Yes quite normal, these engines rely on a complete sealed vaccum system to run correctly. Any leaks in the system across the engine will affect the cars running as unmetered air entering and being picked up in the exhaust sensors will affect the running, usually lumpy idle manifests itself. What is really happening is that the extra air getting in will not have passed through the Maf and be measured, when the extra air is picked up in the exhaust by the lambda sensors its reported to the engine management module and that detects the mismatch between Maf and exhaust readings as the car running lean. To correct what it sees as a lean condition it commands more fuel to be injected at the injectors to richen the mixture. It will continue doing this until it reachged about 20% more fuel, if it fails to correct it the EML will be lit up on the dash to draw attention to the fault. Code reader at this stage will usually show up as high fuel trims.
Diagnostics showing live data of the fuel trims is useful to diagnose for air leaks, high readings are usually down to air leaks.
Tracking down the leaks by checking for holes and splits in all the rubber intake hoses and vaccum lines should be made visually and replace any duff ones found. Also check the rear of the intake manifold where there may be rubber blanking plugs that have split or fallen off their unused vaccum ports. Dip stick O rings have also been known to fail as well as plastic cam covers that develop cracks and crumbling as the plastic ages from repeated heat cycles. Certain way to test for air leaks is a smoke test, doing that can reveal leaks you cant find with just a visual check.
