Why Does Engine Only Run Well With Pressure Tester Connected??.

[DienstXIV]

Posts: 77
Location: Chattanooga TN
Hey everyone, I have a 1975 911S and I have been having trouble figuring out why my engine runs poorly. I checked for vacuum leaks half a dozen times, replaced all vacuum hoses, all fuel injectors and seals, all intake runner boots and seals, I also replaced the fuel pump, cold start valve and had the WUR rebuilt.

My engine doesn't like to have a consistant idle. When warm the RPMs go up and down between 1200rpms and 2000RPMs, sometimes I can tune the idle screw and fuel mixture and get the RPMs to settle at 900RPMs but then I take the car for a spin and it totally changes. Sometimes it will decide to have a steady idle at 1200rpm or 1300 or it just will stall out or go back to going up and down but again. It's very irradic.

I decided to check the fuel pressure to see what that would tell me and see if the fuel pressure might be fluctuating as well. I hooked up the fuel pressure meter inbetween the WUR and Fuel Distributor and with the valve open the engine runs great! The idle problem is totally fixed but when I close the valve everything goes back to the same issues. Anyone know why this would be?

Btw control pressure is a hair over 2.7bar and system pressure is a hair over 4.5bar.

Cheers!
Alex

[boyt911sc]

Originally Posted by DienstXIV

Posts: 77
Location: Chattanooga TN
Hey everyone, I have a 1975 911S and I have been having trouble figuring out why my engine runs poorly. I checked for vacuum leaks half a dozen times, replaced all vacuum hoses, all fuel injectors and seals, all intake runner boots and seals, I also replaced the fuel pump, cold start valve and had the WUR rebuilt.

My engine doesn't like to have a consistant idle. When warm the RPMs go up and down between 1200rpms and 2000RPMs, sometimes I can tune the idle screw and fuel mixture and get the RPMs to settle at 900RPMs but then I take the car for a spin and it totally changes. Sometimes it will decide to have a steady idle at 1200rpm or 1300 or it just will stall out or go back to going up and down but again. It's very irradic.

I decided to check the fuel pressure to see what that would tell me and see if the fuel pressure might be fluctuating as well. I hooked up the fuel pressure meter inbetween the WUR and Fuel Distributor and with the valve open the engine runs great! The idle problem is totally fixed but when I close the valve everything goes back to the same issues. Anyone know why this would be?

Btw control pressure is a hair over 2.7bar and system pressure is a hair over 4.5bar.

Cheers!
Alex

Alex,

Your problem is learning how to use the CIS pressure gauge correctly. Do you know how to measure the fuel pressures without running the engine? Lastly, you should not close the shut-off valve if you want the motor to run. Why? With the shut-off valve closed, your control pressure would equalize to system pressure causing no fuel flow to the fuel injectors. This is Basic CIS 101.

I doubt you are doing the vacuum leak test correctly. Would you share your methods for locating these hard to find leak sources. Thanks.

Tony