Figure 1.

It has been an eventful summer, and with each day I get a little more overwhelmed with the kind of work I used to long for. Diagnostic dilemmas. They seem to be crawling in from every direction. Some are simple and straightforward with the aid of scope and probe, and some are difficult due to intermittency.

One that surprised me started out as a seemingly simple, "Won't start hot," but it really brought me to my diagnostic knees. The worst part of the whole deal was that the car was a 1.3 Liter Geo Metro - simple as it gets.

The owner had done all he could using the dartboard method and then switched to the diagnostic by popular opinion plan. Both to no avail. He was obviously attached to his Geo and determined to get it fixed no matter what the cost. He finally broke down and took it to a shop or two and wound up with a few more parts he hadn't thought of. When it came my turn, I got the "You're-supposed-to-be-the-guy-who-can-fix-it-if-anyone-can" ego boost, and I was off and running. I chose to ignore the "previously been tried" file and treated it like a fresh job.

Figure 2.

I began by letting it run for about 20 minutes and then shutting it off as the owner had advised and he was right. It wouldn't start. I did a quick check with some carb cleaner and determined that the problem was probably ignition related. I pulled a spark gap quick test and saw spark. I now decided to "go technical." I started wondering about things that could cause these symptoms and elude a normally sharp tech and thought I should do some basic testing for light power and ground testing for the ignition system. Seeing nothing, yet, that looked suspicious, I stepped up to my automotive multimeter and checked the power feeds to the coil and injectors with the meter on "Min Max" while cranking. I then tested the voltage drop across the engine and body grounds. So far I wasn't seeing anything that could cause the symptoms.

I let the vehicle cool long enough to restart and grabbed my current probe and scope. The current waveform for the injectors looked normal - squelching thoughts of an injector heating up and sinking current. I dropped the powertrain control module (PCM) and started checking references, ground and power circuits and resistances. Nothing out of the ordinary so far. I let it run again, and this time when I turned it off, I noticed that the injector waveforms were irregular (see Figure 1), and the ignition current had different dwell times and firing times.

Figure 3.

I finally decided that this might be an issue with the crankshaft position (CKP)/cam position (CMP) alignment - possibly due to a timing belt stretch. This might explain why it was acting up on a hot restart. Lately it seems I'm seeing a lot of vehicle platforms using this logic in the powertrain control systems. The crank signal has to occur inside a narrow window before the PCM will allow spark and injector operation. Generally speaking, though, most of the ones I've become familiar with seem to allow one or two sparks and they quit. This system, however, seems to go into an irregular spark mode that fires the coils and injectors but not close enough in time to run and not far enough out of time to backfire. (Exhaust maybe?) This "decoy spark" looks to be another hurdle to a lot of already struggling diagnosticians. If this firing logic catches on, I'll bet it sells more parts than a Vortec engine cranking at 54 psi.

Figure 4.

I hooked the Fluke '99 to the CMP signal on channel A and set it to trigger from this signal while monitoring the CKP signal on channel B. Figure 2 shows the result.

Notice that the CKP sensor signal circuit and the CMP circuit seem to be in correct alignment or at least close enough that the timing belt looks to be aligned. We pulled the cover to inspect it and found that it had been recently replaced and was right on the money.

So much for that theory. I took another shot of the waveforms for the cam and crank signals while the engine was cooled off enough to start.

The spec for the Geo 1.3 cam/crank signal alignment waveform is so far elusive to me. I hope the next person has less trouble finding the image for proper alignment. Notice in Figure 3 that the CKP signal polarity switches while the CMP voltage is high. In all of the running shots, I see the voltage change before the CKP goes high. I did some research on this engine and found out that it has some problems with cam gears wearing.

I pulled the gear from the cam and seeing no problem, I reinstalled it. I did notice that the crank gear, however, was a bit loose. The slot in the crankshaft was worn a little wider than normal and the key was a bit loose in it. My guy welded a little extra beef on the key, then ground it back to a tight fit. Another go at it this time with some confidence.

Twenty minutes later I turned off the engine and attempted a restart.

Figure 4 was the waveform I got after what I thought was fixing the problem.

It cranked the same as before. I saw this as an opportunity to grow my waveform collection a bit more. After rechecking our cam/crank alignment, we finally discovered that the crankshaft had quite a bit of end play. It had worn the thrust bearing to the point that it was allowing the crank to walk back and shift the phase of the CKP waveform (notice the shiny spot on the backside of the engine behind the lower gear see Figure 5).

Figure 6.

We ended up putting in a shim behind the lower cam pulley, moving it closer to the center of the crank sensor, and solved the problem.

You can see in Figure 6 the sudden increase in the frequency of the CKP signal right after the first ignition current waveform, indicating a well-timed firing event. The guy was pretty much in disbelief when we told him we had it fixed.

Look how far the cam signal has moved to the right of the CKP signal in Figure 7 Geo Metro 1.3 CKP/CMP alignment. Crank cam signal 1.3 Geo. Timing signal alignment 1.3 Geo. This will be the figure that best describes the alignment of the CMP/CKP for the one that comes up with this article during a search.

Figure 7.

The waveforms were a definite help in asking for adequate compensation for the time spent on the job. The customer even added a healthy measure of gratuity and thinks he wants to buy a scope and probe.

"Come play with mine a while," I told him. "I just charge a small fee for the overhead in case you get hurt while you're here."

Jeff Bach is the owner of CRT Auto Electronics, an ASA-member shop in Batavia, Ohio. For more information on this topic, contact Bach at (515) 732-3965. His e-mail address is johntjeff22@gmail.com and his Web site is www.currentprobe.com.

RATE THIS ARTICLE What do you think of this article? Your input will help AutoInc. develop additional articles on this subject. Share your thoughts! Your name Your e-mail address

MOST ACCESSED ARTICLES Fuel Injection Service, Not Just Cleaning The Art of Extraction EGR Systems: Operation and Diagnosis Proactive Target Marketing:_Rethinking Your Business Strategy Engine Performance: HO2S Diagnostics MOST E-MAILED ARTICLES Developing Employee Potential How Critical Thinking Can Help Your Business How to Diagnose the Ford Glow Plug What to Look for When Shopping for the Right Shop Management Software Putting a Price Tag on Complaints

AutoInc. Web Site | ASA Web Site | EPA Releases New Auto Refinishing Regulation | Wage-Hour Standards in the Auto Repair Industry, Part 3 | Passing Your Shop to the Next Generation | CARS Show Section | NACE Show Section | Managing Results by the Numbers | Guest Editorial | Tech to Tech | Tech Tips | News Briefs | Taking the Hill | Around ASA | Shop Profile | Net Worth | Stat Corner | Chairman's Message