[Phil Lavoie]

Hi there,

I have a 2008 xterra, off-road, manual 6-speed, with about 194,000 km.

I purchased it in March and the seller showed me the new cats he'd installed, as well as O2 sensors. I was getting a P0420 code lately and figured it was likely the downstream O2 sensor, which I am about to replace, but now I'm getting a P0300 code, and one search told me that "It’s usually too late to do much other than replace the engine when the P0300 code is thrown."

Is that true? I still have lots of power and can feel no difference since I've owned it. But I have notice that it required me to add 2 litres of oil over 3500 kms. There are no visible oil leaks. Related?

I also get a P0448 code for the evap vent - could that be related?

Replace the engine?!? This is really stressing me out! Help!

 

[kms1990]

p0300 is a random misfire this doesnt mean replace the engine it likely needs spark plugs replaced at that mileage and that can cause the p0420 code which may have also caused the previous owner to have to replace the cats as he ignored a misfire that caused raw fuel to dump into the exhaust which will burn up cats fast.

 

[Phil Lavoie]

Thanks, I’ll start with the spark plugs. Should I replace the coils, too?

When you say that raw fuel will burn up cars fast, how fast?

 

[kms1990]

Thanks, I’ll start with the spark plugs. Should I replace the coils, too?

When you say that raw fuel will burn up cars fast, how fast?

depends how much gets in, if you have a cylinder(or multiples) that are not igniting the fuel at all just driving even a few miles can do a ton of damage to them, but you would also notice that as the engine would be bucking and idling bad check engine light would likely be flashing and you would notice a severe power loss if your plugs are simply not operating effectively due to age and wear it may not be burning everything in the cylinder and a small amount is getting in. it may be worth it to do the coils but those usually dont need anything unless they do fail and since these are coil on plug the computer would throw a specific code for the cylinder that is misfiring if a coil failed.

 

[Phil Lavoie]

Well, I'm not feeling any roughness whatsoever, so I'll start with new spark plugs, clear the codes, and go from there.

Thanks a lot!

 

[Phil Lavoie]

So far, so good, but have only driven it about 10km since clearing codes. Funny thing, the 300 code wasn't in the list when I went to clear it. I thought it would have been saved until I cleared it.

 

[kms1990]

gotta go about 100-150 miles to know or in km 160-240km

 

[Phil Lavoie]

Gotcha!

btw, the spark plugs had a normal light brown colour to them. Still doesn't explain where my oil is going; since owning it for 4 months, I've had to add 2 quarts over about 2500 miles

 

[Satito]

The codes will show up as pending initially and then if they repeat over the course of of a certain distance/time it will either clear or confirm the code. If you have cleared the codes yourself they will often take one or two hundred miles to reappear as kms100 pointed out.

Worn spark plugs can still look good color wise. If they are occasionally not firing, the gas is just getting sent to the exhaust with no combustion so no carbon is deposited. If there is a rich fuel mixture entering and the pugs fire or if they are incorrectly gapped and having too weak of a spark then the gas is only burned partially resulting in carbon deposits. No spark = no fire = no carbon. (too much gas or a weak spark = partial burn = carbon)

With regard to the oil loss. Check the oil pressure sending unit. They often go bad, but only leak while the oil system is pressurized and maybe only when under load when pressure is slightly higher. When you park, it doesn't drip. My 2.4L frontier was going through about a quart every 1000 miles until I figured it out and changed the sending unit. When parked and even idling there was just a little but of oily residue off the back of the unit. It's a cheap part and really easy to swap (On Rock Auto it's under $10 if your truck only has an oil light. $40ish if your truck has an oil gauge.

I assume you have gone all the way under and checked for any dripping back by the rear main seal.

You may also want to check the wet and dry compression to make sure the rings are good. If everything still checks out it could be your valve stem seals are worn. After leaving it at least over night have a friend watch and even video the exhaust while you start it up and give it a couple of revs to see if there is any smoke and note the color and any specific odor of the exhaust.
Also check to make sure there is no oil in your coolant.

Just be patient and go through trouble shooting methodically to avoid throwing parts at it.

 

[kms1990]

Gotcha!

btw, the spark plugs had a normal light brown colour to them. Still doesn't explain where my oil is going; since owning it for 4 months, I've had to add 2 quarts over about 2500 miles

hm, that sounds like a head gasket or valve cover leak, any residue on the engine or drips on the ground? watch it in the mornings have someone else start it while you stand behind it and watch for blue smoke does the coolant look dark black?

 

[Phil Lavoie]

There are no visible oil leaks - I even had my mechanic put it up on his hoist to inspect underneath.

Also, I made sure to check for a head gasket problem when I bought it by checking the radiator fluid; I had the cap off and ran the engine and saw no bubbles.

 

[kms1990]

There are no visible oil leaks - I even had my mechanic put it up on his hoist to inspect underneath.

Also, I made sure to check for a head gasket problem when I bought it by checking the radiator fluid; I had the cap off and ran the engine and saw no bubbles.

its likely burning it then. you need to check what the exhaust is doing

 

[Satito]

There are no visible oil leaks - I even had my mechanic put it up on his hoist to inspect underneath.

Also, I made sure to check for a head gasket problem when I bought it by checking the radiator fluid; I had the cap off and ran the engine and saw no bubbles.

A bad head gasket will not always leak exhaust or oil into the coolant. It depends on where the gasket has gone bad. It can go bad between just the oil galleries and combustion chamber, which would result in oil burning and going out the tail pipe. Do you see any whitish smoke when you start or rev the engine? Bad valve stem seals will also do this.

 

[kms1990]

A bad head gasket will not always leak exhaust or oil into the coolant. It depends on where the gasket has gone bad. It can go bad between just the oil galleries and combustion chamber, which would result in oil burning and going out the tail pipe. Do you see any whitish smoke when you start or rev the engine? Bad valve stem seals will also do this.

or blue smoke white+sweet smell= coolant blue =oil