CIS Troubleshooting questions
 

Hi there,

We talking about a bone stock 1974 911, 2.7 with CIS. I’ve been having some troubles recently with cold running...it will not idle unless it is completely warmed up, then it runs fine. I’ve been having to keep the hand throttle up for 30+ minutes or more. I’ve got the CIS test pressure gauge set-up and have been trouble shooting per my Porsche manual and a CIS book. So, here’s what I’ve found so far…all this refers to a cold engine.

System pressure = 41psi
Control Pressure = 22psi
Fuel Flow (as measured at the fuel return line) 17ozs in 30 seconds
Fuel Flow (as measured before the fuel filter and accumulator) 30+ ozs in 30 seconds
All measured at an ambient air temp of 75f

If I am interpreting the Porsche manual correctly…

my system pressure is low – it should be 65 to 75 psi
my control pressure is low – it should be between 29 to 36 psi
my fuel flow at the return line is low – it should be around 28 ozs/30 seconds

When I discovered the low fuel flow I did the test prior to the filter – there’s no spec, but it seems to be a good flow/volume. So my first guess is the fuel filter – which leads to the questions…

Would a clogged filter lead to poor cold running and could it cause the low pressure readings? What else should I be looking at?

Thanks in advance for any help!!

Mike

Yes it will. Mine was plugged and restricted fuel flow to the fuel distributor. In addition, it puts more pressure on the system and the fuel pump. Your pressure is low as you have stated. Replace the filter and see if that doesn't help. Don't make any changes to the system until you do this.

Steve

"A Porsche does more then just go fast in a straight line"

Thanks for the reply - I'll replace it tomorrow and let you know what happened.

Thanks!!

Mike

If it's the original type fuel filter it is likely the same as the NLA 73.5 fuel filter; you may need to refer to HarryD's conversion/update thread. Jim