Covering the 'Basics'Posted 5/15/2007
By Brian Manley
While this discussion unfolded, I found myself nodding in agreement and making a few notes of my recent repairs that ultimately required a sound "basic" understanding of how vehicle systems function. I also remembered the many diagnostic paths gone astray because I overlooked something relatively simple. Early on, technicians were asked to define what they believed to be "basic." Some of the answers included:
Not a bad list. How many times have loose or corroded connections caused a "concern" for the customer that turned into a headache for you? Would you agree with, or add to, this list? I'll run a recent repair by you to see if you can nail the root cause before I did.
1995 Saturn Coupe - 1.9L Engine
This vehicle came to us on the hook. The engine would crank over, but it would not start and run. On occasion, it would sound as though it wanted to fire up, but it never would quite catch. Sound familiar? Are you already making a mental list of prime suspects? So did I.
I performed a fairly thorough visual of the underhood area, looking for evidence of abuse, neglect, tampering, or rabbit erosion (we continue to get our fair share of critters that nest, nibble wires, and cause no-starts). Next, I grabbed a couple of spark testers and hooked them to two wires; we had good ignition fire. I then hooked up a fuel pressure gauge to check pressure and take a sample; pressure was good and the sample looked and smelled normal. At this point I had essentially covered General Motors charts C-4 (basic ignition system checks) and A-7 (basic fuel system checks). Hmmmm ... what to do next ...
I wanted to verify that we had spark occurring at the correct time, so I verified the spark plug firing order and wire routing; it was fine. I began thinking about the CAM/CRANK SYNC line of data on some OBD-II vehicles, and wondered if I could see that on this vehicle's data stream. I plugged in my scan tool and searched, but I had a pretty basic list of data parameters, with no SYNC data. While I scanned the data list, I checked to see if the throttle position sensor (TPS) voltage was more than 4 volts, which will enable "clear flood" mode in many cars, disabling trigger to the fuel injectors, but the voltage was below 1 volt. I checked the coolant temperature reading to see if it was reading -40 degrees, indicating an open in that circuit and enriching the fuel mixture, but it was normal.
Now I surmised that maybe the plugs were the root cause, so out they came. They were platinum plugs with some mileage on them - I have seen my share of platinum plugs cause issues - but fresh plugs made no difference. With the plugs out, I decided to run a cranking compression test; all of the cylinders were 125 to 135 (I am at 6,000 feet).
So, I have verified spark, fuel and compression. I have found no trouble codes and the lines of data appear to be normal. The basic tuneup parts were new or in good condition. (Have you thought of another possibility yet? I know many of you have by now.) For me, I stepped back and looked up the technical service bulletins (TSBs) on my information system, but found no pertinent titles. I then reviewed the "no start" and "basic testing" articles under the engine performance heading, and that is where I remembered a basic test that I had forgotten to perform - an engine vacuum test. I had neglected to hook up a basic vacuum gauge and crank the engine over to see if it was producing any, but after I hooked up my gauge and covered the throttle body (to eliminate the bypass air) I cranked the engine over to discover that it produced about 1.5 inches of vacuum. That is fairly "normal" at our altitude. However, that made me think about air in, air out. I knew it was getting into the engine, and the engine was pulling it in, and we had spark, and compression, and fuel, but I didn't know if the air was getting out of the engine.
Basics, basics, basics. Early on, I should have popped the oxygen sensor out to see if the engine would start, to diagnose a possible exhaust restriction.
1992 Honda Accord - No Start
As the old saying goes, "A picture is worth a thousand words." The diagnosis on this vehicle was a fairly quick one, but with a digital volt ohmmeter (DVOM) and a few pictures, it illustrates the concept of voltage drop quite nicely.
This vehicle came to us on the hook as well, and it would crank but not start. We quickly tested for spark to the plugs, but had none. Early on, I grabbed a test light and began testing the fuses in the battery junction box, which is fed by the large cable on the right of the positive battery clamp. It had no power to any of the fuses, which led me back to the voltage drop test in Figure 2 (good voltage headed to the starter for cranking), and Figure 3 (only 1.8 volts getting to the fuses, PCM and fuel pump). After cleaning off the corrosion in Figure 4, and on both battery terminals, we had a running Accord!
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