Erratic MFI Vacuum Readings

[Wasatch911S]

My 1972 911S with the stock MFI system seems to run and idle fine, however I decided to measure the vacuum of the motor at the port used to control the vacuum advance of the distributor and got some strange readings. The gauge at idle bonces between 6"-15" of vacuum and then goes to a solid 17" when I rev the engine a little. When I look at a diagnostic tables for Vacuum Readings, it indicates that I could have a blown head gasket or leakage between cylinders. I am altitude (5000') so a good vacuum reading here is around 18".

I know an MFI intake setup is a unique in that it appears the two vacuum ports used for the distributor advance only connect to cyls 1 & 4. And of course there is no cyl -cyl connection on a flat 6 if a head gasket was blown. My questions is could this be a normal reading for an MFI engine at idle because of the layout of the vacuum sources or does this indicate that I have a deeper problem I need to investigate because I cannot obtain a steady vacuum reading at idle?

Is there another source of vacuum on a 2.4 MFI motor that would measure all 6 cylinders at together? Thanks for any insight or suggestions.

[Alan.uk]

Hi so I’m reading this correct do you have only one connection on the vacuum line? If so you need two, what’s happening when it’s connected to one is it the vacuum at idle will be lost during its full stroke hence you have an apposing cylinders so the vacuum remains constant. This is why the vacuum at idle flutters but as soon as you accelerate there is enough vacuum caused due to the higher rpm.

Hope that make sense and helps?

Alan

[Wasatch911S]

Alan,

Thanks for jumping in. What you said makes sense in that those two cylinders are opposed and you would think would balance things out, but I do have "T" in the Vacuum line and it is connected to both sides. A line could be plugged I guess. A good place to look. In your experience if both sides are hooked up do you see steady vacuum readings at 900 rpm on an MFI car? I just realized this may be the cause of an idle fluctuation I have been chasing down when the line is connected to the retard canister on the distributor.

[edmayo]

I have seen those tee's plugged up.

[Alan.uk]

Hi, One thing that really caught me out once was a customer had used a washer T piece which has a one-way valve, in the vacuum line so when you check the lines they were clear but connected to the Engine they only worked one way. That took a while to find. Normally when one line is blocked you also hear the vacuum on the distributor click as the vacuum varies. When both lines are pulling correctly there should be a constant vacuum.

HTH
Alan

[Wasatch911S]

Thanks for the advice. I will check both lines independently and make sure there are no restrictions. I do know the previous owner had "locked out" the retard function. He may have plugged a line, or lines, internally.