Bad IAC produce engine stumble at WOT?

[Cynic94rs]

Brought the 94 RS into the dealer today hoping they could better diagnose what has become a time consuming thing to troubleshoot. The car has been down for a few months with the following symptom: Giving the engine hardly any air causes a severe miss/stumble/hesitation off idle. WOT from any engine speed makes the engine bog and the tach needle bounce from 1000 - 1500 rpm, never to improve.

There are no codes, timing's fine, new distributor, wires, and properly gapped spark plugs. Fuel pressure and regulator measurements all to spec...

But now the dealer tells me that the car needs a new IAC valve to start, potentially in addition to further diagnosis and repairs.

Since the problem persists at WOT (during which I thought the IAC valve isn't regulating airflow past the TB at all) I'm skeptical that this proposed $620 "first fix" is really a fix at all. I have no complaints about the way the car idles... it's everything *else* the car is supposed to do that it's no good at anymore (i.e. -- accelerate)

So what do you think? I'm sure it's true that I need the valve anyway. Should I have them order the part and keep working?

All input highly appreciated. -- especially if any of you BP swappers will sell me a working B6 IAC valve for $20.

[wide_mx3]

Maybe MAF? Mine had that problem but wasn't all the time. I just cleaned the MAF and TB and the prob. went away...

[LongJohn]

I don't think it is the IAC for the reasons you stated, but I'm am amatuer so what do I know.

You said that fuel pressure was ok... did you check it at the fuel rail and at WOT? You can have a partially clogged fuel filter and the pressure would be fine until you really start tapping the system for fuel (like at WOT).

Your problem really sounds like bad fuel flow so:

1. The fuel system can't produce enough pressure or enough volume
2. The ECU doesn't think the throttle is open very far even when it is.

Number 2 is possible with a bad VAF. The ECU would think there is little air flowing, so it wouldn't increase the amount of fuel.

Also, my BP has over 2K miles, so I'm starting to feel safe and I'm about to strip down the old B6 and see what went wrong. I'm still using the B6 VAF so I can't part with that, but everything else is up for grabs cheap.

[neutral]

Do you use a stock ribbed intake hose? If so, remove it, flex by hand & check closely for hairline cracks between the ribs. Any cracking means unmetered extra air entering the intake stream and overwhelming (or nearly so) the amt. of fuel delivered. Even mechs can miss this one and the quick fix is a simple seal-up w/duct tape until replacement of the hose.

[LongJohn]

I think the cracked intake tube is always good to look for, but I don't think it would be my first choice in this case. It has a more profound effect on idle conditions than WOT. It only lets in a bit of air, and that bit is a much higher ratio at idle than at WOT.

His problem sounds like the amount of fuel does not increase when the amount of air increases.