Lacking Boost? Chasing a high boost cut-out issue…

One of the most frustrating issues you may face while owning an UrS4/S6 (especially on modified cars) may be high boost cut-out issues.  Being pre-OBD II, these cars are less prone to give you the annoying (yet helpful) check engine light when problems arise.  Then, when you DO actually retrieve a stored engine code it is often so vague that it pretty much confirms what you already knew… the engine isn’t running right.

In my case, I have a full RS2 spec S4 that normally runs 28 PSI max boost.  It had run strong and run well for many years with only the occasional ignition coil failure to dampen an otherwise perfect ownership experience.  I should also point out that I’m one of those slightly crazy people who chose to stick with the factory ignition coil setup rather than the popular 1.8T/2.0T coil conversion option.  My reason for this?  Well, as inexpensive as the conversion kits are, I have certainly heard of a lot more failures with them than with the more expensive stock coils.  Sure, the conversion kits make for easier swapping when a coil does fail but it is nice if you can just avoid that premature failure in the first place.  The original Beru coils are often good for 80k-120k miles and that was good enough for me to stick with them.

In my case, the high boost cut-out issue seemed to be exactly like the weak ignition coil failures I had experienced in the past.  The car would seemingly run fine up to about 14 PSI of boost at which point it would stumble badly and emit a cloud of rich exhaust smoke out the tailpipe.  Since I had replaced three of the five ignition coils in the past I decided it made sense to replace the remaining two coils, especially since I had over 150,000 miles of use on them.  I swapped them and figured that would be the end of my problems – it wasn’t.

Since I had been working nearly 60 hours a week as an automotive service manager for a German specialty shop, my time to troubleshoot my own car was extremely limited.  Ironic isn’t it?  My problem would persist for quite some time before I had time to dig into it further.  As long as I drove sanely it wasn’t really an issue which made me question whether I had either a fuel supply issue or perhaps an internal ECU issue.  I started leaning towards a fueling issue after my car would sometime take a few extra cranks before starting in the morning.  I’ve seen a failing fuel pressure regulator cause that sort of behavior in the past so I swapped in my spare RS2 fuel pressure regulator.  Naturally, there was no change and my sanity slipped further away from reality.

I should also mention that I had gone through a number of boost leak checks and had found a few very minor leaks but nothing significant.  With those out of the way I was down to:  1. Fuel issue 2. ECU issue 3. Engine issue (compression?)

The compression test confirmed that the engine was not yet dead so that was a relief.  Swapping ECUs with another car also didn’t resolve the issue.  It had to be fuel, right?  Well, it seemed unlikely as I had actually had the fuel pump replaced with a stock Bosch unit just over a year earlier.  It was actually the 3rd fuel pump my car had been through in its 160,000 mile life.  The 2nd pump I had been using was a Pierburg unit which was supposed to be pretty much the same as the Bosch but as it turns out it wasn’t.

I decided to order the high flow fuel pump from 034 Motorsport along with their fuel pump relay.  Much frustration was had due to the fact that the relay from 034 was actually wired incorrectly and would blow a fuse each time the car was started.  Nice.  Once the error was discovered and corrected all was well in the world again and full boost runs are back.

Moral of the story?  Sometimes it isn’t a boost leak or a coil.  Make sure you’ve got enough fuel.  🙂