ramcram

This was an old thread that said "start great sometimes and won't even try sometimes".
Well now it's won't start full stop.
No spark. Things I've done;
I had the engine out about 2 years ago to repair engine fire damage. I replaced the engine wiring loom entirely and replaced anything else that looked less than perfect.
I also replaced everything that can be classed as ‘soft tissue’ that could have been heat affected. This included the brake booster and rebuilding the master cylinder.
The fire wasn’t intensive enough to damage the engine bay paint or the radiator or its hoses.
I replaced every hose in there plus the following;
Water temp senders
Air temp sender
Thermostat
Plugs and leads
Both dist caps and rotors.
Both coils
Both fuel dampers
Fuel pressure regulator – 87 model 55psi
S300 chipset
Serviced and tested injectors
Rebuilt MAF from JDSPorsche
Checked throttle position sensor
New idle control valve
Crank angle sensor
New O2 sensor
Porken tensioner
Timing belt
Water pump
Sump gasket
Cam cover gaskets
Full top end refresh
Tested resistance on all plug caps.
After this it started and ran better than a new one and we had a couple of 3 hour country drives. It didn’t get much driving though. I did stat it up and run it every week.
Then one day it wouldn’t start, just crank over. Leave it for an hour and it would usually start up. It would then be fine for as long as it was driven, even stop for a long lunch and it would start up and bring us home.
This not starting became more frequent until it mostly wouldn’t start.
Following suggestions from my previous thread on this, I sent the 2 computers to JDSPorshe for service/testing and John reported ‘good and healthy’. He even fitted them to one of his own cars and drove around Cambridge! Such great service.
Fitted another new Crank angle sensor. No difference.
What I did notice when it wasn’t going to start was that the Tacho went crazy during crank. It will read faster and faster, up to 7000rpm. Taking to John at JDSPorsche I suggested that the Tacho is showing that a faulty signal is being sent. He suggested that the Tacho may be faulty and corrupting the signal. So, I cut the Tacho wire at J11 and the car started every time for a few weeks. Excitedly, I sent the instrument cluster of to a guy who knows these cars and who I’ve used before, telling him my problem. He reported back that there is no problem in the cluster, specifically the Tacho.
So I left the Tacho disconnected and it went out for a couple of runs, a few hours each with no issues and then coming home from ‘the last one’ it cut out and hasn’t started since.
One other strange thing is that the radio goes off as soon as crank starts but comes back on and it wasn’t noticed before because there was no radio fitted. Now during crank, the ‘accessories’ circuit, X, is disabled as is the radio circuit, which strangely come directly from the ignition switch on the 928. So, there should be no radio at all during crank. This made me suspicious of the ignition switch, so I replaced it to no avail.
I swapped out the computers again for known good ones but no. I’ve rechecked all of the relays but no.
I’m stumped. When it goes, it is fantastic, I love it but it needs to go every time!
Help please.

ramcram

Also soldered the power cables at the battery.
Checked the voltage at the CEB EZF relay, 13.2 volts.
Checked all earths.

Mrmerlin

replace the 3 running relays LH, EZF and fuel pump,do one at a time
Inspect the CE panel for loose wires or connectors that might have pushed out
Was the short TPS harness replaced?
have you checked the vacuum lines for fuel vapors?
put a gauge on the fuel rail maybe the check valve has failed.

next time it wont start spray some ether into the intake see if runs

ramcram

Thanks Merlin,
I bought a bunch of new relays from Roger and took out the CEB. Deoxited all the fuses and relays. Checked the board as much as the mess behind allows.
I checked the crank angle sensor with an osiliscope at the computers and got a nice square wave.
I have no spark, so no point looking for fuel issues until that is solved.

