Rodrv6

1988 928 S4
I took the car for a drive today and about 4 or 5 miles down the road the engine quit with no warning. I pulled over and tried to restart. It would spin, but made no effort to fire. I dove under the dash and tried wiggling relays and fuses. The car would occasionally fire up and run poorly, but would die as soon as I pushed the gas pedal. Most of the time it wouldn't fire at all. Without any tools with me, I called for a tow and got the car home. Now, it starts and runs just fine! I'm a little afraid of taking it out again until I've figured out why it quit.
Any thoughts???

SeanR

How old is your crank position sensor?

Rodrv6

Unknown. I've had the car for a little over 2 years. It's probably original. I've replaced (2 years ago) both coils, rotors, caps, ignition modules, plugs and plug wires.

FredR

As Sean says the CPS is an obvious starting point and possibly the secondary board in the LH computer module may be about to take a dump. Both are very common failure points on the S4/GTS and if you do not know the history failure is just a crap shoot now.

I doubt it will be anything on the ignition side of things given there are two sets of coils/distributor etc and the EZK computer is very robust [failure almost unheard of].

Bottom line- if this happened once it can just as easily happen again until such time as the root cause is taken care of. Friend of mine had a mystery problem in that his S4 would suddenly stop and strand him and then later would fire up. I loaned him my spare LH and the problem disappeared.

Rgds

Fred

Rodrv6

Thanks guys. I've gone ahead and pulled the crank position sensor, which had the usual crumbling plug issue. I've got one on order at the local parts store, which should be here tomorrow. I'll see how it works out before diving into the really expensive stuff, like LH computers!

Snark Shark

Sounds a lot like a loose fuel pump relay connection I had. It was arcing and getting hot, eventually too hot to function. Then it would be fine after I let it cool down for a few minutes.

worf928

This--^

... and this.

Replace the LH, Fuel Pump and EZK relays if they are original.

And, check your battery connections. Loose power or ground lugs can provide these symptoms.

Rodrv6

Update:
I replaced the crank position sensor. The car will not start. The fuel pump is running and I have fuel pressure at the fuel rails. The car will start and run on starting fluid, so I have spark. I've checked for continuity between the various relay terminals and the connectors on the EZK and LH units. I've swapped the relays (ignition, fuel pump, LH) with known good relays.
I'm thinking I've got a bad LH computer.......... I have a call in to Rich Van Meeteren, so hopefully I can take the computer over to his place and test it on another car.
Oh, the joys of owning exotic German cars...

davek9

Hi, have you pulled all the Fuses and cleaned them, while your at it check for correct Amp rating, look for any discolored holders and connection blades.

Also doesn't hurt to pull off both ECU connections and clean them too, WD-40 works just fine, don't use an abrasive. Deoxit is also well recommended.

Cosmo Kramer

Pull the connector off the MAF. If it starts but runs $hitty it needs a new MAF. Mine did this last year. Died with no warning.

SeanR

Have someone crank it while you stand by the engine, you should hear the injectors clicking while it cranks over. If not, getting the LH tested is a perfect place to start.

Roger has rebuilt units in stock.

Kevin in Atlanta

I am closer in Roswell. Give me a call - 770-891-7634 and we'll put in in my 87.

Rodrv6

Another update:
I took the LH and EZK boxes over to Rich Van Meeteren. (Thanks for the offer, Kevin, but I kinda wanted to see his place anyway....) Both boxes worked great in another car. Came home and reinstalled them, no start. I did some investigating and found that I had missed hooking up a ground. Once that was attached, the car started and ran! Until it got warmed up.....then it quit. I could restart it, but only with lots of accelerator pedal and it was running very rich and rough. Unplugging the MAF sensor didn't seem to make much difference. I swapped in a used Temp II sensor, but no change. One thing I noticed was that both Temp II sensors were reading about 560 ohms at 175 degrees. They should have been around 200 or less, per the shop manual. I'm thinking I need to get a new one....
Thoughts?

Rodrv6

Another Update:
I ended up sending the LH computer to Roger for testing/rebuild. He put the LH into his car and it seemed to work just fine. After swapping some emails, we decided to go ahead and rework it since it is 28 years old. I received the replacement computer yesterday and installed it today. I also changed the relays that control the ignition computer, fuel pump and the LH.
The car is running great! I've put about 25 miles on it and it hasn't skipped a beat, so we'll see if this is a long term fix.
Thanks again to Roger Tyson for his knowledge and willingness to help!