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A Difficult Diagnosis: When Multiple Conditions Create Similar Symptoms
By: Larry Hammer | Thursday, July 21st, 2022 at 11:45 am in On The Line

A Difficult Diagnosis: When Multiple Conditions Create Similar Symptoms

Customer complaints of rough idle, misfires, crank but no start or long crank time symptoms can be a challenge with today’s technology. Random or multiple misfire codes may be stored in memory. Symptoms that seem like an ignition related issue may actually be fuel related, carbon induced, or in some cases a normal characteristic.

Fuel Volatility

Fuel volatility and oxidation can promote any one of the performance issues previously mentioned. Fuel volatility is the gasoline’s ability to change from a liquid to a vapor. Reid Vapor Pressure (RVP) and the distillation curve are terms that describe fuel volatility. The recommended RVP is usually 9 in the summer and 12 in the winter. It can be tested in the field with a test kit. Fuel rated in the 12 RVP range can promote surging, hesitation and stalling on a warm day, especially following a heat soak. Fuel in the 9 RVP range can result in hard starting, long crank times, rough idle, hesitation, backfiring, or other lean performance symptoms during cold temperatures. The distillation curve is a measurement of the percentage of fuel that evaporates in relation to temperature and can only be tested in a lab. Fuel volatility must vary in relation to climate and geography changes. The distillation curve can have a greater effect on cold drive ability than the RVP. Burning a higher-octane fuel than recommended by the vehicle manufacturer can contribute to rough idle, induction backfiring, surging, and stalling during warm-up, due to the poor volatility of some of the higher-octane fuels. Trying a different brand fuel, preferably a Top Tier rated fuel, is often a recommended approach to resolve fuel related concerns.

Gasoline Direct Injection

Gasoline Direct Injection (GDI) poses a new set of challenges and service opportunities. The formation of carbon is common with this type of fuel injection, as the fuel is injected directly into the combustion chambers, bypassing the intake valves. This injection arrangement eliminates the benefit of fuel wash on the valves, resulting in an accumulation of carbon deposits and promotes sticking valves. Excessive carbon build-up results in performance issues in the form of surging, stalling, hard starts, misfire codes stored, etc. The carbon deposits can create turbulence with the air-fuel mixture in the combustion chambers, resulting in hot spots due to uneven fuel distribution and improper burning of the gases. A major concern is decarbonizing an engine with heavy deposits. Chunks of carbon can dislodge during the cleaning process, resulting in damage to pistons, rods, valves, cylinder walls, oxygen sensors and the catalytic converter. To prevent this, an annual or every 15K mile cleaning may prevent this condition, saving the customer a lot of expense. Ask your Mighty representative about the available products and services.

Extended Crank Time

Compared to port fuel injected engines, the GDI equipped engines incorporate longer crank times, and this should be considered a normal characteristic. This is especially noticeable during cold ambient temperatures. The GDI systems operate at higher fuel pressures, which requires the mechanical fuel pump to build up the required pressure before the first injection event occurs. The crank time can vary from 1.5 seconds to as much as 7 seconds in extremely cold temperatures. Engines consuming E85 fuel may require a crank time twice that of an engine burning 87 octane fuel. During colder weather, an increase in the amount of gasoline is required in the ethanol mix to reduce the start times.

Cold Start Clicking Noises

GDI equipped vehicles may encounter cold start clicking/ticking noises and these symptoms are a normal characteristic. The noises may be heard from the engine compartment and are more evident when standing outside the vehicle with the hood raised. The noise may lessen once the engine reaches normal operating temperature. The noise emanates from the high-pressure fuel pump, as it builds the required high pressure. When the engine reaches normal operating temperature, the fuel pump will tick at a lower rate of approximately one tick per second at idle. The clicking sounds are produced by the fuel injectors pulsing under high fuel pressure

Black Smoke and Rough Idle

Black smoke and rough idle following a cold start is a common complaint on GM vehicles equipped with GDI. These symptoms are the result of a cold start system designed to help reduce cold start emissions. The system uses a dual-pulse injection strategy during cold starts to help reduce the time required to get the converter up to normal operating temperature. The customer may observe black smoke, soot, rough idle and misfires during this cycle. Troubleshooting today’s technology can be challenging, even for the most experienced.

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