Dear mid-60s Buick caregivers,
Last couple outings with my trusty wagon I had noticed that the RPM has porpoising. When idling it could be as low as 700 RPM when the electronic fuel injection system is set to keep it at 800. It was unpleasant enough that I decided to sideline the car until I could look into the problem. I suspected something wrong with the EFI system. The manifold pressure sensor is known to be vulnerable to failure on the
FAST Sportsman. I figured its time had come.
On Saturday, I woke up my 11 year old laptop to see if it still worked. This is the computer I use to adjust the EFI parameters. Fortunately, no problems getting it going after over a year of non-use. Yesterday was the day I set aside to look into this. Alas, all my efforts to troubleshoot the problem were for naught. First, I tried to get the FAST diagnostics to report which sensor was bad. Having failed that, I took a 5 minute log of the engine's operation. I hoped to see the porpoising RPM behavior. I could then look at the values of the other EFI parameters to see if one was also oscillating in the same way. That looked like a likely way to find the culprit. Alas, the engine really didn’t exhibit the RPM swings. After all this effort, it seemed like I was going nowhere fast, so I put Biquette back in the garage and decided to take a careful look at the log using my desktop computer.
I looked at everything. All the sensor values looked reasonable. The RPM was holding within tolerance. To my frustration, everything looked normal. I did casually note that even the voltage was 12 volts. I concluded that I would have to drive the car some distance before I would see the problem and only logging an extended drive would get me to the bottom of this.
I had other things to do, so this project returned to the back burner. About 2 hours after putting Biquette in the garage, I noticed something very strange: the battery was still being recharged. Normally the alternator is so beefy that the trickle charger reports a full battery after about a minute.
Huh? I discharged battery? Oh No! . . .

All of sudden, I remembered that 12 volt reading that I had overlooked. Since I had gone to all the trouble of getting these data logs, I can show it that way. Here is the first minute of the engine running yesterday:
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The voltage starts out at 12.2 volts and drops down to 12 volts after the engine starts. You can click on the image for a higher resolution version.
In contrast, here is the previous log I had taken on October 12, 2018:
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As you would expect, once the engine is running, the voltage is a healthy 14.6!
The morale of this story is an old one: when troubleshooting keep an open mind and pay attention of
all the symptoms before you!
Hopefully that's a quick smile before you undertake your next car related project!
Cheers, Edouard
