What causes a valve to "burn"?

[cn90]

I have an "academic" question for the gurus...

Let's say one exhaust valve (#3) is burnt and one does not have the money/time to fix it.
I am thinking......how about disable one cylinder by:
- D/C the connector to fuel injector #3, this way fuel is not injected into the cylinder causing flooding, ? hydrolock etc. etc.
- D/C the spark plug wire #3 (or ignition coil #3 in direct ignition system), this way there is no spark in cylinder #3.

The bottom line is: there is no activity going on in cylinder #3.

When you think about Honda VTEC engines have the VCM function that can turn off 2-3-4 cylinders during cruising to save fuel.

So the idea of disabling 1 particular cylinder in an engine with burnt valve should work? Yes, No?

[erikv11]

Yes.

This is not really an academic question, it is more a point of standard procedure IMHO. I've done it and if you read through some of the burnt valve posts, it is posted all the time here as an interim solution: unplug the injector, it is fine to drive it around until you can make time to fix it. People have posted about unplugging the appropriate injector and then driving 400 miles to get the car home, etc.

Performance goes down and I wouldn't recommend starting a cross-country tour, but not as bad as you might think. I did notice that performance was actually a little better with the injector plugged in - maybe there was enough compression in the case of my car that there was still some combustion. But regardless, still better to unplug the injector and save the cat from having unburnt fuel dumped into the exhaust.

[cn90]

Question for those who clean the injectors:

- How do you know you need F.I. cleaning service?
- Where do you send them for cleaning?
- Pricing?
- Anyone DIY at home?

[erikv11]

How you know: Aside from leaking, best evidence I can think of is a misfire code that follows an injector and isn't due to electrical. Or a leaky injector.

Where and pricing: www.cruzinperformance.com.

At home: implausible unless you have a sonicator.

Side note: any good cleaning service will test flow rates before and after cleaning, and give you the output. I don't know that I've ever seen an injector that flowed poorly before cleaning. But you get new pintle caps and o-rings and should do the seals too. So the usual assortment of pros and cons.

[cn90]

Some food for thoughts:

1. If oil leaking past valve seal is a potential cause of burned valve, then I have this interesting observation for you guys:
- My car has 150K, if I use 5W30 Oil, oil consumption is about 1qt/1,500 miles.
- I recently switched to Mobil1 15W50 Synthetic Oil, oil consumption is about 100cc/2,000 miles. Hardly noticeable oil consumption. Fuel mileage unchanged.

Anyway, from now on, I will do this:
- Summer: Mobil1 15W50 Synthetic Oil.
- Winter: Mobil1 10W30 Synthetic Oil (instead of my usual 5W30). The engine rebuilders tell me the "modern 10W30" has similar viscosity as 5W30 from 10 years ago.

2. Re: Octane.
Let's say 10 gallons of Octane 91 costs $40 ($4.00/G).
Let's say 10 gallons of Octane 87 costs $36 ($3.60/G).

Does anyone know the "energy density" in each Octane (87, 89, 91)?
My point is: although it appears that Octane 91 is more expensive, but if the car gets more mileage out of it than Octane 87, then the $ per mile travel is similar, then it makes sense to stick to Octane 91 because:
- ? less chance of burned valve.
- does not cost you more (this remains to be proven).

We need some petroleum chemist here to teach us.

[abscate]

You would need a (SWAG) 10% increase in mpg to make the economics work (setting aside the valve benefit for a moment) whereas the real mpg increase will be <2% typically.

[erikv11]

...
Does anyone know the "energy density" in each Octane (87, 89, 91)?
My point is: although it appears that Octane 91 is more expensive, but if the car gets more mileage out of it than Octane 87, then the $ per mile travel is similar, then it makes sense to stick to Octane 91 because:
- ? less chance of burned valve.
- does not cost you more (this remains to be proven).

We need some petroleum chemist here to teach us.

IMHO this is one of those questions one may ask hypothetically, basically just because it is possible, but never would seriously. In other words, hydrocarbon chemistry aside, there are a lot of things to tinker with in a turbo S70, but octane levels is not one of them (LPT or HPT). Run 91 and you don't have to worry that your knock sensors are working, and that you are exacerbating the poor heat transfer that comes from leaky valve stem seals. Or run 87 and speculate. No-brainer in a high mileage engine.

You gotta love the mix in online forums of "hey it's just a few more bucks, why not go with it?" and "we can save another $1.47 every month with this mod, it is a requirement!"

[j-dawg]

- My car has 150K, if I use 5W30 Oil, oil consumption is about 1qt/1,500 miles.
- I recently switched to Mobil1 15W50 Synthetic Oil, oil consumption is about 100cc/2,000 miles. Hardly noticeable oil consumption. Fuel mileage unchanged.

i've noticed the same - from about 1qt every 750 miles to 1qt every 1500 or so, having gone from 5w30 to 10w40. only about half the consumption, but i didn't want to go any more viscous.

2. Re: Octane.
Let's say 10 gallons of Octane 91 costs $40 ($4.00/G).
Let's say 10 gallons of Octane 87 costs $36 ($3.60/G).

Does anyone know the "energy density" in each Octane (87, 89, 91)?
My point is: although it appears that Octane 91 is more expensive, but if the car gets more mileage out of it than Octane 87, then the $ per mile travel is similar, then it makes sense to stick to Octane 91 because:
- ? less chance of burned valve.
- does not cost you more (this remains to be proven).

We need some petroleum chemist here to teach us.

i am no such thing, but in my old nissan maxima (which called for premium) i did compare cost per mile, and it was just about a wash for exactly the reason you describe. i figured if the money i spent for a mile on premium was the same as the money spent for a mile on regular, i might as well buy premium and have the power.

[E Showell]

I'd be interested in hearing what percentage of burned valves occurred on cars using synthetic oil. It strikes me as being just possible that given the generally lower viscosity of full synthetics of the same grade (I'm making an assumption here based on some recent reading) as conventional oil, some of the thinner viscosity synthetic might squeak by valve stem seals and migrate onto valves causing localized hot spots and burning.

Is there any way that this theory (and it is just a theory) could be correct?

[cn90]

I don't know of any study, but my "common sense" tells me to use thicker oil (10W30, 15W50) in a car this age/mileage (15y/150K).

All I know is that: since I switch to Mobil 1 synthetic 15W50, oil consumption is down to almost nothing.
Engine runs smooth like a baby.