'Simple' Geo Metro Gets ComplexPosted 10/8/2007
By Jeff Bach
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.
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.
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).
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.
"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."
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