vacuum 2.9

[werewolf]

This is most likely a dumb qustion. I was doing the sea foam test on my 87 4x4 2.9 yesterday. I used a can of spray in the intake to clean throttle body. then put 5.oz in the engine, it said 1.oz per quart of oil. Then i did the 3/4 can into the intake through the brake vacuum hose. But to my surprise when i pulled the hose off brake buster the engine rpms went up higher. I would think that big of vacuum leak the engine would try to die. It seams backwards to me.

 

[adsm08]

On a MAF system a leak as big as pulling the booster hose might kill the engine.

The 2.9, with the exception of a very few units built for California, used a speed/density system, which is less sensitive to air coming in through places that are not the throttle body.

MAF systems work by measuring the approximate amount of oxygen, not air, just oxygen, coming in by having an intake duct work of a known volume, and then inferring the temperature of the air by measuring the amount of power needed to keep one of the wires in the sensor 200* hotter than one of the other ones. It is very accurate, but it is also very sensitive to leaks after the sensor.

A speed/density system makes a calculation for the same information, but it does it using the temperature of the air in the intake and the pressure/vacuum of the intake manifold. This is somewhat less accurate than MAF, but it is less sensitive to air leaks.

The reason a MAF controlled engine will stall when you introduce a large leak is because it lets in a lot of air that the computer can't see, and that in turn leans the mix out so badly that the engine can't run on it. A speed/density system will "see" that air when the pressure in the intake goes up/vacuum goes down, and so it can compensate a little better.