92 Coupe Stock with Auto Trans.

I was driving home today and came to a stoplight. While sitting there the car sort of bucked. I had the AC on automatic stereo on. Happened twice. I put the car in neutral and it stopped. Went on my way and it happened again when I stopped for another light. After that I drove home again without having to stop.

So my first thought was transmission. When I got home I stopped in the driveway and it did it again. Put it in neutral and the problem went away.

Put it back in gear (Drive with OD) and when it happened I turned off the A/C. Problem went away as soon as the fans shut off. So I switch the gauge reading in the LCD to volts and notice they read 14.0 with A/C off car in park. Put it in Drive again and turned on A/C and volts dropped to 12 something then quickly went to 13.9. Then 14.0 Put it in park A/C off still at 14.0. Increase RPM and voltage stayed at 14.0 Shut engine off then turned switch to run voltage was 12.7

Also noticed a brief dimming of the LCD when the A/C was engaged.

I distinctly remember the voltage was 14.7 when I left for work. Drove with A/C off as it was in the 60's. No issues, on the way to work, or when I went to lunch.

So now I'm thinking they alternator might not be keeping up and causing a misfire when the voltage drops.

Outisde temp had climbed to mid 80s for the drive home not sure if that's a factor or not.

 

Could it be possible that something is wrong with the a/c compressor and when it kicks in and you're idling it's enough to drag the engine down?

 

Possible. Take the alternator out and have it rebuilt and peek over their shoulders and see

 

I think the alternator is fine, a voltage reading the the mid 13's to 14's means the alternator is keeping up just fine. Even if it stopped working completely for a few seconds should not cause any running issues.

 

I know that this doesn't relate to the charging system, but recently mine had the same symptoms you describe. The EGR wouldn't seal all the way, which resulted in a vacuum leak, and in the "bucking" at idle in drive.

Another symptom it had was when in OD with the torque converter locked around 1200-1800 rpms light throttle had the same "bucking" feeling when accelerating. It might be worth testing it with a vacuum gauge and seeing if that's your issue.

If it fails, it's about $40 shipped for an AC Delco OE part, and 30 minutes to get the old one off and new one on

 

Brian- can you explain what "sort of Bucked" means? Some people describe a "buck" as a "slight surge" or "breif stumble."

I know that my AC is a big draw especially at idle. This draw coupled with our cars RPMs decreasing from cruise to idle at stop give me a quick stumble then recovery. I never really thought much of it as the two factors combined made sense and the engine recovered in just a second.

If you were to leave the car in D, with the AC on and sat at a light for any length of time does the engine eventually smooth out or does it consistently run rough?

 

My 94 is about the same.
Can't say anything really is out of the ordinary. The voltage indicated is only constant when the load is constant and RPM. The actual voltage is dependent on load and how much of a full charge the battery has.

When you turn on the A/C, the left side fan comes on, the compressor clutch and the blower fan. That is a large instantaneous draw and the voltage measurement will go down. As that is happening the ECM is telling the IAC (at idle) to increase speed and that takes a second or two for it to really happen. If you sit at a light with all that stuff on as well as the headlights, you are going to see a much lower voltage till you get going and the RPM is higher and the alternator can put out more voltage & current.

When you shut the engine down and read battery voltage it will be 12+ volts as it probably has a full charge. It will probably come down a bit as time goes on.

In addition, a DVM can indicate a different voltage reading from what you see on you digital readout as much a 0.5 volts. Not a problem, just so you are aware. (Mine is 0.4 lower on my speedo).

Too much detail……I've run off. 

I don’t think either that the electrical is causing and of your bucking problem. Could be anything especially with the load of the transmission on the engine. Little water in the gas, some kind of momentary miss-fire, injector, dirty throttle body. How long has it been since plugs and wires.