Cold/hot starting problems and CIS values off
 

So, my cold and hot starts have progressively gotten worse. When I got my '78 911SC a couple years back, it used to start at the tap of the key - almost like it was an EFI.

Got the vacuum and pressure gauges out this morning. Everything seems to be spec but for a low control cold pressure and high residual/leak numbers. It was 78F when I test. Manual numbers to the left, actual to the right -

System Pressure 4.5-6.2 4.6
Control Cold 2.4-3.3 2.1 (yes, 7.7 PSI of Vacuum applied)
Control Warm - stabilized after about 8'
No Vac 2.7 - 3.1 2.9
W/Vac 3.2-3.6 3.6 (again with 7.7PSI)
Leak Test
10' 1.3 1.6
20' 1.1 1.4

So would a bad check valve be the culprit of low control cold pressure and higher leak numbers? One of the few things I have not replaced. Noted the system pressure check was on the low end. Recently went through electrics and had CDI rebuilt, new plugs, wires, cap and rotor. I did notice some pretty definitive arching on cap points and a bit on the tip of the rotor.

Thoughts?

To rule out a bad check valve I would pull the security switch plug at the sensor plate housing temporarily. Then the pump runs already when ignition switch is set on. So you can prime the fuel lines a moment before starting the engine. Does it starts better then?

If the cold control pressure is suspected - most of the cis control pressure gauges have a valve. I would play a bit with to get it barely closed to tune the control pressure a bit higher. Does it starts better then?

Thomas

If you think you may have a fuel problem, maybe use a carb cleaner spray into the intake at startup, right?

I also would suggest to use start fluid before start, but only when my above mentioned tests have been failed...with that extra shot of start fluid you emulate the function of the thermo time switch...

Thomas

CIS Troubleshooting………
 

F1LOCO,

The residual pressure test @ 1.4 bar after 20 mins. PASSED the test. The higher the values the better is your residual pressure. You misinterpreted the test results. A defective FP check valve has no adverse effect on your low control pressure.

Tony

Thanks for the replies. While surfing through I don't know how many posts, I came across one that mentioned potential battery voltage issues causing the longer cranks. Checked my voltage and sure enough it was at 11.7. Put in on the charger over night, 12.6 this morning. Touched the key and bang - started right up. Was running 12.2 after firing it, but once I revved it and excited the alternator diode to get the alternator light out, I was running at 13.3. Took it for a short spin and currently 12.9. Going to see if it drops over a few hours time. Running an Odyssey 925. This is actual my second (replaced first under warranty). I read somewhere I should be running at least a 50ah battery with the '78 alternator, which apparently the 925 is not at 28ah. May have to shop for something to fit the Rennline holder I forked out for when I bought the Odyssey with a bit more amp hours. Too many options it seems these days. Definitely want to stay AGM but may consider the lithium. Just don't want to burn the 911 or the house down.

Keyword battery - I posted a few days my opinion here, don't want to repeat it here: https://forums.pelicanparts.com/porsche-911-technical-forum/1173460-battery-disconnect.html#post12404963

Time will tell if this is your culprit. If the starter doesn't reach for any reason it's normal speed may this make the difference whether the engine starts or not. Keep us posted.

Thomas

Read here under No-Start; https://www.systemsc.com/diagnostic.htm