Solution to \"Bouncy\" Idle
If you cant remember, I have a 96 A4 TQ with turbo upgrade to t-03/28 which is similar to the GT28RS kit, only bigger!
For a few months before and after the turbo upgrade, I noticed that the motor would oscilate between 400 - 1000 rpm either on idle, or just after stopping at a set of traffic lights. At the same time, a loud hissing sound could be heard from the petrol filler. After numerous attempts at trying to figure out what it was, I finally decided to play around with the fuel lines and vacuum return lines.
What I found was that the so-called 'purge' valve (which normally ticks on a standard 1.8T hence previous posts from other people asking what it was - I'll get into this later on in the post), wasn't ticking any longer. This is, in fact, a form of boost control for the earlier model turbo's.
Essentially, the 'purge' valve is vacuum from the plenum/intake manifold (directly behind and underneath the TPS) and the hose can be followed right to the input of the 'purge' valve. The exit hose then travels back through to the rear of the vehicle, leaving an open end at the petrol filler, just above the pipe which accepts your fuel at the fuel station/bowzer/outlet.
I found this to be causing a major vacuum leak under idle, constantly sucking from the petrol filler, which in-turn caused the car to run really rough and oscilate between revs. There was no need to be worried about the vehicle stalling as the ecu would compensate for the loss of revs and actuate the TPS in order to bring the car back to running once again. I also found that driving in cooler months, it would do it more than in warmer months.
The workings of the 'purge' valve essentially vents unnecessary boost pressure rearward, thus, stabilizing manifold pressure in order for the motor to get its correctly optimized air ratio for the fuel. At idle, we dont need the purge valve to operate, but, if you have boosted the car considerably without a remap, then the purge valve will read its standard value and then S**T itself after more boost has run through it. Mine has S*AT itself due to high pressure, and no remap.
A quick fix was to fill or block the return line from the purge valve in order to fool the ecu into thinking that there was boost, but not enough to be vented back out of the manifold. This has helped me and worked 100%. No more rough Idle, no more hissing sound, no more rough running.
I did have some pics of what I am talking about, but, they have disappeared. I will have some more in the next few days in order to highlight what exactly was done to overcome the rough idle.
Some will say its dangerous, others will say its just a quick fix to a lingering problem. Ive found it to work, so therefore I am satisfied with it.
Specs on my car are:
Custom Exhaust Manifold for Garrett/Air Research T-28 exhaust housing on turbo
Custom Garrett/Air Research T-28 / T-03 turbo
Re-routed intake pipe - major cold air intake (RAM style)
Full length Mandrel 3" Exhaust from turbo
Standard SMIC (going!!)
Adjustable Fuel Pressure Regulator (currently set at factory 3 Bar)
Oh, and on another note. A few of you guys/gals are looking at a 1.8T kit from the States. Just make note and be weary that the turbo in the kit says it is a 'Wet Floating Bearing - Garrett type.' This means that its not exactly a Garrett turbo, and is only oil-cooled. The standard KKK-03 is oil and water cooled. I'd prefer to keep the water lines unblocked for the turbo, rather than upgrade to a standard oil cooled unit. We all know that water adds to cooling efficiency, thus better turbo running. You decide!