Aftermarket October 2022

OCTOBER 2022 AFTERMARKET 27 operating status. This basically means whether it is using the 02 sensors input to correct the current fueling conditions or a preset look up table programmed into the ECU, as an example when the engine is cold. When the engine ECU is operating in closed loop it is using the 02 sensors signal and adjusting fueling based on this. When it is in open loop, the engine ECU is using a predetermined look-up table, adjusted for long term trim correction, to control the current fueling requirements of the vehicle. As you can see from the freeze frame data in Fig.2, the vehicle was in closed loop during the fault occurrence meaning it was using the sensors information to determine the amount of fuel delivered. You will also note the short-term fuel trim is at +25% and the long-term fuel trim is at +20.3%, thus giving us a total fuel trim of +45.3%, indicating a very lean condition. Remember, if the ECU is adding fuel (positive fuel trim), it is reacting to a lean condition. The long-term fuel-trim’s purpose is to keep the short-term fuel trim centered about zero. Therefore, this parameter will increase to maintain the sensors’ ability to switch between 0.2-0.8v. One last piece of information to consider is the engine temperature. As you can see from Fig.2, the engine operating temperature was at 85.8°C. It is not uncommon to see a vacuum leak be the cause of a lean condition. However, once the engine is up to operating temperature and all the components have expanded due to heat, the leak can be sealed. We could see right away that we did indeed have a very lean condition at engine operating temperature. I also verified the engine speed at this fault occurs during idle, therefore no load on the engine. To summarise, we had a lean condition at idle with the engine temperature at normal operating temperature and the engine ECU was in closed loop control. Therefore, armed with this information and the information given by the customer we could make some analytical assumptions. Logical deductions save time The customer had not complained of any power loss and the issue was occurring from idle. Therefore, if the fault was relating to fuel pressure, then we would likely have a complaint of poor running, possibly stalling/misfiring so we could put this check further down our probable causes list. I verified he operation of all the engine sensors, including the 02 sensors. The next step, which gave us all the information we needed to know, went like this; By Fig.3 increasing the engine RPM, I found that the fuel trim data would decrease indicating that the air/fuel ratio was getting better. By carrying out the aforementioned test we were decreasing the negative pressure in the manifold compared with the external atmospheric pressure. This reduced the amount of negative pressure (vacuum) internal to the manifold which in turn meant less air was drawn in, thus equalizing the air/fuel ratio, effectively fixing our vacuum leak. When the vehicle is at idle, the throttle plate is nearly closed, causing a larger vacuum in the manifold, thus resulting in a larger sucking of air through the leaking component. When the vehicle is at higher engine RPM though, the throttle plate opens further, equalizing the pressure in the manifold with the atmospheric pressure thus reducing the sucking effect caused by a vacuum at lower engine RPM. All this information was gathered from the driver’s seat with no need to look under the bonnet. A smoke test was my next step to find the vacuum leak I believed I had. After a quick smoke test, I found excessive amounts of smoke coming from the intake manifold area. Removal of the manifold was authorised, and I found a damaged intake manifold gasket seal. Part of the seal was missing allowing excess air to be drawn into the engine via the vacuum leak. Please refer to Fig.3. With all new gaskets installed and the manifold and components refitted to the engine I wanted to verify my fix by looking only at the scan data and not setting the smoke machine back up. As you can see in Fig.4, I was again looking at the fuel trim data. You will also see that the short term is now counteracting the long-term figure. This is a very good indicator that the vehicle is fixed. Once the fuel trim figures stabilized, by taking the short- term fuel trim away from the long-term fuel trim we were left with -10.9%. This figure would likely also decrease to around a normal operating figure of + or – 10% total fuel trim. This is the short-term and long-term fuel trim combined. The reason the short-term fuel trim was now negative is that the engine computer was no longer compensating for the large intake leak and excessive amount of air entering the engine. The vehicle would now be running too rich until the fuel trims counteract the original fault. Therefore the engine ECU was now taking away fuel to return to stoichiometric ratio by reducing the duty cycle to the fuel injectors. Ultimately this was a straightforward job. Understanding fuel trims and loop status drastically reduced the diagnosis time on this vehicle and led me to an accurate and efficient diagnosis. Fig.4 Understanding fuel trims and loop status drastically reduced the diagnosis time on this vehicle ”