> All modern engines are fuel injected, so the intake valve only ever sees filtered air.

How modern are we talking?

There are plenty of fuel injected engines in production/use today that inject fuel into the manifold [1], right before the valve.

Those valves definitely make contact with fuel-laden air, not to mention EGR[2] recycling exhaust gasses into the intake manifold for emissions/efficiency improvements.

What you're talking about is direct injection, which is becoming more and more common, but is not "the norm" if you take the cross section of total vehicles on the road today

[1] - https://en.wikipedia.org/wiki/Manifold_injection [2] - https://en.wikipedia.org/wiki/Exhaust_gas_recirculation

It is probably a typo and they should have wrote exhaust valve. It's thick enough to be an exhaust valve.

The cylinder head has both an intake valve and exhaust valve on each cylinder. One to let in misted fuel and air (delivered via the intake manifold, which is via fuel injection rail or carburetor jets), and another to allow the dispelling of combusted fuel.

That said I've seen carbon build up on intake valves too. Usually from bad valve adjustments or improper seating...bad lifters/rockers etc etc so they port isn't properly sealed in combustion. Nothing to do with being fuel injected or carbureted. It all comes via the intake manifold... The valves know absolutely no difference.

EGR systems are known to be a cause of deposits in the intake-side of an engine.

A blast of walnut shells through the engine will clear those deposits right up. You can also use seafoam as a preventative measure to burn off any excess deposits.

If you want a performance boost, you can get a methanol-water injection added to your engine which should boost hp output while burning off those deposits. Just be careful with meth, if the mixture is not present engine temps will skyrocket leading to knocking and detonation very shortly afterwards. A properly integrated fuel-cutoff system will prevent this but you must ensure your meth controller has this functionality otherwise you are taking a great risk IMHO

With the exception of some, but not all, newer GDI (Gasoline Direct Injection) engines, fuel injected engines have fuel injectors in the intake manifold upstream of the intake valves. Some GDI also have a combination of traditional injectors and direct injectors. It is inaccurate to say "the intake valve only ever sees filtered air."

https://en.wikipedia.org/wiki/Fuel_injection#Multi-point_inj...

> All modern engines are fuel injected, so the intake valve only ever sees filtered air.

Not even close.

There's two sources I can think of off the top of my head for contamination of the intake valves. Others have mentioned the exhaust gate recirculuation, but there's also crank case air that can be recirculated.

A cold engine's piston rings don't fully seal properly against the cylinder walls. They can't or else the piston could drag and scrape the wall when thermal expansion occurs. So when you start a cold engine, a great deal of pressure slips past the rings into the crank case where the engine oil is. Depending on what phase the escape occurs, it can carry gasoline, combustion gasses, etc into the crank case.

In the bad old days when emission regulations weren't a thing, the normal practice was just to let this blow by vent from the engine crank case into the atmosphere. The EPA at some point considered this not so good, and mandated that this needed to stop. The solution that was implemented was to recirculate the air from the crank case into the intake manifold. Unfortunately this means that any particulate is also recirculated as well (evaporated oil is the nasty one) and can gum up the intake valves.

Now what you're thinking about is gasoline direct injection engines, and top tier gasoline isn't going to clean the intake valves because the gasoline is injected direct into the combustion chamber. But there's still port injected engines which are still fairly common, which wash the gasoline over the intake valve and in turn wash the contaminants from blow off the valves and into the combustion chamber. Hence why Top Tier gasoline is beneficial to engines that use valve injection.

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Just a fun aside for anyone that runs a gas car; if you've ever wondered why oil change intervals are either have both a time based and a mileage based interval; it's because of that cold engine start blow by. Oil doesn't degrade sitting in an engine case, but it's performance does degrade when gasoline from a cold engine being started slips past the piston rings and mixed with the oil.

When you're running long miles, it doesn't matter as much. The engine is able to get up to full temperature, which both properly seals the piston rings, but also warms up the oil, which causes the gasoline that has gotten into the oil to evaporate and be circulated back out.

But that doesn't happen if the engine isn't run long enough to get up to temperature; if you're making short little 5 minute trip commutes every day, then the oil just keep getting contaminated more and more with gasoline. Left unchecked, the oil ceases to be able to properly lubricate the engine from the contamination, and heavy wear and tear starts occurring in the engine itself.

A proper oil change interval would take into account all the factors that effect oil life; the engine's temperature when it was started, how long the engine was run, how hard the engine is run at, etc to calculate how badly the oil has been contaminated by cold starts. For people that have an oil life monitoring system; that's exactly what it does. But what about everyone else? Do you expect people to note down the ambient temperature, and the engine temperature every time they start up the car, plug it into an excel sheet and see how much oil life is left?

Hence the need for a simple formula that could accommodate most use cases; the 3500 mile or 6 month interval. 3500 miles for people that were running their cars hard such as towing boats, RV's, etc, which wore the oil out faster then normal commuters, and 6 months for people that were just making a 5 minute commute by car every day while in the middle of winter.

FYI most modern cars account for this now. My ford of 10 years will notify when an oil change is due and it varies between 3000 and 8000 miles in my experience