For one, the O2 voltage was switching as the engine rpms were raised up but were down to zero near idle. The next important PID data was LTFT reporting +30 and it was captured in fuel cell 14 at a high rpm. Normally if this was a vacuum leak at idle. the Fuel Trim cell number would be a 0 to 6 number rather than anything higher. His thinking at this was point was that the engine maybe starving for fuel since one of the major complaints was low power. Franklin proceed to test the fuel pump current waveform at the fuel pump relay, pins 30 and 87, and found the current ramping waveform was normal at 6 amps. He followed that by performing a fuel volume test with our MAC/MityVac fuel pressure and volume tester. The results of the fuel pressure test were a perfect pressure of 62 psi and a volume reading of 0.5 gallons per minute without any bubbles or fuel discoloration in the sight glass. Now he could rule out a fuel delivery problem and concentrate on finding out what was causing such a huge command of 30% LTFT.
|Want more ? Enjoy a free subscription to Motor Age magazine to get the latest news in service repair. Click here to start you subscription today.|
His next step was to remove the Zeus and install the EScan since it provides more in-depth information on driveability problems. The EScan data revealed the same DTC P0171 (Figure 7) with the very important Freeze Frame data. The Freeze Frame data is like a snap shot picture of when the engine acted up and threw the DTC. In this case, we can see that the engine was hot, not moving since mph are 0, MAP at 14 HG, MAF 3.2 lower than the 1 gram per litter at idle and rpm at 588, STFT 35% and LTFT 29%. Now this information warrants further investigation that led Franklin to the (Figure 8) Escan Sharp Shooter Fuel Trim data. He could now view the trim data in a graphic format rather than just as a static number. The fuel trim numbers were high at the low rpms and a bit higher on the chart than idle since the engine would stall out at a normal idle speed. We noticed at this point that the high numbers were not just at idle but up at a 70% throttle and about 4000 rpms. Normally if the numbers are high on the low end of the scale and high all the way through the rpm and absolute throttle ranges, it’s a MAF problem.
But hold on, we have an engine that will not run at idle. This made us think that there had to be a massive vacuum leak. Franklin began to unplug all the engine vacuum lines and sealed them to see if there was any difference. He removed the PCV valve and noticed a slight difference, so he installed a new AC Delco PCV that resulted in the engine to somewhat idle, but the LTFT (Figure 9) was still high.
At this point we thought that the next logical area for such a large leak was at the intake manifold. I took out our leak detection tool that consist of a Coleman propane bottle, valve assembly and flow tube. With the valve fully opened for maximum propane flow, we checked for leaks. In the past, we had come across some intake manifolds on these engines that had gasket issues as well as a PCV valve problems. Working as a team, we checked all intake areas and came up empty, no detectable vacuum leaks anywhere on the engine. The next thing we tried was flowing propane to an open vacuum port and found that the engine was able to idle better. With that result, we confirmed that there had to be a leak somewhere, since adding propane allowed the engine to idle better and the fuel pump passed pressure and volume test.
Our next step was to shut the engine down and connect a smoke machine. Unfortunately, the smoke test did not reveal any leaks, coming up empty handed we thought it had to be something that we were overlooking. I carefully thought about a couple of GM police cars that I had worked on right after 9/11. Those engines had a similar issue that resulted in a vacuum leak that were also difficult to locate. The problem with those vehicles was that the power booster diaphragm was defective causing the high fuel trim readings. I mention that to Franklin and had him smoke the power booster to check for escaping smoke. The test yielded no leaks, so we were still no further along locating the leak.