mj1pate

The recent spate of stallouts (still under investigation) has probably been a good thing. Having driven the little 86.5 "S" 4k miles across the US, just after taking possession had me a little complacent. (How's this for finding a silver lining?)
I'm getting better equipped to handle these kinds of situations with proper Porsche diagnostic equips and am beginning to find my way around the FWSM. Still a ways to go...

To the point:
1) While standing over the open hood, by the side of the road the other day, I reflected back to my Alfa (grey market) Spyder days when having a timing light and an *installed* fuel pressure gauge could tell me in a minute as to what system wasn't getting with the program, or if a fuel system was browning-out. I understand that fuel pressure doesn't come up, until crank starts. Is this the only interdepency between their operation? Is it no more complicated than this? Or is there some sort of reverse dependency in which spark depends on fuel pressure, as well?
2) If fuel pressure waits on spark, is there a real reason for maintaining this interdependency? There are plenty of same period, and more recent vehicles that activate the fuel pump, when accessories are switched on...taking in consideration that fuel deleivery is an open loop. Reason for going here, is that fuel delivery can brown-out with low pressure, while not totally failing and its a little stressful to have to crank or jumper to see fuel pressure, when you're in a less than ideal trouble-shooting location, as occassionally happens. There *may* be safety features in here, but my gosh...the number of technophobe operators in vehicles that activate fuel by the ignition key position, and so few flaming cars....

Thanks

Mike

RngTrtl

I know I am not answering your question but I have some suggestions...Replace the fuel pump relay, the contacts inside could be corroded and not allow anough current to pass through and "brown out" the fuel pump as you speak. Just as a thought, I think that the FP relay waits until the starter engages (and enginges get spark at the same time).

Alan

Mike

1) No - fuel pump requires cranking or engine running - thats all... no reverse dependency.

2) Reason is purely safety - 928's can deliver more fuel than most other cars of that vintage + Porsche is conservative (a good thing mostly). If for any reason the engine dies fuel will be cut off. e.g. in a crash no (more) fuel will be pumped onto a hot engine...

Sounds like either a pump (runs good at high rpms?) or engine sensor issue

Alan

mj1pate

Thanks everyone...
I'd like to install an underhood fuel pressure gauge, connected to the test port on the front fuel rail. I need the correct fitting for the test port, to provide safe sealing. Can anyone identify how to specify the correct connector for the fuel rail test port?

Thanks

mj1pate

Ahh....slly me. The gauge can be tapped right into the (new) front cross over hose that connects the two fuel rails.

sharkmeister85

Might be easier to tap it into the existing test cap on the fuel rail. Been done before. http://www.928motorsports.com/parts/fuelrailgauge.php
Glenn

Lizard928

Mike the factory computer will activate the Fuel pump for abotu 4 sec when you first put the key into the ign, so there is FULL fuel pressure when you first crank, however without seeing spark the relay will not be activated, so first fix your spark, THEN if you still have fuel problems look there.

As well does this car have a factory alarm and have you bypassed it?

mj1pate

Thanks Lizard...
will do. Yes, it is an alarm car. I suspect that alarm has been de-activated, as the hatch pin switch has been eroneously "closed" for some time and the alarm system seems none the wiser. But....I'll verify this. I know that there are detailed threads on deactivation.

mj1pate

Thanks Sharkmeister...have just ordered it!

PorKen

My '86.5 doesn't activate the pump until there are enough rpms. No pre-start pumpidge. I have swapped the EZF (plain) relay, and EZF, to no avail.

The rpm cut-in is pretty high, too. My little Nippon starter really has to spin it up. That brings up another pet peeve. The A/C compressor is activated while starting? That's silly. I usually have to shut off the A/C to fire up with the mini-starter installed.

PorKen

I took a look at the LH2.2/EZF wiring diagram, and there is no starter signal to either the LH or the EZF. The LH controls the fuel pump relay.

It looks in the diagram as if there once was, or was planned for, a starter input. There's a space in the diagram for the wire from the CE plug W, term. 22, to term. 4 on the LH.

PorKen

I'm looking at how to force the fuel pump to run while cranking, or just have a switch to prime or lock it on for testing.

Here's some reference info for the '86 CE.



Each plug is marked 1-5 next to each terminal. In the WSM electrical diagrams, the terminals are called out as W11 or Q22, etc. The left side of the connector, if looking at the CE are the 10s, the right 20s.

In the pic above, the red letters indicate a terminal that is unwired on US cars, but powered. Q14 goes to the fuel pump, and the cold air bypass valve on Euro LH2.2s. W22 would go the starter input on the LH (#4), but that circuit is unused on 928s? Some other makes must use it, or maybe the euros, but my LH board is empty in the center of the board where #4 is wired to, it's marked for a couple of diodes and resistors.

The engine must be turning fast enough for the crank position sensor to generate a valid signal to the EZF, which in turn sends a reference signal to the LH. When the LH gets it's signal, it turns on the fuel pump relay via the (-) wire, W15.

John Speake

The fuel pump is energised when processed rpm pulses are fed from the EZF to the LH ECU. When the LH ECU recives these pulses, it pulls one side of the FPR coil to ground. The other end of the coil is fed from 12v.

So to arrange a "prime" switch you just need to ground that terminal of the FPR.

On some MY87 and later LH, the fuel pump is activated for a couple of secs after ignition on, before the engine is cranked. But this is rare, in my experience of seeing over 500 units.

PorKen

Interesting. My '81 L-Jet runs for a few seconds when the key is turned to 'on', but I think that is done through the special fuel pump relay (the rev-limiter is in the relay too).

Looking at the 84 and 85 wiring diagrams, it looks like at least the LH2.2 euros are wired for the starter signal to the LH.

John Speake

OK, on pin 4, LH2.2. I've not checked out that part of the circuit on the Euro version of the LH.