John Speake

The CPS signal shouldn't be a square wave, it is a rather spikey waveform as per WSM Vol. 1A page D28-11. The higher spikes correspond with the missing teeth on the timing ring. It is these spikes that the EZ-F synchronise to.

ramcram

Thanks John. I don't know why I said that because I can't actually remember what the wave form was. It was on the previous CPS. I just followed the test schedule, like a monkey. I've found the Schedule that was used. It's a Tech spec from Porsche and we will attach it tomorrow, from page 1 through . . .
Have you any idea why we would be getting feed back into the radio circuit during crank, with a new ignition switch as well?
Also, why the Tacho reads so vigorously?

John Speake

I've no ideas on the radio interaction during cranking.

The rev counter can behave like that when there's no signal from the EZ-F to pin 1 LH.

Mrmerlin

Umm did you use all Bosch electrical parts for the refresh?
IE a new CPS made by Bosch?

I have found that ignition parts and other electrical parts in general to cause lots of running issues if they come from other MFGs like a FAE crank sensor for example ,

same goes for the ignition switch I hope you got it from Roger .
Its also possible that the computer chip connections have failed this could have led to flooding then fouled plugs, and it will not start with bad plugs I would suggest to put a stock set of computers in the car dont mess with the chips,
check the ignition system for spark then remove the plugs and put in a fresh set

ramcram

thanks merlin

ramcram

I've always found it wierd that Porsche run the radio power supply directly from the ignition switch!
Why would a radio be that important?
Why not from the X rail along with other accessories?

John Speake

Because the radio is designed to come on with the ignition switch in the first (aux) position ?

ramcram

Well yesterday’s testing results using the Porsche publication pictured WD 491 120.
The oscilloscope curve for pins 7 and 19 on the EZF plug was exactly as pictured in the test book but I lack the skill to be able to determine if the voltage was 2.4?
The resistance across EZF terminal 23 and earth [temp sensor 2] was 1.4kohms with an ambient temp of about 18C. The test book states 1.4-3.6kohms for temp of 15-30C. So, this is within spec if the ambient temp was lower. However, this would not stop the engine from starting. The reading was spot on at the sensor, so the resistance is somewhere in the loom.
The rest of the test were all good, except that it won’t start and still has no spark.
If [as described in the test book] the double plug [green and white coax wires] above the CEB is separated and battery voltage is applied to either male pin with the ignition on, a large healthy spark will jump across a 4mm gap on a spark plug connected to the relevant coil. So that means that the coax wires from the EZF to the ignitors and the coils plus their circuits are ok. Yet cranking with the spark plug so connected to a coil gets no spark still?
If I had not had the EZF & the LH units tested by John, as well as swapping the EZF for another known good one, I would say faulty EZF!
I would love to know how to force this spark across the 4mm plug gap on a coil from the other side of the EZF. That is, with the green and white wires connected properly in circuit with the EZF, trigger the EZF to make the spark?
One thing I don’t like is that the volt meter in the dash reads about 1.5 volts lower than actual battery voltage at the jumper post [which is almost the same as the voltage reading on the battery itself]. I know that the dash meter is only indicative but the whole cluster has only just been serviced and calibrated by a guy I trust, so I’m going to remove it again and chase the voltage drop.
The mystery of the radio coming on [remaining on] during crank may be that the radio guy who installed it recently may not have used the original power source. I must check this.

harveyf

I've had my 86.5 fail to start and then mysteriously come back to life on 2 occasions. I feel your pain. You've changed out a lot more stuff than I did. Don't know if that's good or bad. As Mr. Merlin pointed out, even new parts can cause problems. He didn't come right out and say it but you might want to swap in a set of new plugs, in case yours are fouled. I think that might have been one of my problems. Fallout from chips with richer F/A mixtures.

Regarding the dash voltage gage, mine read low also. There is a rheostat on the back of the gage that you can use to adjust the gage reading to match that of the battery.

ramcram

Thanks harveyf.
I have a spare set of plugs. they are fun to change on the 86!!!
I think I need to look a little more closely at the crank angle sensor wiring also.

zekgb

Following with interest as experiencing the same issue.