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IMR-Merlin's Avatar
 
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79 SC hard start question

Cross posting here because you guys are my friends and might help.

I have a question on a 79 SC that I bought last week.


I am having a hard start issue with it. I first thought it was a cold start or fuel enrichment issue. After digging through the car I found the ignition timing was about 10 degrees retarded and fixed that. The idle was then sitting @ 2k RPM, so I adjusted that down and also fixed the intake boot that was popped off of the throttle body.

Last night everything was fine, started first crank, timing and idle were double checked and even checked for timing advance under RPM increase.

This morning, I have the same no start/ hard start issue.

It's fuel related and I haven't worked on a CIS car since I sold my last VW 16v about 20 years ago!!

My question is, what keeps the system pressure up during cranking? When I crank my car, the fuel pumps disengage.

Are they supposed to keep running under crank or is there an accumulator that keeps pressure.

If I consistently bump the key, I can get the car to eventually putt, putt, putt on every key bump, but under any type of prolonged crank, there is now fuel getting to the engine.

Thanks in advance for your time

Brendon

Here's a pic of the car when it rolled into the yard. It's definitely a 50 footer....


Old 07-03-2015, 05:03 AM
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How's your fuel pressure? There is an accumulator on your left side of the engine compartment.
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Old 07-03-2015, 05:45 AM
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I'd guess accumulator as well brutha - just aft the fuel filter, simple swap out

you could try the one in your 930 before buying one so you aren't throwing parts at it
Old 07-03-2015, 05:52 AM
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I have one left over from the 930, I will toss it in.

Thanks Paul

Quote:
Originally Posted by krasuskyp View Post
I'd guess accumulator as well brutha - just aft the fuel filter, simple swap out

you could try the one in your 930 before buying one so you aren't throwing parts at it
Old 07-03-2015, 06:22 AM
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Fuel pumps stop running during cranking? Not sure if that's supposed to happen.
Old 07-03-2015, 06:23 AM
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That is what I was wondering also. I think someone might have wired the pumps into the wrong circuit.
Old 07-03-2015, 06:28 AM
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Quote:
Originally Posted by IMR-Merlin View Post
That is what I was wondering also. I think someone might have wired the pumps into the wrong circuit.
My thought too. There are only a couple circuits that stay hot during cranking. Most of the circuits that are hot during "run" are not hot during "start". I'd think that the pumps should be wired to a circuit that remains hot during "start"
Old 07-03-2015, 06:39 AM
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Going to have my wife crank the car and I will check for 12v on the 30 terminal of the pumps....
Old 07-03-2015, 07:04 AM
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Oh boy, this is going to be a long weekend...

I pulled the fuse panel cover and discovered this lovely artwork....

They hardwired the pump to a fuse that is hot with key on, but not under crank....

You can see they spliced one blue wire to the chassis ground and the other to the fuse.

These run directly into the front pump, so I am assuming they did some other fine mojo on the rear pump.

I know there is a post somewhere on here to update the fuel wiring. Anyone know where it is?


Luckily I have my 87 G50 donor here to pull parts from....





Old 07-03-2015, 07:46 AM
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Ok, did some more digging.

I get chassis ground and 12v possitive to the fuel pump relay 85 and 86 terminals with key on, but not during crank.

I can't seem to find any terminals in the fuse panel that have 12v during crank unless they are "hot" at all time.
Old 07-03-2015, 08:43 AM
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I don't believe that the SC has a rear pump...turbo only feature IIRC. There is definitely a circuit or two that are hot on "run" and "start" key positions, or there should be. I ran into this one the hard way when I wired up the Electromotive ignition in my 930. 1st go I wired it to a circuit that was not hot on cranking. Obviously car won't start.
Old 07-03-2015, 08:52 AM
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The SC fuel system has only one pump which is located in the front.
I would bypass the ignition switch and see if the engine will start. The crank contact on these is a common problem with age. Also the SC cold start system is active at a much lower temperature than the 930 so if the CIS daisy chain is not getting power the engine will not start easily.
Check out the CIS Primer on my website if you have time as it will shed some light on the differences in the 2 systems. www.RarlyL8.com
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 07-03-2015, 09:15 AM
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Quote:
Originally Posted by IMR-Merlin View Post
Ok, did some more digging.

I get chassis ground and 12v possitive to the fuel pump relay 85 and 86 terminals with key on, but not during crank.

I can't seem to find any terminals in the fuse panel that have 12v during crank unless they are "hot" at all time.

That's nasty looking!

I believe Porsche used the same color codes on most of these year ranges. If so, the wire off the key (terminal 15) that's hot for 'on' and 'crank' is about a 14 GA Red/Black stripe to the fuse block.

Looks like you have a unused fuse location that you could convert to a second fuel pump fuse, then feed a second fuel pump relay, if you have a rear pump. Is this a turbo conversion, or just a NA SC engine?
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Last edited by WinRice; 07-03-2015 at 09:48 AM..
Old 07-03-2015, 09:45 AM
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Correct, there is only one pump in this car. I was assuming there were 2 in all CIS cars.
Old 07-03-2015, 10:34 AM
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Only the 930 has 2 pumps, the low powered N/A engines don't need the volume.
The pump does not run in the crank position, only in the run position. The cold start injector does all the work until the air meter plate takes over.
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 07-03-2015, 12:12 PM
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Quote:
Originally Posted by IMR-Merlin View Post
Oh boy, this is going to be a long weekend...

I pulled the fuse panel cover and discovered this lovely artwork....

They hardwired the pump to a fuse that is hot with key on, but not under crank....

You can see they spliced one blue wire to the chassis ground and the other to the fuse.

These run directly into the front pump, so I am assuming they did some other fine mojo on the rear pump.

I know there is a post somewhere on here to update the fuel wiring. Anyone know where it is?


Luckily I have my 87 G50 donor here to pull parts from....
Sheeit mahn....that's nothing. My pumps run all the time, and with both pump relays removed. Makes no damn sense at all and will take me God knows how long to sort it out. But right now I don't give a rats ass....she runs, she drives, she gives me a woodie regardless.
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Old 07-03-2015, 01:38 PM
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Looks like a fun car. Looking at the shots I suspect the wiring has been streamlined over time.
I run my pump on the key on. As memory serves it is one of the major B+ as another advised.
My cold start is tied to the starter solenoid wire. I believe large yellow. easy to identify, it's on the solenoid.
You should take off the aircleaner and inspect that pop off valve. This is an assumption yours has this on the airbox. My experience is they all leak around the O.D. therefore causing the symptoms they are sold to react to, lean start. Assuming it is experiencing what you describe the valve may now be damaged and need replacement.
Hang your CIS gauges and check your pressures.
Old 07-03-2015, 01:46 PM
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The valve is in great shape, looks new actually. I got it to run again today, but there is a high idle now.

So high, I can't bring it down with the idle screw.

Need to prep for tomorrows BBQ now, but I might jump into it a little later.

Mark, this is the least of my worries.

After I solve the cold start issue which if FP related.

I have some others:

No power window
No headlights
No horn
No directionals
No hazzards
Wipers only work with FUEL PUMP fuse installed
Drivers door auto locks
No heater blower


That's just the stuff that sparks and smokes when you play with it. LOL
Old 07-03-2015, 02:01 PM
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You have a classic case of Gremlins
Google "how to drive out Gremlins"
Old 07-03-2015, 04:03 PM
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High idle = air leak. Check the AAR for power and function then look for O-rings, hoses, and runner sleeves; they all can cause extreme high idle.

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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 07-03-2015, 05:05 PM
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