00 V70 XC SE What's up with the security screws on the MAF?

[instarx]

All the parts stores sell the bit sets also for around $10. OEMs /suppliers used tamper-proof on critical systems-critical being the ones the Governments required, like emissions control equipment.

Ahhh, now that makes a lot of sense. The MAF directly effects emissions, so it may have to be tamper resistant by law.

[dosbricks]

I went with an AIP Electronics MAF, which I chose for price.

I do recognize the importance of OEM when the parts are safety related, but a MAF is just no big deal....

MAF is telling your ECM the amount (quantity) of air being pulled into the manifold and using that info the ECM sets the fuel/air ratio and fine tunes it based on the feedback from the O2 sensors. Pretty critical device, especially considering the very tight parameters of a Volvo.

Good luck. I tried the aftermarket route once. It was money wasted in the long run.

[rspi]

The '94 - '97 cars have the T20 6 point security bit but the '98 has that funky 5 point bit. I got one from Pepboys for about $5.

[Atis]

A MAF's sensor plate is a precisely fitted/designed device to the actual housing (i.e. plastic tube) as it is very sensitive to airflow. If the mechanic messes around with it, it can goes out from tolerance.
It could be that you will not notice it during driving, but the measured values will be off. In case of constant engine rpm, the O2 sensor will compensate it, but during non-steady state, your engine will run too much lean or rich.

[misha]

Atis....modern Maf sensors are actually electronic microchip which is fitted inside of the insert.

You can only fit it one way...you can't go wrong.

There are no settings and no tolerances which can be disturbed like on mechanical maf of previous generation.

I disassembled one of those few years ago and i precisely know what is inside of that insert.

[Atis]

I know them well...
In our MAFs there is a ceramic plate for the sensor and the compensating resistors, (inside the airflow) and another ceramic plate with the circuits (inside the plastic part). What I was thinking is that it could be that an over tightening can damage the ceramic plate or result a non optimum placement of the resistor. I mean fractions of millimeter. They are REALLY sensitive for the correct placement inside the tube.
Beside these (as it was previously mentioned) the most likely reason for the safety screw is indeed not to mess up the emission.

[instarx]

A MAF's sensor plate is a precisely fitted/designed device to the actual housing (i.e. plastic tube) as it is very sensitive to airflow. If the mechanic messes around with it, it can goes out from tolerance.
It could be that you will not notice it during driving, but the measured values will be off. In case of constant engine rpm, the O2 sensor will compensate it, but during non-steady state, your engine will run too much lean or rich.

I see your logic in the need for security screws, but I think it is more a matter of a legal requirement than an actual need to secure it in a super critical position. In reality the sensor placement in the sleeve isn't that critical as long as it is the same sensor. I am an engineer by training and have many years experience in designing ducts and measuring airflow and mass airflow in duct work - and moving the sensor a millimeter or two in the collar isn't going to make a bit of difference.

Have you seen that square grid on the upstream side of the MAF assembly? The common explanation is that it protects the sensor and keeps it clean, but that is incorrect (really, how many BB-sized dirt particles need to be filtered out downstream of the air filter... none!) The grid is actually a device to straighten the airflow through the sensor-tube. By making the flow laminar the designers create a duct with a fairly large sweet spot in which the sensor unit can be positioned.

You may also notice that the exposed heated wire in the sensor unit is a long one that extends from near the center of the duct toward the edge. This orientation also reduces the need for critical placement of the sensor since it measures airflow in a cross-sectional area of the duct and not simply at a point (which would make positioning critical).

[Atis]

I fully agree with you that the most likely reason for the screws is the legal requirements.

I know the design of the MAF (obviously not 100% depth). In the 850/S70 Volvos there is a hot film MAF and not a hot wire MAF. Although it does not matter in this case, as it works as you described, measure an "average" value. I was told by the responsible design engineers that it is really sensitive for placement, therefore I was thinking on that scenario. The grid was designed in a way to make the airflow laminar, just as you mention and I know that they worked a lot just on designing that grid.

[misha]

It's very sensitive and very fragile to the touch.

That's the reason why it's important not to touch the film when there is a need for cleaning it.Just spraying with a special maf cleaner and drying before re-installation.

[vjaneczko]

You can always use a Dremel or a thin saw and cut a notch into the screw head so you can use a standard screwdriver whenever you wish.