What causes a valve to "burn"?

[E Showell]

I remember doing a seat of the pants "grade-of-gas versus price at pump" comparison. When I used to run regular (which I no longer do). The difference in gas mileage versus cost was such that at any spread of about $0.20 gal. or less between regular (87) octane and 89 octane, it made sense for me to buy the 89 since the increased mileage offset the increased cost. At a delta of > $0.20, it was not economical from a mileage/cost standpoint.

[mecheng]

I remember doing a seat of the pants "grade-of-gas versus price at pump" comparison. When I used to run regular (which I no longer do). The difference in gas mileage versus cost was such that at any spread of about $0.20 gal. or less between regular (87) octane and 89 octane, it made sense for me to buy the 89 since the increased mileage offset the increased cost. At a delta of > $0.20, it was not economical from a mileage/cost standpoint.

Did the valve burn on your NA model? I found most have been turbos. You were using 87 octane when it happened? I didn't think the NA needs higher octane.

What made you suspect your 02 sensor is gone, these cars usually trigger a light for 02 sensors at around 100,000 miles.

[E Showell]

Yep. Burned a valve on the NA using 87 octane. Unconfirmed theory on the bad O2 sensor. What made me think that was it was increasingly good gas mileage that I could not attribute to a change in fuel -- for example going from winter grade to summer grade gas, a change in driving habits, etc. -- and I mean a significant improvement -- like an additional 4-5 mpg. I was getting 29 mpg stop and go with a manual trans. right before the valve burned. I am not aware the sensor was ever changed and the valve went at 229,500 miles or thereabouts, so it is possible I was on the original front o2 sensor at that mileage. I never got an o2 sensor code, thus my hopythesis that it failed so gradually that the car adapted to the failed state.

Something I recently read here may, however, lead me to recant. I remember the fateful day -- I had a long highway run at c. 85 mph in 4th gear. For whatever reason, I did not notice that I was only in fourth. So I had a long, hot run of say 10- 15 min. at 85 mph in 4th using 87 octane fuel. It is possible that combo generated enough heat to toast the exhaust valve regardless of any other possible contributing factors.

This may be a cautionary tale to anyone considering an "Italian tune-up." If you're going to do it, make sure you fill up with premium first to keep the combustion temperature down.

[mecheng]

@E Showell: Good info from your post, as we all know heat is the cause but the cause for the excessive heat is unclear and therefore the root cause is unknown. Maybe these cars don't like sustained high strain situations. Others have reported the problem after driving up hilly terrain.

As for the 02 sensor, I'm not sure I agree because I know my car had a lazy P0133 code when I bought it. The PO had the code between driveclean test cycles which in my area is 2yrs. When I bought the car, I tried cleaning the sensor to no avail. The ECU was smart enough to know that wasn't good enough so I changed it. Once I did, I noticed about 1-2 mpg increase which is in-line what others say and makes sense that the Engineers put the car into fuel rich mode once the sensor is faulty to prevent damage. Not the other way around. The PO did say he always used Premium.

Once you rebuilt the head, did you change the O2 sensor and notice the drop in fuel economy? I assume you still have the same engine? Was the engine burning oil on start up or while it was running?
One theory is oil deposits on the valve are the cause. Did you ever use fuel injection cleaner?

Even though the Comp ratio is higher on the NA you wouldn't think they need anything more than 87 octane as they are not producing that much heat. A turbo on a hot day on the other hand is a completely different story.

That is my theory why the O2 sensor can be ruled out; otherwise it would have happened to my car.

[E Showell]

Not burning any oil that I am aware of. It's a long story, but the engine is a new-to-me used engine after a head rebuild gone awry and yes, it got newish O2 sensors (the old replacement ones [c. 11k miles on them] transferred over) from the engine with the rebuilt head.

I generally run 2-3 bottles of Techron through the fuel system per year -- let's say one every 6-7k miles.

My mechanic warns that overly frequent use of an injection cleaner like Techron can fry the injectors, so I try to be moderate about it.

[mecheng]

I use Lucas Fuel system treatment because it is oil based and lubricates all the seals, fuel pump, etc.
How well does a oil clean, not sure but same goes for all fuel injection cleaners; they could all be snake oil. At least I know I won't be damaging/drying seals, injectors. I use it 2-3 a year.

[E Showell]

Techron is basically Stoddard solvents from what I can tell from the label.

[atucker1]

My mom's 850 had a burnt exhaust valve at 117,000. She always used 87 since that's the minimum according to the manual. I also seem to remember it had the valve stem issues (plenty of blue smoke) and nobody could ever figure out why it was burning oil.
My current 850 (185000) has zero (I mean zero) blue smoke in the exhaust, even after idling. The previous owner always used the Mann filter and took meticulous care (so I assume premium gas).
Just goes to show how important taking care of your car is. It makes a difference.

[theWIFES_S70]

What an incredible read. I am really amazed by the guys here at MVS... Ya'll are the kinds of teachers and classmates and friends I wish I'd had all throughout my life! Just read through this whole thread. If you're a newbie or thinking about a P80, read through this. Know what you're getting yourself into!

- Fuel injector cleaner, Lucas
- High Octane Fuel
- Keep up with your oil changes (preferably synthetic)

And pray that the previous owner did the same.

[abscate]

Blew exhaust number 3 on my 1999 T5 at 260k
Car was driven conservatively but Ill confess I didnt always use high test, 87 octane maybe 50% of the time.

Valves were badly coked up from leaking valve stem seals - car ran dino for first 140k until I learned that synthetic is not just expensive oil We live and we learn to do better.