Fouling spark plugs and burning oil 2006 GMC Sierra 2500HD 8.1L

2007 GMC Sierra

Engine burning oil

This question is in regard to a 2006 GMC Sierra 2500HD (8.1L gas engine) with 197,000 miles on it. It had around 60k or 70k miles when I bought it in 2012. It has had a rough life since then – lots of hauling and towing. It has had a long-running problem of burning oil (I would have to add some oil every month), but the oil-burning has suddenly gotten much worse, and I am adding oil every few days.

Every mechanic I’ve ever taken the truck to always says “that’s just what old trucks do, they burn oil”. Another problem (which I assume is related) is that the spark plugs get ruined frequently and have to be changed. I’ve had to change them at 124k miles, 130k (wires only), 170k, 183k, and 197k. At 197k, I told the mechanic I was tired of spending so much on spark plugs, and we decided to try a cheaper plug and see what happens. This, in addition to a full tune-up/fluid exchanges, fixed the symptoms I was seeing (intermittent shudder when idling, ‘check engine’ light coming on when going uphill over 45mph, engine misfire code P0300).

But then I only made it 1200 miles before those same symptoms returned. Same P0300 code, one ruined spark plug, cylinder 2. A mechanic replaced that one plug, and I made it another 80 miles before things went south again yesterday. This time it’s 2 plugs, cylinder 2 and another cylinder (he didn’t tell me which one). So we’re replacing all plugs with the better double platinum ones.

Any ideas what is going on? Am I just doomed to replace the plugs every 15k miles from now on? I love this truck and don’t want to get rid of it. For anyone interested, there are more details below, in case they provide any additional clues. Thanks in advance!

Full Maintenance History

  • oil changes are done every 5k miles
  • oil cooler lines replaced in 2013
  • major tune-up at 124k miles (transmission flush, coolant flush, 4×4 service (front/rear differentials, transfer case), spark plugs, fuel induction system service, air filter replaced)
  • 130k, replaced spark plug wires, cleaned mass air flow sensor
  • 132k, replaced fuel pump assembly
  • 169k, code P0171, cleaned mass air flow sensor
  • 170k, replaced spark plugs, plug wires (mechanic said plugs were really bad and covered in oil)
  • 174k, replaced air filter
  • 183k, replaced spark plugs and air filter
  • 1955xx miles, replaced serpentine belt, belt tensioner, and fan clutch; mechanic said spark plugs are misfiring/worn out (but I couldn’t afford to replace them yet); mechanic said “we took off the valve covers and looked inside of the cylinder heads, in case the valves are gummed up. But we didn’t really find anything except the piston rings are a bit burned out.” He recommended trying Lucas oil stabilizer and Lucas fuel cleaner (which I did) and using premium gas and higher quality synthetic oil from now on (which I did). He said the engine is burning lean, so better fuel will help that. Told me to get a tune-up as soon as I can.
  • at 197k, ‘check engine’ light came on every time going uphill over 45mph; code was P0300 (engine misfire; no other codes); idle is very rough; replaced spark plugs/wires with cheaper copper plugs as an experiment, did fuel exchanges (differentials, transfer case, transmission (& filter), coolant, power steering fluid); cleaned the air filter
  • 1200 miles later, replaced 1 spark plug; added “Engine Restorer 8-Cylinder” to the oil, as recommended by the mechanic
  • 80 miles later (today), replacing all spark plugs with the right kind. Mechanic says it will take a while of driving at high speeds for the “Engine Restorer” to work?

I travel a lot, so I rarely get to use the same mechanic. Whenever I go to a new mechanic, they usually tell me that my last mechanic was BS-ing me and/or didn’t know what he was talking about. I’m a car dummy, so I have to just take the word of these mechanics, and I never really know when I’m getting the run-around.

Thanks to anyone who made it this far in my post!
(attached photo is the cheapo copper spark plug after 1200 miles)

Oil Fouled Spark Plug

Print a copy of your history and present it to your next mechanic, they will appreciate knowing what you have already gone through.

Chevy engines burn oil. This is true but they shouldn’t foul the plug in 1200 miles. You have something else going on.

Definitely fouling the spark plug. So the engine is going to store the P0300 – Engine Misfire code when this happens. That is normal during this situation. There are several things that can cause this code and situation.  The good news would be that you have tried a few things that tell us which cylinders are the problem cylinder.

Compression Test

The next thing to do would be to have a compression test performed. This information can sometimes explain a lot of whats going on. And this will confirm the problem cylinder/s.

Head Gasket

If two cylinders next to each other are significantly lower than the rest there is a possibility of a blown head gasket. However a blown head gasket would also be followed by white exhaust smoke and coolant loss. Since you didn’t mention any of those symptoms we will move on.

Fuel Injector

If there is specifically one cylinder that keeps fouling out the spark plug there is a chance of the fuel injector being stuck OPEN(ON). When this occurs the cylinder rings will wash out(unseat) and foul out that spark plug. After time the the excessive fuel being dumped into the cylinder with washed out rings will allow the fuel to seep into the crankcase and contaminate the engines oil. When this happens you will notice excessive exhaust smoke and start fouling out additional spark plugs.

When looking at the oil on the dipstick it may also have a fuel smell. A quick test can be to remove the spark plugs from the effected cylinders and cycle the fuel pump on and off a few times. While keeping at a distance to prevent fuel from shooting out at you, look to see if the cylinder fills with fuel. If you find the fuel injector is stuck open you will need to replace it and and then perform an oil change to remove any contaminates oil.

Hole or Crack in Piston

You didn’t mention it but i want to cover engine ping. If you experience the engine pinging when climbing a hill or under heavy acceleration you may have caused internal damage. Continually driving the vehicle under these conditions will burn a hole in the piston. If this is the case you will have 0 compression and oil foul the spark plug quickly.

Worn Valve Guides and seals

When the valve seals or guides are worn you will see a puff of smoke at start up. Sometimes lasting until the engine reaches operating temperature. This usually occurs after the vehicle has had time to set and the oil drains down past the guide seals and into the cylinders.

Engine Misfire Causes

  • Faulty spark plug (s)
  • Faulty ignition coil (s) – Engine Coil
  • Ignition coils harness is open or shorted
  • Ignition coils poor electrical connection
  • Clogged or faulty fuel injectors
  • Injectors harness is open or shorted
  • Injectors poor electrical connection
  • Improper spark plugs
  • Insufficient cylinders compression
  • Incorrect fuel pressure
  • Intake air leak