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By: Larry Hammer | Thursday, February 16th, 2023 at 12:26 pm in Tech Tips


Troubleshooting today’s technology can be challenging for even the most experienced technician. Making an accurate diagnosis, rather than throwing a lot of expensive parts and labor at the symptom can be a challenge. How would your shop handle the following customer complaints?

a) Crank but no-start

b) Hard starts c) Long crank time

d) Misfire symptoms

e) Multiple misfire codes stored including: PO300, PO301, PO302, PO303, PO304, PO305, PO306, PO307, PO308

When symptoms like these occur, many technicians focus entirely on ignition related concerns such as spark plugs, ignition wires, coils or possibly fuel injectors. While these are all possibilities that could promote the symptoms, the technician must look beyond these areas of consideration to make an accurate diagnosis. Given the mentioned symptoms, would you have followed the same diagnostic pattern? Would you have considered that the symptoms may be a fuel or carbon related condition? Other conditions such as fuel volatility, oxidation, excessive carbon or sticking valves can promote the same symptoms. Let’s explore some of these conditions that can promote the same performance symptoms, but often fail to be on the technician’s radar when performing a diagnosis.


The Reid Vapor Pressure (RVP) of the fuel must be adjusted to compensate for seasonal changes in the weather. The required RVP for a summer fuel is rated at 9 RVP. Consuming fuel with a 9 RVP during winter conditions will likely result in a rough idle, surging, hard starts or backfiring conditions. The required RVP for a winter fuel is rated at 12 RVP, which is necessary to provide adequate vaporization. If this fuel is used on a hot summer day the fuel will turn to vapor, especially following a heat soak. Many refer to this as vapor-locking. Seasonal changes require an adjustment to the vapor pressure of the fuel. It is not uncommon to encounter drive-ability concerns during this seasonal transitional period for the fuel suppliers. Kits are available to measure the RVP of the fuel. No attempts should be made to correct the symptoms with additives, as they are not effective.


Fuel oxidation results in the formation of gum and varnish. The oxidation process begins when the reaction between hydrocarbons and oxygen creates compounds that affect the composition of the fuel, resulting in the formation of gum and varnish that can restrict valve movement. The residue often shares the appearance of molasses and will restrict the movement of the valve stems in the valve guides. This condition is especially a problem on low mileage engines with tight tolerances. The condition is more pronounced once the engine cools down and the gum-like residue solidifies, forming a sticky, gummy or waxy-like substance. Fuel that has been stored is highly susceptible to this form of contamination. To help circumvent these forms of contamination, antioxidants are added to the fuel.

The formation of the gum and varnish deposits can be determined by observing valve movement, which usually exhibits a slow or no movement, or by performing a compression test. Vehicle manufacturers recommend a TOP TIER rated fuel to insure the proper detergent levels. The detergent level in this fuel is 2-3 times greater than the minimum standard set by the EPA and the Canadian General Standards Board.


Engine performance complaints including misfires, rough idle, long crank times or no-start symptoms can be the result of carbon build-up on the intake valves and in the combustion chambers. Misfire codes may be stored, and the vehicle may run rough for a few minutes and then run perfectly. This symptom is due to the accumulated carbon absorbing the fuel much like a sponge, creating a lean fuel condition. When the carbon becomes fully saturated, the engine will run perfectly until the next cold start. Those same carbon deposits can collect on the fuel injectors, affecting their spray pattern, resulting in hard starts, misfires and excessive levels of hydrocarbons, carbon monoxide and oxides of nitrogen. Excessive carbon deposits can result in an increase in the compression ratio, which can require the use of a higher-octane fuel to suppress spark knock. This is especially a problem with turbocharged engines, as the boost pressure increases the compression ratio to a level resulting in violent detonation. Higher compression ratios can result in preignition of the air-fuel mixture resulting in detonation. Carbon knock can result in damage to the internal engine components.


With Gasoline Direct Injection (GDI) the formation of carbon deposits on the intake valves is in relation to how fuel is delivered to the combustion chambers.

On a Port Fuel Injection (PFI) system, fuel is injected into the intake manifold upstream of the intake valves, which provides a fuel wash on the intake valves, reducing the formation of carbon deposits. With the PFI system fuel detergents and supplements added to the fuel tank can be beneficial in removing carbon deposits on the intake valves.

With the GDI system fuel is injected directly into the combustion chambers at pressures that can exceed 2000 psi. Unlike the PFI system the GDI system does not spray fuel directly onto the intake valves, resulting in the formation of carbon deposits, due to the absence of fuel wash on the valves. With this system, fuel additives placed in the fuel tank offer no benefit in preventing the carbon from accumulating on the intake valves. However, those fuel tank additives can help clean the combustion chambers.

Intake valve contamination occurs as unburned fuel and oil vapors are drawn back into the intake manifold via the positive crankcase ventilation valve to be consumed in the combustion process. The intake valves become coated with these chemicals. Intake valve guide and seal seepage further complicate these concerns, as the oil is baked onto the valves. This is especially a problem with higher mileage engines and the wear factor, which promotes oil consumption and blow-by gases. Valve overlap is a contributor, as some combustion gases are forced past the intake valves, promoting carbon build-up. Exhaust gas recirculation is another contributor of contamination.

Caution: GDI equipped engines have a longer crank time than engines equipped with PFI. The GDI system operates at higher fuel pressures. The mechanical fuel pump must build up the required pressure before the first injection event occurs. Cold ambient temperatures require a longer crank time. E85 fuel will extend the crank time.


Methods for carbon removal vary from one vehicle manufacturer to another. Most all agree on the fact that large concentrations of carbon being dislodged can create damage to the engine and turbocharger. In addition to piston, cylinder wall and turbocharger damage, they can damage the oxygen sensors and catalytic converter.

Some vehicle manufacturers recommend removing the intake manifold for cleaning with chemicals, while others recommend removing the cylinder heads and disassembling the valves for cleaning with a wire brush. Some methods of cleaning involve blasting the carbon deposits with walnut shells.


Instead of waiting until the engine components are contaminated with carbon and the engine performance symptoms require some major repairs, consider an induction clean-up. We recommend this service on an annual basis or every 15K miles to prevent the accumulation of major carbon deposits.

Ask your Mighty Rep about his complete line of intake manifold and valve cleaning chemicals. The cleaning process involves introducing special chemicals into the intake manifold down-stream of the mass airflow sensor. They do an excellent job of removing those harmful contaminants without creating a hydro-lock condition, which can damage connecting rods, pistons and bearings.

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