I'm experiencing a definite skip, but the only trouble code I have is 31 - Gear Shift Position Sensor. Is there a trouble shooting sticky for this engine posted on the forum?

https://charm.li/Buick/1993/

How many miles on the car?

How many miles on the current spark plugs?

How many miles have you owned it?

Thank you for your reply.

I am not the owner of the car, just the current user. I managed to get the car running again after it had sat in a garage for 17 years.

There is just shy of 185,000 miles on the car.

I do not know the mileage on the plugs, but I pulled them and they do not appear eroded. The gaps and electrodes look nearly new. They were, however, black and sooty.

Since I initially posted, another issue has manifested itself, or it may be the same issue that is causing the skip, but becoming worse. I have the symptoms of a failed automatic transmission modulator valve: rough idle, white smoke at the exhaust, a hard shift in reverse, loss of transmission fluid (I had recently added ATF before I began using the car. It looks like it has lost 0.5 to 1 quarts since I added ATF.) However, as best I can determine, there is no modulator valve on this car. All I found that was similar in function to a modulator valve was a control solenoid.
If there is a modulator valve on this car, can you please direct me to its location? I searched but did not see anything like it.

I haven't heard that all of the bad fuel and water and junk were drained out of the tank and the fuel filter replaced after 17 years sitting, has this been done?

So it is running rich.

What is the current gap on the plugs?

I believe this car has a 3-speed 3T40 transaxle with a TV cable. No vacuum modulator on this car.

Does it leak anywhere?
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No transmission leaks that I can find. There is small engine oil leak somewhere, but not bad enough to be messing with that yet.
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Thank you for confirming that there is no vacuum modulator though. That helps.

Also, yes, I replaced the tank, the tank sending unit, and fuel pressure relief valve. I've had it running for a few months now. And I recently replaced the injectors because one seemed to be leaking fuel. But now this issue has popped up.

I wonder: Is the air filter plugged up?

Great question. I checked it before my initial post and it is almost as new.

Also, you asked about the plug gap. I measured a couple today. One was .055, another was .057. I've pulled all the plugs and all were black and sooty.

General appearance of all plugs.

Plug gap.
At least 4 plugs had oily threads.

I did perform some more diagnostics today. On a hunch that there may be a clogged PCV valve, I removed a pipe that runs between the front valve cover and the throttle body. I plugged the open end of the pipe with tape and left it out.

. I started the engine and it idled better than it had been, but still smoked. I made a short video of the tailpipe here:
https://streamable.com/mlr0av

Then I replaced the pipe and started the engine. The engine barely idled, then stalled. And it still smoked. Short video of the tailpipe here:
https://streamable.com/x74nig

So there does seem to be excessive crankcase pressure. However, I do not see a PCV valve coming out of the engine.

Looks like moisture to me, like maybe a head gasket leak or something.

In those videos, has it been running long?

How is coolant doing?