Vacuum Leaks and Running Issues: Setting the Record Straight

I cut my teeth as a mechanic back in the '70s, mostly working on carbureted American inline-6 and V-8 engines. Whenever there was a misfire at idle (running on less than all cylinders)—and I knew I had spark, fuel, and compression—I started looking for a vacuum leak.

It was common back then for a vacuum hose to come off near one cylinder and cause that cylinder, and only that cylinder, not to fire at idle. The reason was that these engines were carbureted. All of the fuel entering the engine was being mixed with air way up at the carburetor. If a vacuum leak near one cylinder was present, that cylinder would suck straight air (no fuel) via the vacuum leak, which would throw off the air/fuel mixture to that one cylinder—thus making it too lean and causing it not to fire (misfire).

Fast forward to the '80s and the introduction of electronic fuel injection (EFI) with an injector at each cylinder (multi-port EFI) replacing the carburetor. No longer was the fuel being mixed with air up at the carburetor. Instead, the total amount of air being drawn in by all cylinders was measured by a device up where the carburetor used to be: the air flow meter (AFM). The information from the AFM was transmitted to a computer electronically, which told the injectors how much fuel to inject.

With this system, it is not possible for a misfire to be blamed on a small vacuum leak, because any small vacuum leak—no matter where it is—simply reduces the flow of air past the AFM, which, in turn, reduces the flow of fuel from ALL injectors. The resulting reduction in fuel causes ALL cylinders to run too lean, not just one. And there is more to the story...

Carbureted vehicles back in the '70s were called "open loop" systems. If anything went wrong upstream, there was nothing downstream to measure the results and tell the carburetor to make an adjustment. The EFI system in all Vanagons includes an oxygen sensor downstream in the exhaust system. This makes it a "closed loop" system. If there is a small vacuum leak that causes a general lean condition, the oxygen sensor detects it—then it sends its signal to the computer, which corrects the mixture.

Thus, when you are trying to chase down a running issue on any engine with EFI, don't bother looking for a small vacuum leak to blame. It would have to be a pretty massive leak in the intake system to make a big enough difference to the air flowing past the AFM to actually cause any noticeable running issue. Any mechanic worth his weight in 1970's salt can tell you that!

S. Lucas Valdes