'88 5 speed. The engine has been intermittently cutting out when I hit a bump or rough spot in the road, usually at around 45 - 50 mph. Today it is cutting out at low speeds (5 - 10 mph). There isalso now a buzzing from the passenger footwell which I assume is a relay. Which one should I look for? Thanks and Merry Christmas.

 

My first guess would be intermittent fuel pump issue.

I'd start by swapping the fuel pump relay for a new one.

 

Then, ground strap. Replace, not clean. Use autozone for $7. If it solves it, replace with new 928 strap ($70).

 

Hilton:

Im soon to chase down the same issue myself...I dont think its the FP relay, I tested that kind of death, and it takes a moment or so once that fuse is pulled..a slow death over a second or so. My 'instant death' when a foot sits on my CE panel cover is instant off, and then instant back on when the offending foot is removed.

 

The engine death due to fuel cut out is faster if you're driving, not idling - pretty much instant if above 4k rpm's in second (well ok, so maybe I shouldn't be taking speed bumps at that speed, but hey, I like to run factory ride height ).

My suggestion was based on fuelling being anecdotally the most common cause of engine cut out on S4's - specifically relating to the fuel pump circuit or pump itself karking.

In your case Jeff I'd be concerned about driving the car until you've pulled the panel and figured out where (and what) its grounding when pushed on. If its high current you're running the risk of a fire.

Rod, what other symptoms if any?

Lights dimming or voltage gauge varying?
When its cut out, does it re-start immediately? Or sometimes fail to restart for a while?

Fuel pump failure can be gradual, depending on the mode of failure, so it could be warning signs. Chris's suggestion of ground strap is a good one especially if you're somewhere with a damp climate.