Karman-vortex Unit And Volume Air (vaf) Sensor

[K9crew]

A few weeks ago I changed the distributor caps, rotors, wires, fuel filter, oil etc and I was pleased with the results since the miss at idle and under load disappeared and generally the car was running very well. The only problem was that the engine would stall at idle in N or P. It got so bad that the engine was stalling EVERY time I put the shifter in P. On Sunday morning I decided to check all of the vacuum hoses, wiring harnesses etc that I had messed with during the previous maintenance ... everything looked ok. AWJ had suggested checking the fuel pump ECM so I followed the link that he posted and did some reading in the factory repair manual. I wasn't optimistic since the car wasn't exhibiting the typical symptoms that precede a failed ECM but then I noticed a reference to the Volume Airflow Sensor (VAF) ...

"The ECM determines airflow volume by applying a reference voltage to the VAF sensor, which contains a Karman-Vortex unit. The ECM uses a signal to control fuel injection duration and spark advance".

As part of the distributor installation I had to remove the air filter, MAF sensor housing and associated ducting. I happened to notice a metal honeycomb screen after the air filter but before the MAF. I decided to remove it at the time to improve air flow since I thought the mesh was merely a coarse screen to catch debris that made it past the filter. To cut a long story short I reinstalled the screen on Sunday and the car hasn't stalled once since then. :) So now to my question ... has anyone else removed that screen and experienced drivability problems afterwards? Is that mesh the Karman-Vortex unit that is mentioned in the manual? It will certainly diffuse and straighten the incoming air but I was amazed that its removal would have such an effect.

[AWJ]

That's getting to black box territory. :magic: I've never messed around with it. The turbo set-up in ours has a SC400 maf. I haven't done much playing with this. But that screen obviously does something. You didn't mention that you were playing with the maf before. It's always that kind of stuff that bites us I guess. I've done many similar things before and learned a lesson or two. ;) Glad you got it there.

The whole thing is a vortex measurement device. I've heard of people swapping the sensors with 1UZ sensors but never removing the screen. It seems that is was just affecting your idle. So that may be part the equation that meters air at no load and after wards the system goes over .... nevermind - I'm not gonna try. The stuff confuses me. All I know is you can replace it with a vpc or map system standalone or modify the signal slightly with high and low settings on a fuel controller by following the directions for your application.

[sc300man]

You hit the nail on the head with this one. My old MKIII supra had a very, very similar VAF (although Supra guys call it an AFM, Air Flow Meter). In fact, your VAF is a popular upgrade, due to it being bigger, for our AFM.

Anyway, the screen is there to straighten out the air before being metered by the sensor. I personally know about a dozen people who removed the screen, myself included, and noticed the same problems you encountered. In all cases, when the screen was reinststalled, the problems went away.

As for freeing up some airflow/HP. I know of one guy who dynoed with and without the screen with no difference at all. As you have figured out, just leave it in.

Josh