Yo J,
I don't know about the back pressure causing that.
Following by Walker Exhaust:
Vacuum Test, General (Click Inspections); "...The intent of the vacuum test is to determine if there is a blockage or restriction in the exhaust system. It may or may not indicate converter problems. Ideal backpressure levels vary from one application to another. Different size engines have different flow rates and so do their exhaust systems. Check manufacturer specifications. Readings also depend slightly on where in the exhaust system they are taken. This test can be performed using any suitable engine vacuum source, including the carburetor, intake or injection manifolds. After the pressure gauge is connected to a vacuum source, readings are noted at a variety of engine speeds. To perform the vacuum test at the intake manifold, start with the engine off, and disable the EGR valve by removing or disconnecting its hose or one of its solenoids. Connect the pressure gauge to the manifold and take an idle reading with the transmission in neutral. Turn off the engine and disable the EGR valve by disconnecting its vacuum hose. Plug the hose. With the transmission in neutral, take another reading at 2500 to 3000 RPM. Read at variety of other engine speeds. To perform the vacuum test at the injection manifold, start with the engine cool, and remove the air injection check valve. Connect a rubber adapter cone to the check valve connection. Then, connect gauge hose to the adapter. As engine speeds vary, readings should initially drop slightly, then rise to within 2 to 3 inches of the vacuum level established at idle. Any sudden drop of over 10 inches of vacuum may indicate a blockage problem. Erratic swings of the vacuum indicator may indicate periodic blockages caused by loose components temporarily blocking the exhaust system. Remember that vacuum levels are also affected by factors other than exhaust system restriction, including valve and ignition timing..."
High exhaust back pressure (greater than 1.5 psi at idle and 3 psi at 2000 rpms on most vehicles
- Using the O2 Sensor ports, check for excessive backpressure ahead of - and behind - the converter.
- High backpressure behind the converter indicates a restricted muffler or resonator.
- High backpressure ahead of the converter indicates a restricted converter.
- High backpressure at the exhaust manifold indicates a blocked Y-connection.
- Excessive backpressure will adversely affect AFR, leading to excessive emissions.