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SC starting issues....


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After my Le Mans wheels let me down by STILL no running right before like a young puppy, leaving an oily puddle for me to find, I'm forced into Plan 'B' which is my SC. It has always started on the first piston on the first turn, however lately (last 12 months) it is super reluctant to cold start. Each turn of the key it fires very briefly, after multiple tries it gets closer to catching and eventually goes. Subsequent hot starts after a hour or even two are 100% fine. 

Q; is this residual fuel pressure leaking away overnight? If so how best to restore pressure before attempting a start? Run pump on key? Run pump at fusebox? Lift arm in airbag with ignition on? Something else? I'm leaving tomorrow morning, so no pressure.... (bad pun).

Thanks, Chris

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I don't have the gear to measure cold pressure..... But, in breaking news (and after 40 spirited miles yesterday) it started 1st 2nd and 3rd times on the button from cold...... In other words, how it always was until about 12 months ago. Cinders you SHALL go to Le Mans! Perhaps it was just teasing me. 

If you see an AA breakdown truck heading for Chateau Chanteloup near Sille Le Philippe, you'll know who it is!

Chris.

 

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Fingers crossed all will be well.

If unfortunately it does breakdown insist it gets shipped back to the UK for repairs, as they will want to take it locally. When my ECU (we didn't know at the time) started playing up at the Le Mans classic, it was taken to Porsche OPC Le Mans. 3 weeks later they still couldn't figure it out and left the car in a mess. Got it brought back to the UK and off to Jaz, who found the issue within a couple of days. 

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Ok kids, the problem has returned. Fortunately I made it to/from Le Mans but it was again super-reluctant to start when cold. It would fire on every turn of the key but only very briefly, then crank and crank without firing. So the trick is just turn the key, if it doesn't start within a second, turn the key back and re-try until that initial fire is enough to 'catch' and the engine will run 100% fine as if nothing had happened. However, before my trip I tried it every day and it started 1st time from cold. It will always hot start, so what is it about 1 in 5 cold starts?? I'm wondering if the CDI unit fails when volts drop due to cranking? I read somewhere Jonny's CDI unit is good down to 8.5v but what about the originals? Coil same perhaps? Both get warm during use....

Still scratching my head..... BTW Le Mans was good! 

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I agree, however why does it start perfectly from cold 3 out of 5 times? What alters the mixture that dramatically from one day to the next? No harm in trying the procedure above of course, I have the CIS mixture tool.....

Chris

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There is a wire from the starter to the coil I think (not 100% where it goes) that ensures some extra juice during a cold start. I discovered this when installing a WOSP starter. The WOSP instruction said this wasn't needed and you didn't need to connect this but I found my cold starts were awful and I had to crank much much longer before it caught for cold starts (hot starts were still ok), Once connected back to normal starts. Just a thought.

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It's a yellow wire that receives voltage when cranking and goes the the thermotimer switch in LH cam cover. From there (when cold and only for a few seconds) it powers the 7th injector in the inlet plenum. It's only required when it's proper cold and mine has been disconnected for a few years now with no apparent difference as the car stays under it's duvet when it's that cold! 

Chris.

 

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8 hours ago, Chris T said:

Ok kids, the problem has returned. Fortunately I made it to/from Le Mans but it was again super-reluctant to start when cold. It would fire on every turn of the key but only very briefly, then crank and crank without firing. So the trick is just turn the key, if it doesn't start within a second, turn the key back and re-try until that initial fire is enough to 'catch' and the engine will run 100% fine as if nothing had happened. However, before my trip I tried it every day and it started 1st time from cold. It will always hot start, so what is it about 1 in 5 cold starts?? I'm wondering if the CDI unit fails when volts drop due to cranking? I read somewhere Jonny's CDI unit is good down to 8.5v but what about the originals? Coil same perhaps? Both get warm during use....

Still scratching my head..... BTW Le Mans was good! 

Try a different fuel pump relay - might just power fuel pump for an instant, then drops the power so no fuel.  If the engine catches, then the fuel relay is using a different outlet, so no issues.

 

Could also be a lean mixture - do you have a pop-off valve - if so, make sure that it is well seated.

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  • 3 weeks later...
Posted (edited)

Well, I'm beaten..... Symptom confirmed as no power to fuel pump while cranking, and from here on you'll need a wiring diagram to keep up..... Fuel pump relay; 87a has ignition switched power, no power while cranking. 87 has power from terminal 50 only while cranking to avoid air flow sensor stopping play while cranking. 30 goes direct to pump (but 35 ohms to earth 'cos WUR also in circuit). 86 has ignition switched power to operate the relay and 85 stops the pump if grounded, correct operation for air flow sensor contact. I've tried a couple of different relays although not another red one. Same symptoms. I'm stumped.

