Mr Earbrass

Greetings all. I have a 1987 928S4 that has become somewhat frustrating. When I turn the key the engine turns as it should and fires right up. I let go the key and it ceases to run. After a few tries it will continue to crank, but with no firing at all. Sometimes it will start and run normally until I put it in gear (automatic) then the engine shuts down (sometimes). Battery is relatively new and kept on a tender when I am not driving the car for any length of time. I did a full intake rebuild 2 years ago - new CPS, Knock sensors, fuel lines, vacuum lines, sent injectors out for cleaning,- you all know the drill. I originally thought grounds, so I cleaned them when the symptom was less common and I was not afraid to drive the car off my property. I checked the 14 pin - which was clean, but now needs rebuilt since I messed with it and the plastic cracked (though not so bad that that is the issue). Next fuel relay, swapped it out with no change in symptoms. But now I noticed that the tach does not move when cranking, so I thinks faulty CPS. Also on closer inspection if the tach does move when cranking that is when the car will fire up - if only till I let go the key. My question to the forum is would it fire at all with a bad CPS? Or if my ECU was failing or some other part between the CPS and the tach, would it display the same symptom - no tach - intermittent tach? Can one get to the CPS connector to the harness without pulling the intake manifold?
Other items that may or may not be related but with german cars who knows:
The P light does not come on in the dash though all the others work.
The car has a battery drain if left a couple weeks without running or on the tender.
When it does run, it runs strong and good - no squirrely idle speeds or anything.
The symptoms started with difficult hot starts.

Thanks in advance for any help.

Kevin in Atlanta

Ignition switch?

davek9

I'm leaning toward a ECU issue, has either one ever been replaced, can you get your hands on two loaners, IIRC on the '87 the LH (fuel computer) had issues and all fail sooner or later.
Also if you have an Auto, remove the Kick-down relay and see if that helps, they too can cause the FI to stop working.
Edit: yes the CPS can be changed on an S4 w/o removing the intake

Also does anything not work when the Key is in the "run" position, like windows, HVAC bower, headlights, that would be a clue if it's a bad switch.

dr bob

The CPS doesn't know whether you are cranking or running. It reports the gear teeth passing by to the EZK (ignition) box, which runs the tach and signals the LH to run the fuel pump and inject fuel. If the tach bounces while cranking or the engine fires, the CPS is usually fine.

There are some popular symptoms that point to ECU failure. Some report that they hear the injectors clicking before they start cranking. If the LH has been rebuilt then move this down the list.

Your symptom of fires on crank, no fire when you let off says that something is dropping out as the key is turned back. The circuit in question is #15, hot with key in run or start position. Turn the radio on next time you are trying. .If the radio is on while cranking but off when the key goes back to run position, that may be a sign of the ignition switch failing. The "cranks sometimes" symptom may also point to the ignition switch.

Before you go any further, think relay-relay-relay. LH, EZK and Fuel Pump relays. If you haven't replaced them before, Just Do It now. Clean and secure the battery terminal connections, including the minor wires that connect at the positive post. These are on the annual electrical maintenance schedule that most folks ignore until something doesn't work. The smaller "minor" wires provide power to the fuel pump and injection, the ignition system, and to the cooling fans. Nothing really important.... Soft wire brush (I use a detailers' brass toothbrush) to remove any grey oxidation. Snug everything up when you reassemble. Then a coating of Vaseline to maintain some protection from corrosive battery vapors.

If still no good results, remove the electrical portion of the ignition switch and clean it. You can use Deoxit or your favorite contact cleaner. With the plug removed, you can cycle the switch with a small screwdriver as you clean. Return the switch to off position so it indexes on the lock tang before reinstalling. The switches are known to deteriorate with age and corrosion. If you decide to replace it, get a known good new one from one of our trusted vendors. There are some cheap knock-offs that just don't last, so if you see one that's priced too good to be true, keep shopping. The switch is shared with the 964 and other models so they aren't exhorbitant.

Mr Earbrass

Thanks for all the answers mates. Problem resolved with an ignition switch. For future readers here is what happened and how solution was found.
Car: 1987 928s4 auto
Mileage 128,000
Symptoms: Originally car would just quit running for no reason. Sometimes at highway speed, sometimes at stops, sometimes going slow, but would always start right back up and continue on as though nothing happened. This happened not too regularly but since it always restarted not too big a deal and it was one of those awful things that can't be fixed cause it ain't broke when it is back in the garage. Next the car would do the above - still occasionally, but then, if driven to a destination, refuse to start without some work. Sometimes starting fine then when shifting into gear it would shut down. It always got me home, but now I didn't trust her. I started cleaning all the grounds (it should be noted that I pulled the engine a few years ago and replaced all the sensors, vacuum lines, fuel lines, cam belt and assorted related bits, that looked anywhere near marginal or hard to get to) and replacing relays with no results. In the end the car would always start when one turned the key, but die when the key was released becoming harder and harder to even get the intial start as one proceeded.
But it was glorious summer and time for fast motorbikes so I let the project sit.
Now as the weather here turns colder I started thinking how nice it would be to have my car back. So I go out to the garage and turn the key and the engine fires to life only to quit when the key is released, but I notice on subsequent starting attempts that the array of warning lights sometimes come on and sometimes do not (typically when the engine chooses not to fire). So I suspect the ignition switch, order one from the friendly folks at 928 International. Install it and I now have a car again!