Diagnostic Procedure with ICE and GDI

August 7, 2019


Diagnostic Procedure with ICE and GDI

By John Forro

The internal combustion engine has never had a clean design. This is why technology is moving away from ICE as fast as possible. Starting with hybrids, converting to electric vehicles. This will never be a 100% efficient power plant because every one of those piston rings, regardless of condition, has a ring end Gap. I know we’re supposed to stagger them with everything else but we never have 100% efficiency going on inside of a gasoline engine or a diesel engine as a result of those piston rings. Manufacturers have tried to add subsystems to the vehicle. They plan to try to clean up the fact that we’re not dealing with 100% efficient gasoline engine.

This program is not dedicated to GDI (Gasoline Direct Injection), but we need to take a note how dangerous the GDI can be. Spraying the fuel directly inside the cylinder the gas filling pressures could be in the Thousand or higher PSI range. So a little piece of information that I wanted to throw out there is, you never want to open up the high side pressure of a GDI system without first relieving the available fuel pressure. Using the VE graph, that are actual fuel rail pressure at the time happens to be over 1400 PSI. If I were to just take a couple of wrenches and start loosening up one of the fuel lines. That could injure the technician quite severely. The easiest ways to do this is to have a factory scan tool. A lot of times you can bleed down the fuel pressure or you can simply just go ahead, remove the fuel pump relay out of the fuse box, crank the vehicle over, and allow that pressure to drop normally.

Stoichiometric Graph

Looking at your scan tool you should see 30/50 PSI. Once set, It’s safe to go ahead and work on the fuel system on. So the conventionally, the internal combustion engine has always been used for sensing and adjusting. We have to go ahead and toggle that fuel mixture rich and lean, back and forth, over and over again to achieve the stoichiometric graph. In a nutshell, anytime we had too much fuel in the system we need to go ahead and immediately the oxygen sensors would tell the computer to go ahead and lean out the fuel mixture and it does this by shortening the injector pulse width time that it’s on. We need it to also be running lean at times because we were dealing with the three poisonous gases of HCCO, and NOx. Well, in order to clean up HCCO, we need to add elements of oxygen to those gases. If we had two elements of oxygen THC we turn it into H2O which is just water, and if we had two elements of oxygen to CO we turn that into CO2 (carbon dioxide) which is harmless for us to breathe.

The Take-Away

So the whole idea is to clear up the NOx. The easiest way to accomplish that is to go ahead and richen up that fuel mixture. The way the car manufacturers have developed all this to go back and forth is they start using a three-way catalytic converter. The ICE is nothing more than an air pump. Remember that when you are doing those diagnostic procedures. So anything that restricts its ability to breathe will and can cause any type of drivability complaints. This could be a stumbling complain could be a rough running complaint, it could be a misfire complaint, including poor fuel economy, and no-start. So keep that in mind when you’re trying to approach some of these diagnostic jobs that just dealing with an air pump outside.



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Using your scan tool and labscope to accurately decipher engine PIDS, mechanical issues, fuel issues and ignition issues. Instructor John Forro will show the students how to get the most out of your diagnostic process to get to the root of drivability issue without spending needless time performing many diagnostic tests.

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