This is a problem relating to multi-port fuel injected vehicles. Previously, fuel entered the intake manifold through a carburetor or fuel injectors positioned in a throttle body. The fuel passing through the intake manifold kept the interior of the manifold free of carbon buildup. Multi-port fuel injection differs in that the injectors are in the lower manifold runners, close to the intake valves. Air and positive crankcase ventilation (PVC) induced carbons are the only things passing through the manifold. The PVC valve deposits carbon on the walls of the intake manifold referred to as “coking.” Coking begins to be problematic when sufficient coking material accumulates, restricting airflow and fowling the injectors. The rate of coke buildup depends first on the climatic zone, and secondly by the mileage induced wear on the engine. The climactic zone matters because the colder the engine, the longer it takes to reach normal operating temperature and seal the rings. Mileage induced wear refers to the piston ring and valve guide wear, which creates higher levels of crankcase pressure. The increase in pressure forces more carbon through the PCV valve and into the manifold. By removing the buildup of coke in the manifold and injectors, the idle will smooth out, the throttle response will quicken and the fuel economy will increase.


  • Drop in fuel economy
  • Rougher than normal idle
  • Slower than normal acceleration


  • Abundance of coking in the intake manifold
  • Carbon covering the throttle plate in the throttle body
  • Fouled injectors

Procedure For Cleaning The Intake Plenum

This requires the use of an upper plenum cleaning kit. The kit consists of a pressurized bottle of “Upper Plenum Cleaning Agent, an “on and off valve” and a “spray nozzle.”

  • Place the vehicle in an open area due to the smoke and fumes produced by this process.
  • Remove the rubber air duct attached to the throttle body. Never attempt to run the cleaner through the mass airflow sensor as it will destroy the hot wire, rendering it useless.
  • Open the throttle and look past the throttle plate into the manifold. If it is severely coked it will require two bottles of cleaner.
  • Install the “S-shaped” nozzle in the throttle body with the nozzle centered at the top of the throttle plate.
  • Push the rubber air duct over the nozzle tubing to hold it securely in place.
  • Connect the hose from the nozzle to the valve and install a bottle of cleaner. Hang the bottle from a stringer under the hood.
  • Start the engine, increase the throttle to 2500 RPM and hold it there through the entire procedure. If the throttle can’t be advanced from under the hood or is an electronic throttle, a helper will be needed to hold the throttle. The RPM must be high when the bottle opens or the engine will stall.
  • When the RPM stabilizes at 2500, open the valve and allow the injection of the cleaner.
  • Continue to maintain the RPM until most the smoke dissipates, then shut the engine down. Remove the nozzle and see if one bottle was sufficient to remove all the coking. If not, repeat the process with another bottle.
  • Remove the nozzle and install and tighten the air intake duct.

Fuel Injection Cleaning

This requires a bottle of fuel injection cleaner, a valve with a pressure gauge and a hose to the fuel injector rail.

  • Open the main fuse block and remove the fuel pump relay.
  • Thread the bottle of fuel injection cleaner onto the valve and pressure gauge combination and hang it from under the hood.
  • Pull and block off the hose to the fuel pressure regulator to prevent the cleaner from returning to the tank.
  • Install the hose from the cleaner on the fuel pressure test port on the fuel injector rail.
  • Open the valve on the cleaner and turn the pressure control knob to 45 PSI.
  • Start the engine and allow it to run on the bottle of cleaner until it stalls when the bottle is empty.
  • Turn the key off and install the fuel pump relay.
  • Remove the fuel injector cleaner hose on the fuel rail.
  • Install the vacuum hose on the fuel pressure regulator.

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