Monday, Jan 28, 2008 at 00:54
Hi Matt,
Yes, I agree, but the problem here is uneven air distribution. The MAF tells the ECU how much air is entering the intake system, but not each individual cylinder. The amount of fuel being delivered to each cylinder is equal, so if you have one or two blocked intakes, they will be rich.
Overboosting will increase fuel, but the additional air helps cool the intake and combustion process and doesn’t necessarily mean that it will be rich. I’ve seen more ZD30 pistons crack from excess pressure than melt or have burn holes from excess heat.
Personally I feel that Nissans 40K air filter replacement schedule is poor. It should be more frequent, but in many cases MAF contamination comes from EGR and crankcase blowby.
The ZD30 EGR valve has 4 stages from closed to ¼, ½ and fully open. The ECU controls boost and EGR under certain conditions. Boost decreases, even under load, to increase backpressure and enhance EGR. If the EGR valve jams open, not only is boost pressure lost to the exhaust, but excessive EGR occurs under deceleration. So, not only will you get contamination, but air that has been metered by the MAF will escape through the EGR valve causing it to overfuel.
This is another problem that potentially is a time bomb, because EGR valves generally eventually fail.
Another interesting theory is that oily crankcase fumes make their way toward the air filter and promote MAF contamination when a hot motor is shut down. Fitting an oil/air separator can go a long way to reducing oil in the intake.
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