Chris.

 

Edited by Chris T
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If only so simple….. one wire goes to pump from terminal 30 of relay. Shoving 12v down that makes the pump work. Besides, it runs when ignition on, just not while cranking….

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1 hour ago, Chris T said:

Well, I'm beaten..... Symptom confirmed as no power to fuel pump while cranking, and from here on you'll need a wiring diagram to keep up..... Fuel pump relay; 87a has ignition switched power, no power while cranking. 87 has power from terminal 50 only while cranking to avoid air flow sensor stopping play while cranking. 30 goes direct to pump (but 35 ohms to earth 'cos WUR also in circuit). 86 has ignition switched power to operate the relay and 85 stops the pump if grounded, correct operation for air flow sensor contact. I've tried a couple of different relays although not another red one. Same symptoms. I'm stumped.

Chris.

 

Nearly ... Pin 85 goes to ground via the flap valve, but looses ground (and closes the relay) when the contacts are open.

 

Do you have power to pin 87 while cranking ?  If you have power while cranking and pin 30 makes the pump run, then the issue is the relay.

If you have no power at pin 87 while cranking, check  the yellow wire at the ignition switch (pin 50) - there should be 2 - the other going to the starter

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I do indeed have power to 87 while cranking, 87a while not cranking. When 85 is grounded Via safety switch and operates the relay throwing the switch connecting 30 to 87 from 87a where it rests. As it is, 85 is never grounded as safety flap valve is disconnected. this I guess means relay is never operated, so 30 stays connected to 87a at all times.... and not powered during cranking.

Strange, as 2 separate Porsche specialists and others here advised disconnecting the safety switch as it's a potential fail point. However if that's the only way 85 ever sees ground then it's the only way the pump receives power while cranking....?

Now have I got it right Peter? Off to the garage to try my theory! 

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My mistake - the relay is grounded when the flap is open and closes when it is shut, so the microswitch is normally open at rest.  The starter feeds current via 87 when cranking and when the engine is running, the flap valve is open, the normally open microswitch grounds the relay, so power flows via pin 87a.

 

 

If you have power to pin 87 while cranking and pin 30 makes the pump run, then the issue is the relay.  If you bridge 87 to 30, the car should start but stall (no fuel) once you let the key back to the run position.

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Peter, you were right first time. Air flow flap microswitch is grinds 85 when shut, thus operating the relay (diagram shows relay at rest 30 connected to 87a) to make 30 (cranking power) connected to 87. As a safety device it stops the pump running by switching 30 to 87 (now not powered) should the engine stop. So it's my assumption it needs the switch to operate properly OR for those who have disconnected it there is enough residual pressure to fire the engine.... SO. my suspicion now falls upon the one way valve on the fuel pump or pressure accumulator. 

I'm sure 99% have switched off by now, so I'll email you direct when I get to the bottom of it.

P.S. I'm cracking on through those theory modules, 4 today! 

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8 minutes ago, Chris T said:

 

I'm sure 99% have switched off by now, so I'll email you direct when I get to the bottom of it.

 

Please don't - I for one are interested in the solution!

 

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1 hour ago, Chris T said:

Peter, you were right first time. Air flow flap microswitch is grinds 85 when shut, thus operating the relay (diagram shows relay at rest 30 connected to 87a) to make 30 (cranking power) connected to 87. As a safety device it stops the pump running by switching 30 to 87 (now not powered) should the engine stop. So it's my assumption it needs the switch to operate properly OR for those who have disconnected it there is enough residual pressure to fire the engine.... SO. my suspicion now falls upon the one way valve on the fuel pump or pressure accumulator. 

I'm sure 99% have switched off by now, so I'll email you direct when I get to the bottom of it.

P.S. I'm cracking on through those theory modules, 4 today! 

Next time you go to cold start, remove the relay and link 30 to 87 to run the pump for a few minutes.  You then have a primed system (hopefully takes the one way valve & accumulator out of the equation), then quickly refit the relay and try to start.

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No need, fuel pump runs when ignition switched on but running for a few seconds doesn't significantly make any difference. 

 Q; to standard SC owners ie. safety air flow switch plugged in, does the fuel pump run with ignition turned on, pre cranking? If my reading of the wiring is correct it should not (85 earthed so relay deployed meaning 30 is connected to 87 waiting for power when cranking).

Chris.

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Posted (edited)

No, the pump doesn't run before cranking on an SC.  It only runs during cranking and when the air plate is lifted (air flowing).

My guess is the wiring is not correct.

Edited by Jonny Hart
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