Today while putting everything back together from removing my front cover, I got a little over-anxious to see if it would fire back up ( if I had the timing on, check for any oil/coolant leaks, etc..) and I started the car without the AFM on. The car started, idled, and reved. Although it idles and revved very poorly, spuddering, etc.. it did hold an idle on its own, and did rev. Now this is my 1st S5 car, so it may be different, but I know none of my other S4s would even start with the AFM not plugged in. I thought the ECU used the AFM position to trigger the injectors..

 

I accidentally left the AFM unplugged on my old S5 N/A once before. It idled like crap and the check engine light came on. I quickly realized my error.

So, yes, they can start with the AFM disconnected.

 

My GTU would crank and idle without getting an AFM signal but after driving a distance I would have to keep light pressure on the gas to keep her running as she would not hold her idle.

 

if the AFM is left disconnected it has a fail safe setting in the ECU to allow it to idle and rev up a few K to possibly drive the vehicle, but if you rev it too high it loses power and dies. this is a built in limp mode for the ECU.

 

that must be only for the s5, I know on the s4's that **** wont even run...

 

yes wen i had a GTUs i forgot to plug in the afm and it idled like crap and hellah smoke came out of the exhaust

 

No, the CAS tells the ECU when to open the injectors; the AFM is used to calucate how long to open them for.

 

The biggest difference between the S4 and S5 that I can see causing this is the addition of a full range TPS.

On the S4, once the TPS has gone wide open, the ECU really has no way of telling what the engine's doing. It could use the manifold pressure sensor combined with the engine RPM (FD style), but if it had the ability to do that, why use an AFM in the first place?

With the S5, the full range TPS could be used to approximate AFM values (through some sort of conversion function). It won't be *good*, and will fall apart at higher power output, but in a low power range it's obviously good enough to keep the engine running.

-=Russ=-

 

Doesn't the ECU control the fuel pump relay on the s5, and on the s4 it's the switch in the AFM? IIRC that's why a s4 would never start is because the fuel pump would only run during cranking and then stop.

 

I think you've hit the nail on the head there Rat.

 

... yea, and that minor issue of "fuel flow."

What happens if you jumper the fuel pump test connector & try to start a S4 without an AFM?

I'm going to have to try that tomorrow.

-=Russ=-

 

Thats what I was trying to say on my 1st post...I know on my S4s if you held the afm closed it would get no fuel and not start, it needed the engine vacuum to pull open the flap.

 

I found some info on how Toyota ECU's control the fuel pump, and it would be a logical assumption that the S5 ECU does this the same way.

Instead of using a switch in the AFM, the fuel pump relay (aka circuit opening relay) is controlled directly by the ECU instead. It closes the relay any time it receives a cranking signal from the ignition key or there is an "Ne" signal coming from the CAS, indicating the engine is turning. If the engine stops turning, the signal from the CAS stops and the ECU opens the circuit opening relay, killing the pump.

It seems the S5 ECU can run the engine poorly at low load without an airflow signal. The same is probably true for S4's, if you jumpered the fuel pump check connector to bypass the AFM safety switch.

 

It'll flood, I tried this while troubleshooting some issues with my stock wiring harness.

 

If you start an S5 with the afm unplugged; it will crap out at about 3k, and it will throw up the check engine light..

 

my s4 would start with the AFM unplugged but would dump out at about 3k RPMs, started it a few times until i realized i left the AFM unplugged.