car starts without AFM but wont run with it? WTF HELP PLEASE
 

I have an NA s5 that I recently installed an intake hose kit on.

When I go to start the car without the AFM plugged in the car will start and immediately rev up to 8000rpm.

however when the afm is plugged in it'll rev up and then die.

Any idea what my problem is?

If you have the sotck TB elbow still, there is a elbow and a nipple capping it off coming out of the bottom, and a air line that goes to the bac i think. Make sure those are connected still and the nuts on the tb elbow are on snug. make sure you have everythingclamped on the pipe.

revs up to 8000rpm? Check the throttle body if its going that high. Sounds like the pates are open.

What is this "intake hose kit"?

yea, sounds like a massive vac leak from something that came unplugged or was overlooked. 8000 is pretty high for even a giant-ass vac leak

I agree with it seeming like a vac leak, also the plates weren't open at the time, this is very confusing.

I can no get the car to start and keep it alive for a bit with the gas but you pretty much have to goto WOT just to keep the car running.

all signs point to vac leak, I unassembled and reassembled the intake manifolds and found n o t h i n g.

this is becomming a huge PITA.

any help appreciated.

wait, if you get the car running and hold down the gas with the AFM plugged in, then it will only run at WOT? or without the AFM plugged in?

when disassembling/reassembling the intake, did you knock the throttle body or drop it or anything? I had a similar problem that really seemed like a vac leak that i couldnt find, until i realized the throttle shaft was slightly bent (it was bent when i picked up the engine, i didnt think anything of it cause it still seemed to close fully, just stuck slightly when closed). One good tug on a thick wire wrapped around one end and it worked great

with the afm plugged you really have to give it a lot of throttle to keep it running, and if you let off it dies

I'll check the tb

Well, sounds like metered air is bypassing the TB if the plates are closed, but even at that it, the largest hose would only bring it to around 3000 rpm.

You sure they are closed? The engine CAN NOT rev to 8000 rpm without A LOT of air. Meaning only one thing. The throttle body is held open some how.

A huge vacuum leak would not allow it to rev that high since the air would have to be metered. If you had a large vac leak the engine would simple not start or die out the second it starts.

a friend of mine suggested that perhaps the TPS is out of wack thinking the car is at WOT< this would bypass the use of the AFM as it would be running on a map in the ECU, I'll adjust my TB to spec, hopefully that's the problem.

hmmm..... after much deliberation, i have come to the conclusion that...


















your cars broke.

the car should start without the afm, since when it's starting the AFM is closed anyway, and only opens up after the car begins to run. I was thinking it might be an issue in the fuel system until the 8000 rpm bit, which is what's throwing me here. Just to check the fueling, barely crack open the afm with the key set to "on" (car not running), and see if the fuel pump turns on (the same way the check connector would work). Check the TPS. When it is running, spray carb cleaner around the intake to check for leaks (it will rev if some finds its way thru a leak into the engine)

I don't think you're right. With the AFM unplugged the car will start and run. Just not very well. IIRC instead of running off input readings from the Air Flow Meter it runs off a given map in the ECU, which is not as accurate as the AFM, but can run.

If it runs when unplugged, but then skyrockets to 8000 rpm with the AFM plugged in, I would lean towards a problem with the AFM. Perhaps the door is sticking on the inside?

James

EDIT: Just read it's an S5. I'm not familiar with the S5 AFM sensor, so it may not have a door like the S4's. My advice would be to THOROUGHLY check for vacuum leaks, test the AFM, and make sure 100% your throttle is not sticking somewhere... whether it's the cable, linkage, or the plates.

it goes to 8000rpm with the afm unplugged

If the AFM door was stuck open, the computer would dump enough fuel in there to promptly flood the engine. And about the AFM being plugged in vs unplugged, I know if i unplugged it in my car (S4 TII), it would promptly die.

S4 and S5s, during startup for the first few seconds, have a map in the ECU and I believe uses the pressure sensor instead of the AFM.

After that the ECU switched to the AFM. If your car has no problems, unplugging the AFM will instantly kill the motor since the ECU has nothing to run on. There is no "map" in the ECU for the motor to run off of while the AFM is unplugged.

Wrong. The ECU does not bypass the AFM because the TPS reads at WOT. If your TPS was reading 5+ ohms while the plates are closed, then your TPS is certainly bad.

:jump: :suitdance :ylsuper:

Figured it out, had a huge vacuum leak between manifolds, I lose :/

That is really strange that it would even run with a vacuum leak that large.

Although, thinking of it, I can kinda understand why it wouldn't not run with the AFM with a vacuum leak that large. But hey, good job finding the problem :)

vac leak
start up fuel rich.... lots of air going to engine, rev up
then off start map, no fuel signal from afm, car dies.