CIS troubleshooting question...
 

Hi all,

In regards to my 1974 911, 2.7 liter, stock. I’ve been troubleshooting some problems with the CIS and I’ve run into a big one – I think. The System Pressure is low by about 1 bar. I put a few shims in the pressure regulator – in the fuel distributor – but that did not seem to make any difference. I would add that fuel flow – as measured at the fuel return line – is low. While not a test procedure in the book, I measure fuel flow out of the fuel accumulator and it is considerably more than I get from the return line. I have replaced the fuel filter – that did not have any impact. I’m thinking weak fuel pump, but before I go out and spend $350 on a new one I thought I’d ask for some advice.

Thanks in advance!!

Mike

Are you using a CIS pressure guage and test rig to verify the fuel pressure. If you are not I would suggest blowing the $100 bucks and use it to check the fuel system before spending money on parts you might not need.

Hi Pat, I am using the CIS Pressure Gauge - that's how I know my system pressure is low by 1 bar.

Thanks!

Mike

The K basic fuel pressure should be 5 BAR/ 73 PSI as per the Bosch CIS fuel injection management book.

I'm getting about 3.5 bar. Question is - what components might cause low system pressure? What should I be looking at?

Thanks!

Mike

This may or may not help!
I have CIS on a non P car.
From the manual, the flow of the pump at the return should be 750 ccs in 30 seconds minimum.
For comparison the system pressure on this car is 5.1 +/-.4 Bar
Jeff

Are you sure that you are hooked to the high pressure side. The first time I tested a CIS I was measuring the control pressure side which could be as much as 1 bar lower on cold starts. The line to the cold start valve is a good place to get system pressure. If it is still 1 bar low and there is no blockage in filter, accumulater, line etc then you need a new pump. This is how these roller pumps fail. They start losing pressure. And on a CIS low system pressure causes all kinds of problems from starting to running. Good Luck.

I am measuring the system pressure from the line that goes to the warm-up regulator - but I don't remember which side I had the valve on to test for system pressure - I'll check that again tonight. I'm not getting 750 cc's in 30 seconds - it's more like 600. And I don't think there are any blockages - I get good flow out of the fuel filter.

Question - Is there an alternative to getting a replacement Bosch fuel pump – they are sort of pricy? Is there another model/manufacturer? It looks to me like a simple inline fuel pump would work – as long as it could make the pressure/flow required.

Thanks!

Mike