ramcram

My 1986 5.0l ROW starts and runs like the day it left Stuttgart, when it feel like it! Turn the key and the engine turns over a couple of times and it starts, idles and runs perfectly. Next time it may well just turn over and over without a hint of firing. I can try it a few times, give up, go back an hour later and it may well start as if there was never a problem. When it has started, I can drive it all day and it will never miss a beat or fail to start first time but the next day, it may well refuse to start.
When it won't start I notice that during crank the Tacho is erratic. Sometimes the needle doesn't move at all and sometimes it will flick all over the place.
Background.
Prior to my ownership, it had suffered an engine fire. Not enough to damage anything except the 'soft tissue'.
I removed the engine and replaced everything replaceable [thanks Roger Tyson] and repaired a few burnt wires plus replaced the entire engine wiring loom. Injectors were tested and serviced and a new in-tank fuel pump. Fuel pressure and delivery pass with flying colours. New coils, leads, plugs, dist caps and rotors. All engine sensors, fuel regulators, hoses, the works.
Where do I start looking?

joejoe

How is the MAF?

ramcram

new from JDS Porsche

WallyP

"New" does not necessarily equal "good".

My first suspicions would be:
Intermittently faulty crank position sensor or its connection.
Loose/dirty ground connection, especially the ones on the back top of the engine.
Intermittent ignition switch - less likely, but possible.

martinss

I will be interested in the response to this thread as I am fighting exactly the same issue on my '90 S4. All the symptoms are the same. The only additional thing I see is that when the CE cover is off I can observe that the LH and EZK relays click on and off alternately a half dozen times or more before stopping when the ignition is turned on. If they do this it will not start. If they don't do this, the car will start when the ignition is further turned to the start position. Not to hijack this thread, but I have cleaned all ground (including the ones on the bellhousing used by the computers, fuses, relays, battery terminals. etc.)

davek9

The Tach "o" moving and still not starting is showing a pulse from the CPS, however it may not be a good square wave (need a scope for that).

If it were mine, I'd start with replacing all three main Relays ( FP, LH, EZF ), or you could place a timing light on a plug wire and see if it triggers?

Sounds like you are not getting Fuel as you have not flooded the engine nor mentioned smelling raw fuel.
IIRC you need Spark First before the Injection will work.

Dave

Wisconsin Joe

Basic diagnostics. When it doesn't start, are you getting spark and injector pulse?

I'm going to guess not, and I'm going to guess that you have a bad connection to the CPS. The intermittent nature of the problem and the odd behavior of the tach are what I'm basing this guess on.

ramcram

Wally, it is one of John's rebuilt MAFs, so 'new' wasn't strictly correct. the crank sensor is new, read 'does not equal good' but we did check that with an oscilloscope when the motor was first refitted. when the motor was out every possible connection was cleaned with deoxit and checked with an ohm meter, so I would doubt these connections are the fault but the ignition switch hasn't been looked at??

ramcram

I've just bought a bunch of new relays from Roger, so I think I'm with you there.

ramcram

Haven't done any testing at all at this stage. I have just found and printed out the 'Troubleshooting Porsche 928 Starting Problems' for LH equipped cars.
I'm hoping the wisdom of this group will save me days of time wasting.
Having said that, the other day my mate was here [also a mechanic] and it was in a no go mood with several failed attempts to start it. I was sent done the back to listen for fuel pump activity while he turned it over and I could be convinced I could smell fuel. I didn't hear much because the bitch started first touch of the key.

WallyP

Kneeling by the rear bumper while someone operates the starter can do several things:
- Determine whether the fuel pump is operating at all (not whether it is/they are pumping, just whether at least one is running).
- Determine whether there is a raw fuel odor from the exhaust, indicating that the problem is likely to be too much fuel or too little spark.
- Provide an opportunity for a quick prayer to Dr. Porsche or the deity of your choice.

ramcram

I'm not the praying type Wally, just the hoping type!

soontobered84

Well of course it did. Please tell me you didn't expect anything less.

davek9

Exactly, we have an 87 S4 that does that, as soon as you have everything setup to look, it starts!

glmoore0001

I may be way off base here, but I owned an 80 928 that did the exact same thing.

The only thing that made it right was to get out of the car, put the key in the drivers side door and lock, unlock.

The car was sold shortly after due to an enormous speeding ticket, so I never figured out the problem.

I always figured that the car had a ignition kill circuit that was not getting reset when unlocked.

Best of luck.
Gary