Blues-shark

Previous Background:
- dismantled engine bay to valley
- preventative maintenance of all vacuum lines, rubber tubes, fuel system etc.

Recent Background
- started OK with approx 8 minutes of running
- stopped with fuel pump not running
- jumped the fuel pump relay slots and fuel pump ran OK
- changed the fuel pump relay with no start
- finally I re-seated the fuse by twisting the pins (convinced that this must have been the problem) it ran but later stopped
- strangely started as soon as I repaced the ignition relay as I figured that this was the only additional factor in the circuit (convinced this must have been the problem)
- ran beautifully for about a 1.5 hour drive (ie everything was hot)
- stopped for about 30mins and attempted to start, it ran for about 1 minute (plus I heard the fuel pump running ie wasn't just using old gas) then stopped
- could not start it again ie the fuel pump is not running when I turn the engine over.

Attempted all the previous things that made it start before ie reseat of the fuse (plus checked the continuity from behind with multimeter - OK), changed both the fuel pump relay and ignititon relay and jumped the fuel pump relay pins to check the integrity of the fuel pump and the pump ran.

Are there any experts who can help me with this as it appears that there were two independent problems ocurring in series and now nothing helps to make the fuel pump run when I turn the engine over. Is the common factor the brain? Why do independent fixes seem to help for a short time ie is it just bad luck in terms of timing?

Cheers
Neil
928/red/'85/auto <img src="confused.gif" border="0">

Dennis Wilson

Neil,

Sounds as if you are fighting a corrosion problem. Have you cleaned your grounds? Recommend you start with the grounds above the fuse panel. Then the ground from the chassis to the block and then in the back of the car. Additionally the connections is the back of the fuse panel may be corroded. BTW it is a maze back there so don't pull more than one connection at a time. BTDT.

Dennis

Max

Look through the; Attention New (and not so new) 928 Owners post at the top of the list.

I wrote one that is close to the bottom of the above list called "Intermittent Electrical Problems (86-95)" that maybe of some help.

If I were you I would do as Dennis suggested as well.

HTH

Max

Blues-shark

Thanks Dennis and Max,

I really appreciate both of your responses. I will certainly do as you suggest especially in terms of cleaning the earthing points. I will also add some copper gel. How is it that a cable bolted to metal can lose contact at all and why does it cause such an intermittent problem? Is there some electrolysis occurring?

Cheers
Neil <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

Max

Yes. My belief is that some of our electrical problems first started when the ground points on our cars developed corrosion in and around them.

A dirty gound point:

1. Will cause a heat build up at the connectors, fuses and the relays that are mounted in plastic or to a plastic board (Central Control Panel, CCP)

2. Will also alter sensor readings and helps to send the wrong signal to the different mystery boxes on our cars.

I first discovered this soon after I bought my 89S4, the radiator fans, headlights, rear A/C blower fan and a few other things would work intermittently.

After many, many hours of looking over the wire diagrams and probing and testing sensors, wires and connectors verifying voltages and Ohm readings, I found that most of the ground points read high resistance and where affecting some of the sensor reading. This help with some of my problems but not all.

The heat damage to the CCP and some of the connectors had already been done. I could get everything to work as long as I held the particular relay or a connector in a certain spot or deformed the blades on the relay, connector or fuse.

Before the motor went south I had finally got everything working as designed, knowing that unless I replaced the CCP and a few connectors and connections, the electrical gremlins would someday return.

HTH

Max

Blues-shark

Hi Max,
Thanks for sticking with me on this one, I appreciate your time and the fact that you are generously giving your hard earn't experience. Just to give you some feedback I carefully cleaned the two front, engine, CCP and rear ground points. I couldn't find the one under the instument panel; and the one at the right rear was too difficult to get to due to the trim. Despite not getting to them all, I am very confident that each point that I cleaned has minimum electrical resistance plus I can advise that there was low corrosion evident. Unfortunately, each progressive start test gave a negative result. I get some Zen feeling that the problem does not lie in an ground point despite the fact that your advice seems very sound and logical. I am having the car towed to a mechanic tomorrow morning and I will definately keep you aprised of the result. Once again thank you for the advice and based on your quotable quote I am confident that no band-aids will be required in terms of future grounding ...I just hope the mechanic is as good as you. Sorry for the time zone difference in terms of feedback.

Cheers
Neil <img src="graemlins/roflmao.gif" border="0" alt="[hiha]" />

Max

Neil, There are many things that will keep a 928 from running or running right. The Porsche Shop Manual (PSM) has some very good and specific instruction on pre-troubleshooting. One of the most memorable ones is to start all tests with a freshly charged battery.

Being that the 928 is heavily equipped with electrical devices I always advise cleaning all the ground points (as discussed earlier, including the engine and battery grounds), then the electrical connections at all the sensors and switches and verify that each one is capable of sending the correct signal. After that, it is necessary to verify that, what ever receives the signal was able to get the now verified sensor readings through the wire harness (s).

My earlier post was meant as a precursor to many other tests that needed to be done. It was my fault for making it sound like a fix all. We must remember to always start with the basic checks before moving on to the more complicated diagnosis procedures.

1. Spark
2. Fuel
3. Compression, Exhaust and leak-down
4. Charging system

Spark, needs to be checked at both coils, all the plugs noting the strength at each (A scope works best for this).

Fuel, Flow needs to be verified at the fuel distributor (where applicable), fuel rail, fuel injectors and the retune line at the tank. Fuel Pressure also needs to be verified, which can be done either at the fuel distributor or the fuel rail (where applicable) (specific gauges are required).

Compression, Exhaust and leak-down tests are one of the simpler ones that most anyone can do with a relatively inexpensive tool (s).

Charging system, most parts stores have a small test stand so that an alternator can be test (for free). It’s not a bad idea to remove your alternator every few years and having it tested if you can do the R&R yourself. The PSM even goes as far as saying that a trashy signal from the alternator can cause electrical problems (a scope it a must for this test).

Neil, It looks as thought a lot of work was done to the top of the motor. Did the car run as designed after this was done?

Keep us posted as to what is found.

Kindest regards

Max

Blues-shark

Thanks Max,
Once again appreciate the detailed response - I am happy to report that the car is now purring. The mechanic changed the timing sensor and the brain relay. I have learnt a lot of lessons from this and have tucked away all of your good advice. Once again many thanks I owe you one of the below.

Cheers
Neil <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

mj1pate

Hey;

Just curious; when you referenced "timing sensor", did you mean the crank position sensor? Its certainly down there just behind the intake and its connector into the harness is subject to bumping. The crank position sensor connectors become notoriously fragile with age.

Regards;

Mike