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Registered User
Join Date: Oct 2010
Location: Southern NH
Posts: 3,553
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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.... ![]() |
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Registered
Join Date: Jul 2006
Location: Honolulu, HI
Posts: 9,864
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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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The fun - '06 Carrera, '79 930, '06 S4 Avant, '16 i8 The mundane - '24 Tesla Model 3, '22 Tesla Model Y, '19 Tacoma |
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Banned
Join Date: Nov 2001
Location: top of 3rd
Posts: 4,336
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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 |
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Registered User
Join Date: Oct 2010
Location: Southern NH
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beancounter
Join Date: Jan 2008
Location: Weehawken, NJ
Posts: 3,593
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Fuel pumps stop running during cranking? Not sure if that's supposed to happen.
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Registered User
Join Date: Oct 2010
Location: Southern NH
Posts: 3,553
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That is what I was wondering also. I think someone might have wired the pumps into the wrong circuit.
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beancounter
Join Date: Jan 2008
Location: Weehawken, NJ
Posts: 3,593
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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"
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Registered User
Join Date: Oct 2010
Location: Southern NH
Posts: 3,553
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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....
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Registered User
Join Date: Oct 2010
Location: Southern NH
Posts: 3,553
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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.... ![]() ![]() |
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Registered User
Join Date: Oct 2010
Location: Southern NH
Posts: 3,553
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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. |
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beancounter
Join Date: Jan 2008
Location: Weehawken, NJ
Posts: 3,593
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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.
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Crotchety Old Bastard
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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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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds '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 Last edited by RarlyL8; 07-03-2015 at 09:19 AM.. |
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Registered
Join Date: Jun 2005
Location: Colorado
Posts: 712
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Quote:
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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1986 911 Turbo 3.3L, K27HFS, Tial 46mm, TurboKraft Intercooler, 964 Cams, Monty Muffler, MS3Pro Evo, M&W Ignition, Zietronix WBO2 Data Logger, Wevo shifter, coupler and motor mounts. Last edited by WinRice; 07-03-2015 at 09:48 AM.. |
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Registered User
Join Date: Oct 2010
Location: Southern NH
Posts: 3,553
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Correct, there is only one pump in this car. I was assuming there were 2 in all CIS cars.
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Crotchety Old Bastard
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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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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds '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 |
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Registered
Join Date: Jul 2008
Location: Central Washington State
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Quote:
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Mark H. 1987 930, GP White, Wevo shifter, Borla exhaust, B&B intercooler, stock 3LDZ. |
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Registered
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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. |
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Registered User
Join Date: Oct 2010
Location: Southern NH
Posts: 3,553
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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 |
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Registered
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You have a classic case of Gremlins
Google "how to drive out Gremlins" |
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Crotchety Old Bastard
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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.
__________________
RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds '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 |
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