BlackRx7

have you checked the idle stop screws?

Fingers

Nope. How? And why the transition after 30 seconds if it's something that's adjusted? Wouldnt it be like that always?


AFM sticking? Not likely. Why such a pattern then? How would it know when the time is up and it should start running like a bag of **** again? And the engine bay doesnt warm up in 30 seconds. So the AFM has no way of warming up.


I'm insisting its some sort of solenoid that's doing this.

Hailers?

inboost

I think you've got an Air Control Valve problem.

Specifically I would target the switching solenoid valve. My guess is that the diaphram on the valve that controls port air has ruptured, and when the switching solenoid turns on it sucks air pump air into the manifold causing unmetered excess air to enter below the throttle.

I might be wrong as this solenoid is typically only active when the car is warm and under medium to heavy load.

Still, I think it would be a worthwhile check to see that all the valves hold pressure or vacuum (according to their function, see the FSM page F2-57).

As a fellow S5 TII owner I wish you luck!

Fingers

I think i've got one too it makes a buzzing noise constantly (regardless of temperature). Do i need to replace it?

Fingers

Does anyone have any insight on the acv and whether or not it could cause a problem such as this?

Fingers

TTT

Fingers

ttt

Fingers

Checked the timing. even with the jumper in, the timing jumped back and forth about a 1/4 inch with the car staying at a steady idle, rather randomly, too.

Bad TPS perhaps?

ducetius

ACV has caused similar problems on my car S5 TII.. it is definitely worth the effort to test it. The FSM has some tests plus there are one or two good threads around here for testing. HAILERS has a good write up somewhere here just search for acv

SureShot

I have been chasing/monitoring a similar gremlin.
Sometimes it would idle perfect, then for no apparent reason, start surging up to 1500, stumble , fall to 400, and surge again, sometimes stalling.
When I went to get tools & meter to start checking, it would idle perfect.

I had a big improvement show when I replaced the firewall to bell housing ground and added an alternator to body ground. If those grounds are not perfect, the ECU's sensor grounds try to carry some of the spark & alternator load. The ECU tries to recalibrate & goes nuts.

Still sometimes is wants to surge and after 5-12 cycles idles steady again.
I currently suspect something weak or intermittent in the ECU's internal tach signaling.
I have an NA, so there is a tach signal at 3600 & 5400 for the 6PI & VDI. Three weeks ago the 6PI & VDI both quit.
I tested the ECU & found that they both still clamp, but at 7500?

inboost

I too have seen the timing jumping when the set connector is grounded. I find this to be odd as it was my understanding that when the test connector is grounded the ECU is put into a test mode that fixes items like timing and idle speed control so they don't move.

Perhaps I'll visit the grounds this weekend

Fingers

Here's anothe interesting tidbit. With the air conditioning on, the misfire DISSAPEARS. Mind you... the afterburn popping upon decceleration still exists.

I have a feeling the BAC valve is functioning properly, as it is providing the extra air for the idle (idle goes up when ac goes on... by about 50-100rpm). But that extra air is making the car run alot better. Meaning, it's running pig rich. Would a bad tps cause fucked up readings throughout the RPM range? AFM perhaps?

Fingers

Does anyone else have any opinions?

ZachSpazz

I think your TPS could be flaking out on you. S5 '7s rely on the TPS to control almost all aspects of the fuel curve, and if the TPS is goofed up, it could be telling the ECU to throw extra unneeded fuel into the engine at those specific RPMS.

Fingers

how do I make sure? It's not exactly cheap. I've tested it over and over with a multimeter, but then again, the resistance curve of the TPS could be entirely wrong.

ZachSpazz

Do you know anyone nearby with a S5? All S5 TPSs are the same, and in the span of 10 minutes, you could have theirs swapped, which would immediately tell you if the TPS is bad or now.

Fingers

Yes... yes i do NOS FC3S... i'll ask him if i can borrow it just to see if that's the problem.