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Probing a Misdiagnosed No-Start-ColdPosted 4/15/1998By Jeff Bach
"Are you Jeff? They tell me you can purge the demons from my car. You're its last hope. If you can't fix it, I'm taking it for a long drive on a short pier. I'm tired of sinking money in this thing! I've come to the conclusion that it has to be possessed! It's got everything new in it and it still runs poorly and won't start half of the time when I go anyplace. I've already spent over $1,000 on it and it still does the same thing it did when I first took it in. Do you think you can help it?" "You do not by chance work for the federal government, do you?" I ask. "No, why?" says the obviously distraught young woman. I reply, "You have the logic down. Throw money at the problem until it goes away." "That's what I feel like I've been doing," she says. "I've taken it to three garages and each one told me it was something different. The first place said all I needed was a tune up. But $223 later, they tell me I need a new fuel pump. After another $185.50, they say maybe I need a new coil and module. I took it to another place and they said it looked like the computer might be the problem, but they could not be sure. They charged me another $206.70 to put a new computer on it, and it still does it. I took it to the third place and they said I needed to have the injectors cleaned; $143.31 more and then they thought maybe it was the coil module 'thing.' I finally agreed, and even after $312.70 more down the drain, it still did it. Then they told me it probably had a wiring problem and I would have to take it to the dealer. I was telling my friend about the problem at the doughnut shop when one of the guys in there said to bring it to you. He gave me your card and said if anybody could fix it, you guys could. So now I'm here." I started by asking her who referred us to her. She did not know his name - only that he drove a Cadillac. (I like to give a reward to customers who refer business to us.) "Does it restart OK when it's warm?" I ask. "Yeah," she says, "The only thing wrong is that it runs bad all the time, and when it sits overnight, it won't start in the morning." "How do you start it in the morning, do you have to jump it?" I ask. "No, I have to spray starting fluid into the air filter three or four times and then it will stay running," she replies. "But when it's running, it feels like you're riding a hobby horse down the railroad ties." I chuckle to myself as that was the only time I had heard that description of a driveability condition. I write it on the order. "Could I get your name?" I ask. "What is this gonna cost me?" she answers. "Depends on what it turns out to be," I say. "Usually a problem like this can be diagnosed in two to three hours. Then I will call and let you know how much it will cost to fix it." She asks if I am sure I can find the problem and I tell her we deal with these kinds of problems all the time. "If you can fix it for under $200, go ahead," she says. "If it costs more than that, call me." She leaves. I pulled the car into the shop. It felt like it had a dead cylinder. I hooked it up to the ignition scope. The primary and secondary patterns showed no problem with either system, but a power balance test revealed the No. 5 cyl to be the dead one. I thought perhaps someone may have cracked a spark plug, but that was not the case. My next test was going to be a compression test, but I thought I would first look at the injector pattern with my current probe. Since this engine is bank fired, I knew I would have to check the current flow of three paralleled injectors at once and hope that if there was a problem with resistance, I could spot it. I connected the lab scope to the trigger side of each injector group and took the pattern for the voltage signals first. The pattern for bank No. 1 is shown in Figure 1. No startling revelations there. Next, I got the voltage signal pattern for bank No. 2 (Figure 2). The injectors looked the same to me. I then collected the waveforms in figures 3 and 4. Realizing that each of the patterns represents the current waveforms for a group of three injectors, it is necessary to do some figuring to determine which pattern is the correct reading. After all, one pattern may have a high resistance due to a bad connection or a low resistance from a shorted injector. The maximum peak from the amp wave in Figure 3 is 270 mV. Converted, that becomes 2.7 amps on the amp probe's scale of 10 amps = 1 volt. Given an injector supply voltage of 13.8 volts, we now know the average resistance is 5.11 ohms - about what it should be. Using the formula for figuring resistance in parallel:  You can figure what the resistance should be for each bank. These injectors typically average 16 ohms which, when the formula is applied, gives us a total resistance of 5.1 ohms; and 13.8 divided by 5.11 = 2.7 amps. Although there are subtle differences in the voltage waveform patterns between the shorted injector bank and the good one that some experts are able to distinguish, I could not tell which one was bad (Figure 5). When comparing the current waveforms in Figure 6, however, one need only to have watched Sesame Street to have the knowledge necessary to distinguish the shorted injector bank from the normal one - assuming one knows that a shorted injector draws more current; hence, the larger wave. Having diagnosed the car as needing injector work, it now became necessary to pinpoint which injector(s) to replace. This engine being a 2.8, I needed to pull the plenum to gain access to the injector rail to do individual resistance checks. The No. 5 injector was the only shorted one. The test revealed that it had only 8 ohms. Feeling certain that I now knew what had been causing all the trouble, I contacted the owner and told her of my findings. She agreed to the estimated repair cost stipulating that I would guarantee that the injector would fix the problem. Being familiar enough with this engine to know that one shorted injector will rob the whole bank of the necessary operating current, I was confident that the injector would cure the no-start-cold problem. She returned later to pick up some things she needed from the car and wanted to see what kind of "magic machine" that I used to find her problem. I showed her to the stall where her car was still hooked up to the lab scope and current probe. I printed out her waveforms and showed them to her. "Why didn't those other places do that?" she asked. "I don't know," I replied. She asked for a copy of the waveforms to show to her boyfriend. I labeled them. "Are you sure this will make my car start in the morning and not run so rough?" she asked.
As I handed her the picture I remarked, "Smooth as a hobby horse on a baseball diamond." (Figure 7).
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Copyright (c) 1996-2009. Automotive Service Association®. All rights reserved.
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