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Discussion Starter · #1 ·
Well lads,

The auld lad is having a problem starting his 911sc when its warm.

There is no problem starting it when its cold, it never fails, but say for example after a drive if he turns it off it could take up to a half an hour to start again when its warm.

We've been told by other sources that its a problem with the fuel pump or general fuel system but i dont know how true this is.

Just said id throw it up here to see if anybody could shed a light on the problem

Please note the car is a 1979 Porsche 911sc with a 3 litre flat 6 engine.

Advice/Info would be greatly appreciated.

Thanks in advance

Dave
 

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I have my 944 in with a garage right this minute getting the same thing figured out. Mine would always start but would need a few pumps of the accelorator to get it ticking over. I was thinking the injectors may be leaking or something. I realise the two cars are worlds apart but maybe its something common. Is there a strong smell of petrol off it after turning it off?
I replaced the FPR but that didnt help much.
Ill let you know tmro whatever it turns out to be.
 

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Discussion Starter · #3 ·
QUOTE (Oisin @ Apr 22 2011, 01:27 AM) <{POST_SNAPBACK}>I have my 944 in with a garage right this minute getting the same thing figured out. Mine would always start but would need a few pumps of the accelorator to get it ticking over. I was thinking the injectors may be leaking or something. I realise the two cars are worlds apart but maybe its something common. Is there a strong smell of petrol off it after turning it off?
I replaced the FPR but that didnt help much.
Ill let you know tmro whatever it turns out to be.

Yeah dad was saying that it seems to be a common problem as he was reseaching it on some other forum. No theres not a smell of petrol as such oisin.

Sure let me know how ya get on anywy mate, thanks for the feedback
 

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They sound like different issues?

If I understand correctly, Dave, yours starts and runs fine but if stopped after been fully warmed up it can sometimes struggle to restart?
Oisin, yours seems to struggle to start from cold and keep going without encouragement?
 

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QUOTE (Mike @ Apr 22 2011, 10:01 AM) <{POST_SNAPBACK}>They sound like different issues?

If I understand correctly, Dave, yours starts and runs fine but if stopped after been fully warmed up it can sometimes struggle to restart?
Oisin, yours seems to struggle to start from cold and keep going without encouragement?

Mine would be pretty much the same, starts perfectly fine from cold and runs as it should, but when you park it up for 5 mins and start it again, it will struggle for fuel (it wouldnt start atall if you didnt give it a pump of the accelerator), once started, it runs perfectly again. Im about to head over to that garage now, fingers crossed its something simple.
 

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If it's K jet check the accumulator at the pump, it keeps pressure in the system for re starting. When you turn it off listen near the pump and if it clicks every few seconds you know it's loosing pressure, I had the same problem before on a K jet and replaced the accumulator, problem solved, expensive enough part though.
Hope this helps
 

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I called the garage today, seems I went for the wrong end of the fuel rail. It was the FPD (fuel pressure dampener), not the FPR (fuel pressure regulator). It had deteriorated with time and was flooding the engine with pretty much a full on flow of fuel with no pressure dampening. The part comes in tomorrow so should be reassured by then.

Ive shamefully plagerised all this from rennlist, but its good reading and explains it really well so thanks to whoever Greg is.

FPR vs FPD (Edit as Geo mentions below, this is related to the L-jet system, not K-jet)

QUOTE In both systems, the injectors batch fire once per rev, twice for each 4 stroke cycle. In other words, two squirts per putt. Half the fuel is injected each time.

It is a "batch fire" system - that is, ALL the injectors fire at once. "Sequential" (the other system) precisely timed to attempt to fire most of the fuel charge at the open valve *can* get you another 1-2% HP/TQ, an (why manufacturers do it) is because you get slightly better emissions.

On a 928, you'll notice that the "damper" or "dampener" is located at the "IN" end of the fuel rail. This seems odd at first, because the damper is there to absorb pressure waves in the fuel rail from the injectors, therefore it should be at the end of the rail, no? No. Since the 928 does batch fire, the injectors all fire at once , ergo no big waves.

However, the Bosch roller cell style fuel pump does generate some pressure waves - so the damper is really there to absorb those waves, and to act (sort of) like an accumulator. It builds up a "pressure reserve" before all them durn injectors fire again.

The regulator does just what you think it does - it regulates the pressure the injectors 'see' by allowing a certain amount of fuel to be returned to the tank. The vacuum reference allows more fuel to return to the tank during periods of high vacuum (like ~25 kPa absolute) like you see at idle, or during over-run.

How? The damper is just a spring loaded diaphragm and so is the regulator. Though the 928's damper is vac referenced, most aren't. Think of it as a trampoline in a tube. Or (almost exactly) like one of those anti-hammer jugs you put on your hot water lines. Tube contains a flexible membrane on one side, and a spring on the other. Fuel flows under the membrane, membrane flexes at some predetermined rate to absorb the 'jumping' fuel pressure.

Presure regs - though the more pedantic call them 'relief valves', which is technically correct since they are on the "OUT" end, not the "IN" end... I call 'em regulators, because that's what Bosch calls them.

Regs are kinda like dampers. Fuel flows under a diaphragm. This time the spring is a bit more serious in resistance, and the vac ref is really helpful to allow the regulator to keep the fuel at a consistent pressure thoroughout the load range.

Maybe try find a cheapo second hand FPD and see how it runs after..
 

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Nice read Oisin, never really thought as to how they worked. always wondered about the vacuum line, when you see some do and some dont
 

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That info does not apply to K-Jetronic Oisin, K-jet is continuous injection rather than batch. I think the early 928 used K-Jetronic but the later models used L-Jetronic which is probably what our friend Greg is referring to.
 

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QUOTE (Geo @ Apr 26 2011, 08:05 AM) <{POST_SNAPBACK}>That info does not apply to K-Jetronic Oisin, K-jet is continuous injection rather than batch. I think the early 928 used K-Jetronic but the later models used L-Jetronic which is probably what our friend Greg is referring to.
Damn you Greg! Whoever you are.


I put that note in above, thanks geo.
 
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