400i Vacuum Leak Test

For the brake booster disconnect the hose at the engine end and plug the vacuum port see if the idle changes.

First thing you need to do is determine which bank is not running correctly. If both banks probably not a vacuum leak.

Propane while I am watching an exhaust analyzer is the best way in my experience.

 

What symptoms are telling you that you have a vacuum leak? That should tell you WHERE to test.

An old school method is to use something like shaving cream on the suspected leak and see if it gets sucked in. I think There is a foam for that. McMaster-Carr has several options: leak detectors | McMaster-Carr
Ken

 

Common symptom on both BBi, 400 and 412. First off any running issue requires a complete check to be sure its all tuned properly before beating your head into a wall. 400i's have such a well deserved reputation for ignition advance issues if it was sitting in my shop Id be sure of that first. Basically if the fuel mixtures and the air flow ratios of what is coming in past the throttle plate and what is coming in through the air bleeds is not correct it will do that. Do you have a 4 gas analyizer at your disposal? I consider that essential for tuning CIS. You should be able to snap the throttles wide or nearly wide open and have it drop straight to the correct idle RPM.

Also how many miles since it had a timing chain replaced and the cams properly timed?