928 S4 engine shuts off while driving.
 

This bizarre behavior was observed on by 87 928 S4 auto.
First time, driving on the highway around 65 mph, the engine shut off, all gauges seem to work - speedometer, temperature etc.
Put the hazard light on and pulled over, shifted into park and turned the car off. Then I put the key back in, crank and drive away - drove normally.
Car sat in the driveway for about 2 hours.
I go to run a local errand and the exact same behavior, except the speed this time was about 35 mph.

The car is driven couple of times a week and the gas in it was not old.

Any ideas on what could cause this ?
Thanks.

Fuel pump, or change the three 53 relays known to fail. They're cheap.

Or something loose. Check your ground strap for corrosion. And batt connections.

As posted above check both ground straps for corrosion and clean all other grounds.

Replace the 53 relays with high quality replacements, not the cheap chinese ones.

If an ignition component was failing it would affect half of the cylinders first and cause what many mislabel as the limp home mode.

You could have a bad connection to the crank-position-sensor due to it being cracked and crumbling. This problem will be intermittent as you describe.

Would a bad ignition switch do this?

Follow Joe's guidance for relay-relay-relay replacement. Lift the battery ground strap at the rear apron while you pry on the relays, save yourself some smoke and crying. My vote goes to the fuel pump relay as the culprit, based on personal experience, but the effort to replace the three critical relays is functionally the same as replacing just one, so plan on all three and sleep better for another thirty years.

Replace the EZK ignition relay, the LH injection relay, and the fuel pump relay. As Blake recommends, use high-quality replacements, rather than the lowest-cost-supplier stuff masquerading as good. Our favorite vendors can help you out, as they are careful to source from known-good suppliers.

Get some spares and store them in the car behind the tool panel, along with your other critical bits like a switched jumper and a couple relay pullers (paint can tool with the end bent over to 90º). Throw the old ones away.

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Jim Bailey moonlighted at 928 International during my early years of 928 stewardship. His sage advice and guidance made working on the car a lot easier for many. His "relay-relay-relay" mantra sticks with us to this day, offering an easy and often successful way to eliminate a bunch of possible causes to some interesting running issues.

It can, but the OP's statement about the dash lights staying on points away from the switch itself. The ignition switch uses the same 15 contact that runs the engine systems (fuel, spark) to power the instrument cluster indicators. It's not impossible that they would somehow be able to operate separately, but it would be a real stretch.

Exact same problem.

Here you go:

https://rennlist.com/forums/928-foru...ermittent.html

The final solutions was discovered at the end of that winding thread. LH computer brain!

Following along, since my 86.5 is doing this currently too.

My 89 did this intermittently and it ended up being the engine and computer grounds. When the computer would find a ground, it would run great. Whenever it didn't find a ground, it would shut off the engine just like I had turned the key off. It always happened, it seemed, in the worst possible places (construction zones with concrete barricades and no place to pull off the road, etc.).

My main engine ground under the car on the passenger side may as well not been there. I removed it, cleaned both ends and the mounting points, and reinstalled it. I haven't had trouble since.

YMMV

There's no harm in replacing the relays, but I'd be afraid to send a car, with these symptoms, home from my shop and call it "fixed", by just changing the relays,

Most relay problems are on initial energizing....pretty rare for one to fail, intermittently, while going down the road, and then be instantly cured by restarting.

How it shuts off can help narrow it down... shut off like you turned the key off? or stumbled, bucked, and struggled for a few seconds before it quit?

This--^

Also, what's the history of this issue? Did it start subsequent to other work performed on the car?

What's the general condition of this 928? Has the crank position (engine speed) sender been changed in the last 5 - 10 years? Have the grounds ever been 'gone through?'

This was my issue at one point. Replaced CPS.