To have an engine, gas or diesel, start easily and consistently requires that many things must be in order. Some things will be peculiar to either gas or diesel but most requirements are common to both. Many of the essential needs are interdependent; for example, compression and cranking speed. Both must be within tolerance but as compression decreases, cranking speed will increase – all other things remaining unchanged.
Adequate compression is needed. Compression is the term used when air is taken into a cylinder (intake stroke) and then the piston is pushed into the cylinder (compression stroke) reducing the volume of the combustion chamber and compressing the contents of the cylinder. When starting, the starter motor provides the force required for the compression. As the pressure within the cylinder increases, the air being compressed leaks out a little. The higher the compression, the faster the leak and the slower the cranking the more time for the leak to persist. The internal condition in the engine contributes to the speed of the leak. Bad rings or valves can contribute greatly to poor compression. Low cranking speed can give the same results.
Compression test results are expressed as “pounds per square inch” (PSI). The readings should be close to the same across all cylinders. Gasoline engines should have compression readings between 90 and 120 PSI and diesel engine pressures will range from 340 to 475 PSI. Diesel compression is much harder to measure and requires better measuring equipment.
Low compression with proper cranking speed requires that the engine be repaired. Sometimes that repair is as simple as adjusting the valves or doing a valve job. More often in engines with higher running hours new piston rings will be required, i.e.; a major overhaul.
If compression is low because the cranking speed is low, there are a lot of things that must be checked and corrected to crank faster.
1- The starter motor must be in good shape. There are bearings, brushes and electrical components that age or wear over time, resulting in diminished cranking speed.
2- The batteries must be adequately rated and in good condition.
3- All related wires and cables must be in good condition with all the connections clean and properly tightened. Voltage can drop dramatically if a connection is oxidized, even if the retainer appears to be tightened firmly. The wiring and connections to the starter switch and dash panel must also be good.
4- The grounding of the starter to the engine block must be clean and tight.
5- The starter motor solenoid must pull in solidly and have clean internal contacts.
6- The engine lubricating oil should not be overly viscous.
The fuel must be relatively fresh, of the proper Cetane or Octane and free of water or additives.
Gasoline engines must have:
1- Properly operating choke.
2- Good spark plugs of the proper heat range.
3- Good low voltage components. (Points, distributor cap, rotor, condenser and wiring)
4- Good high voltage wiring and components. (Plug wires and coil)
5- The carburetor must be clean and properly adjusted.
6- The ignition timing must be to spec.
Diesel engines must have:
1- A good injector pump.
2- Good injectors.
3- The injection timing must be to spec.
4- Good glow plugs or manifold heaters – where installed.
5- Good preheating actuators and wiring.
All the above (and probably more) should be checked and discrepancies corrected. One or more items could contribute and fixing just one might improve starting, but not make it the best possible.
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