To understand the problem you have to understand the operation of the hot wire MAF sensor .Thanks...
I am going to trust you guys because this engine is touchy and has its own issues. No need to poke the bear.
You might be interested because K&N would love to hear from you.
They requested evidence to support this from dealerships and no dealership provided it.
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Briefly a thin wire inside the unit is conducting a fixed current, hence it becomes hot.
The rate of air flowing under the influence of the engine flowing through the unit cools the exposed wire and that in turn demands more current to instantly get it back to the same value.
That varying air flow & wire cooling rate is translated into a signal which the PCM can evaluate into a fueling map.
When you switch off the engine, the PCM sends a jolt of current through the sensor wire and it glows momentarily . This is the auto cleaning feature built into the unit to ensure accurate fueling upon the next start up.
Obviously then oil or any contamination on the sensing wire will affect how the engine performs when its running..
Since by itself its self cleaning, then post evidence of wire contamination may not be present under examination , unless its excessive.
In today's world of very precise engine controls, the MAF plays a vital role affecting EGR and turbo operations, any deviation can affect how the engine performs, in fact on the Sprinter it is sometimes necessary to "adapt" the air filter to the engine in question.
In the world of auto manufacturing if the K&N was superior in design & filtering capacity then all the manufacturers would be using it; but as we know fibreglass reinforced paper element has yet to be beaten in performance and durability. Which can be instantly changed out with minimal fuss & cost penalty when required at a specified point or earlier if operating in dusty conditions ,which might be a daily task in extreme conditions.
I wager the K&N demands you keep enhanced vigilance on air filtration over the standard paper factory fit ,and as we know changing or cleaning filters is the last thing an owner wants do do.
To emphasis the point. when living in dusty Baghdad it was an early sabbath day chore to clean the blasted Landrover oil bath air cleaner, before getting your cozzie for a Brit Club poolside boozy breakfast with a bucks fizz.
Then along came my new Series 3 WITH paper element air filter.
Just a few minutes to dust it off, and it was ready to go, enjoying in turn the best parking spot and poolside umbrella table at the club.
All the more reason to get with the program by using the latest and greatest, not to mention enough punctual time to pick up a busty Russian girlfriend for a day at the club .
Experience:- Russian ladies like to be cosseted with attention.
AIR FILTERS BEING FURTHEST FROM MY MIND Plus the immortal words :-
"Dennis, Not everything is so cold in Russia !"