DaveK

I've seen posts in the past which say oxygen sensor and air flow sensor are ignored when the Wide Open Throttle switch activates.

I was talking to my mechanic yesterday about my recent running problems. He previously told me that the air flow sensor is a possible cause, but I told him yesterday that I didn't believe it could be because I had the same problem when I put the pedal to the floor, and that the air flow sensor was ignored under these conditions.

This was not something he was aware of - and he questioned whether it made sense. He pointed out that the load on the engine was completely different depending on revs / what gear you are in - so for example, flooring it at 4K rpm in first is a lot different to flooring it at 2K rpm in 5th. The amount of air sucked in by the engine would be completely different and ignoring the air flow sensor input just because the WOT switch has activated doesn't seem logical.

So my question is : How do we know that these sensors are ignored at WOT? I think I know because I've heard it here - almost certainly from Adrian, but Adrian - how do you know? Does the DME use other inputs than the WOT switch when it decides to ignore the air flow sensor? It seems like it would have to - that WOT alone isn't really enough?

Adrian

Because Porsche and Bosch say so is the only answer I can give you. Your mechanic is not correct totally. When the pedal is to the floor the amount of air acting on the vane opens it to full position. Pedal to the floor the throttle valve is wide open and sucking very hard and the initial suck is sufficient to open the vane to the full open position. The reason the signal is ignored is because the vane vibrates at this position because you mechanic is correct when he says that the amount of air physically entering the engine will vary but for the vane type airflow sensors they are not able to detect such changes and just vibrate in the turbulence. This is why the MAF (mass airflow sensor) was installed on the 928 and then the 993.
Ciao,
Adrian
964C4

johnfm

I think at WOT it ignores measured air flow and uses 'lookup tables' - it knows the revs and with WOT @ known revs it can calculate fuel requirement. I'm sure Adrian & Colin & a few others know more.

DaveK

Adrian,
Thanks - your reply has actually made me think of another question. We often see claims of power gains for MAF on 964s, and I know this has been discussed several times before. But from your answer, when you are trying to go as quickly as possible (i.e. foot to the floor, WOT) the inputs from the air sensor will be ignored (on a 964) regardless of whether it's the original system or MAF. In addition, although I understand that the stock air flap will cause some disruption to the incoming air, this will surely be at it's minimum when the vane is fully open - which you say it will be a WOT. To me, this suggests that at WOT, MAF would make very little difference - readings are being ignored and interruption to the air flow is not that much better than a standard air flow sensor when fully open? This almost seems to confirm that MAF can't give you much power gain - at least not at WOT.
Unless the vibrations of the stock air flow flap are very severe?

Christer

Dave

I am not sure that with a MAF the signal is ignored. I seem to remember reading something about that - but can't think where. Or we can just wait for Tony Taylor to let us know as he has one installed.

DaveK

But if it isn't ignored that would mean MAF requires DME updates, and I've never heard that.

When I looked at MAF before, at least some of them had connectors so that to the DME, it still looked like the original air flow sensor.

DougB

I've performed MAF conversions and reprogramming on Motronic systems and I've not seen any Motronic version that ignores the air flow sensor at full throttle. The older L-Jetronics were sad to do this.

The only time the airflow sensor is ignored is during cranking.

-doug

Adrian

Dear Doug,
The 964 is actually fitted with an L-Jetronic system with a very basic Motronic engine management system. However I do agree upon reading our posts that the use of the word ignored creates an image that may not be 100% accurate. The airflow sensor input wide open signal is 4.6V. The DME takes this 4.6V in conjunction with the WOT microswitch as saying okay the vane is fully open and we will ignore any variations to this 4.6V whilst the WOT microswitch is activated. It takes the 4.6V all the time and is always monitoring the input from the vane because when the pedal is lifted the vane output is again included in the calculations. So what I should say is that variations in output from the airflow sensor at WOT are ignored. The original signal of 4.6V is still there of course.
To answer the MAF issue, you have it in one. The Bosch 2.1 (cut down version) Motronic fitted to the 964 series cannot read the MAF signal. Therefore it has to be converted back into an airflow sensor signal and the smae conditions apply so any advantage of the MAF is basically lost in the conversion process. Now I know there are companies out there that disagree with this but there is more than sufficient factual evidence available that the simple installation of a mass air flow sensor on a 964 engine is a waste of your hard earned money.
Ciao,
Adrian
964C4

Ade - C4 91

Small side question; does anyone know why the WOT microswitch is often positioned so that can't ever engage? I've heard of several instances where this is the case - and mine certainly was when I bought it. Was this by intension or done by someone during servicing?

Ade

DaveK

Ade - I don't think the WOT switch can be moved - and I don't think it will be touched during servicing. The problem comes from accelerator pedal adjustment - you can alter the height of the pedal and if you have it too low, the pedal travel will not be enough to engage WOT switch.

DaveK

I should probably clarify - obviously WOT switch is not on the pedal. But if the pedal doesn't travel far enough, neither will the throttle linkage which is where the WOT switch is.

Ade - C4 91

Hi Dave,

Sorry, you are correct saying that the WOT microswitch cannot in itself be moved, but the spring metal Cam on the throttle assembly that engages it can be (which is how the WOT is adjusted).

It should be engaging at a certain throttle opening degrees (57deg I think from memory?) but the Cam itself can be loosened and moved about quite a bit so that it doesn't actually engage at all. This was the situation on my car, and it was way off, so there is very little chance that it was from the cable loosening.

Ade.