A propane enrichment tool is excellent to find minor air leaks. With the tune-up tachomete
Pumping losses are the work that is required to move the air/fuel mixture into the cylinder bore and then evacuate the spent gases on the exhaust stroke. Thermal losses are the heat energy that is sent to the engine coolant, engine block, and out the exhaust pipe. Internal friction is the last loss and that is the fuel used for the engine to turn the crankshaft, camshaft, valvetrain, and water and oil pumps. For this reason, an engine needs a certain amount of idle speed just to keep going but anything beyond that is unnecessary and wasteful.
The mixture strength or amount of fuel and air required to run the engine is based on the fuel’s stoichiometric rating. This is the recipe of fuel and air required for the most efficient chemical conversion of the gasoline. For street gasoline without any ethanol added, the value is 14.7:1. That is 14.7 parts of air to one part of fuel. When the gasoline has 10 percent ethanol, the stoichiometric value drops to about 14.3:1. If you are interested in learning more about gasoline and its interaction with a Pontiac engine, reference the Sept. ’11 issue of HPP, which includes a primer and fuel test.
Causes of Idle Instability
Chevron Techron is an excellent tool to remove deposits from the fuel injectors and intake
The main cause for a high idle speed is to mask instability—either a roughness or swing in engine speed. The main culprit for idle instability is uneven fuel distribution in the intake manifold. If each cylinder is receiving a different air/fuel ratio, then the amount of power produced during each expansion stroke (the proper name for the power stroke) will be altered. Distribution problems are usually rooted not in the intake manifold design, though some amount of variation can be found there. The real issue lies with the mixture delivered by the carburetor or fuel injector and then the variation in the created versus delivered mixture strength.
Created mixture is the one that exits at the carburetor throttle plates and is found just as the charge enters the plenum of the intake manifold. Charge describes the homogenized fuel and air. Delivered mixture is what ends up in the cylinder bore and is ignited.
Many things can alter and can be responsible for this difference. The carburetor serves three purposes: throttle the engine, atomize the fuel, and emulsify the fuel. Throttling describes controlling the amount of airflow entering the engine. Atomization is breaking the liquid fuel into very small particles. Emulsification is mixing the fuel with air. That is why a carburetor has emulsion tubes or orifices in it. But the fuel is still not ready to burn in the cylinder.
An EGR valve that is not closing all the way will dilute the charge with exhaust gas and i
A phase change called vaporization needs to occur. That is when the fuel changes from a liquid (in very small particles) to a rarefied or gaseous state. The vaporized fuel is what combusts. Vaporization occurs in the intake manifold with a carburetor and in the cylinder head intake runner in a port EFI system. It requires heat for the phase change to happen. It is identified as the latent heat of vaporization.
If the carburetor does not do a uniform or good job of atomizing and emulsifying the fuel then the vaporization rate in each cylinder will be different. Heavy fuel particles will stick to the intake manifold runners leaving behind a high level of hydrocarbons. Then as the speed of the charge in the manifold runner increases or more heat is applied, it will pull away some of the wetting from the manifold wall and alter the delivered mixture. Thus, the created versus delivered mixture will be different.