

Only some basic condensation on cold morning startups. Never while driving.

Restriction in the air intake system. Maybe foreign material or oem component loose or able to move and restrict air flow and move again to clear air way. Remove everything and wiggle test. Clear opening to fender well. Run vehicle with air filter and or housing removed if possible.

 

[Fless]

WAG for something else to try -- with a scanner check the fuel alcohol content. Since you run gas only, it should be 0%. Above 10% it will overfuel due to thinking it needs more for the ethanol.

Are the plugs oil-fouled at the firing contacts, or ?? Pics of said oil on threads? Cylinder side or external side?

What are your short- and long-term fuel trims when this happens?

 

[Jerome Nelson]

You are only getting the rich codes? Nothing else?
Have you done/had a compression check done?
Where are you gapping the plugs?
Is it all the plugs getting fouled or just certain ones?
It could be possible after 20 years that the injectors are sticking and over fueling, but that is something a shop would have to check.

Thank you, Mudsport! Great questions!
No codes - except for the P0172 (and rarely the P0172). No "missing" codes, which I find odd. But, the master mechanic I took it to said that the "missing" isn't enough for it to trigger the code.
I didn't gap the plugs - they were pre-gapped. However, I just looked up the plugs I bought and one complaint was that it wasn't gapped properly. I'll pick up a gap tool and check that.
All of the plugs are fouling about the same and evenly.
I replaced the fuel injectors as well.
Thanks for your great questions!

 

[Jerome Nelson]

Is everything you've replaced so far oem parts? A lot of aftermarket sensors don't work very well or work for long.

The spark plugs & wires were AC Delco.
The injectors and coils were aftermarket. As the truck is 24 years old, I didn't want to spend the X10 price for those. However, this could be causing the problem. The diagnostics the master mechanic ran didn't show any issues though. Great question! Thanks.