Leaking transmission recently replaced my transmission oil pan gasket

With that many miles, I would bet on a leaking rear main seal. That is a seal basically between the engine and transmission. When it goes, it will start leaking engine oil from that area. Replacement usually requires pulling the transmission out.

It might be a good idea to lift the truck up and get under it with a flashlight and see if you can spot where it is coming from. Occasionally you might get lucky and spot a loose bolt or something causing the leak.

And are you absolutely sure that it is not transmission fluid? It could be leaking if the gasket is not perfectly situated or if the bolts are not tight enough or were over-tightened. You can try spraying some degreaser all around and letting it drip and dry off, and then looking for the leak again.

It's a leaky gasket for sure. What I always do is apply a very very thin line of RTV, and then put the rubber gasket on. All is not lost though. You can put a very clean (preferably new) catch pan under it and drop that pan again, allowing it to drain the new fresh fluid into the pan. Be sure to scrape every single bit of RTV off of the pan and the transmission. If you leave one bit of it on there, you will surely have a leak. Put that thin bead of RTV on the pan, then put the gasket on top. Then carefully put the pan back up, making double sure that the gasket doesn't come out of alignment anywhere. When tightening your bolts, start with the ones on the corners and apply evenly (i.E. Two turns of one bolt, then 2 turns of the opposite bolt, then two turns of the next bolt, then two turns of the opposite, and so on). Then pour your fluid back in.

I always just use the blackish/brownish junk. And when it put the pan up, I give it a few hours to cure before putting any transmission fluid in there.

Nah it should be ok. When you check the fluid, you're supposed to warm up the transmission a bit (drive it around the block or something), park it on level ground, leave the engine idling, and then check the dipstick. That's the most accurate reading you'll get and its what manufacturers call for.

If the service engine light is on, pull the code(s) from it using a jumper wire or paper clip in the diagnostic connector (under the dash, left side). Here's a how-to guide for GM's along with the codes: https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

Generally, if the TPS is bad, you should get a code 22. Code 21 basically indicates that it need adjustment. See what codes come up and we can go from there.

Also, be sure that you have the fluid level where it needs to be, and make sure that you used the correct fluid for your truck. Check the fluid level using the method that I outlined a couple of posts up from here. What fluid to use should be engraved on the dipstick (probably Dexron II) and is also in the owner's manual if you actually still have it.

The good news is that adjusting that sensor is a simple process. Here's a how-to that I found on good old Google: https://itstillruns.com/replace-sensor-system-gmc-vehicles-7560145.html

And that little trick for pulling codes will only work on OBD1 systems (which would be almost every car from 1995 and older). If the diagnostic connector is sort of trapezoid shaped, then you'll need an OBD2 scanner.

No, you just need to adjust the one you have.

Yes, the article is technically about replacing the sensor, but if you read all of it, you'll see it also tells you how to adjust the new sensor. Just skip the part about replacing the sensor and adjust the one that's already there. Simple as that. And yes, it says GMC but this process works on almost every GM car from the 80's into the mid 90's, and especially Chevy Trucks (basically the same as GMC trucks, minus some bells and whistles).

From the article: "[Loosen] the bolts just enough to allow the new sensor to slightly move back and forth for final adjustment. Plug in the sensor connector and clip wire-piercing probes on the center and left wire. Turn on the ignition and set the voltmeter to the 12-volt scale. Adjust the throttle sensor, with the throttle completely closed, to 0.5 volts. Tighten the small bolts.

Replace the air cleaner and lower the hood. Start the GMC and perform a test drive to make sure the system is operating correctly."

By wire piercing probes, it means probes for your multimeter that pierce the wire a little bit, allowing your meter to contact and read the voltage running through the wires. You can also stick paper clips down into the connectors (from the side that the wires come from) and then connect your meter to the paper clips

LOL, I live in the south too. Been here since I was 12 (23 years now). Anyways, yes loosen those screws/bolts just a little bit. Loose enough that it can be moved around by hand, but still tight enough that when you let go of it, it stays in place. Get it to where the meter reads 0.5 volts, and then tighten it up.

The way I understand an automatic transmission like yours, yes too much fluid could cause the pressure to be too high, which could cause all kinds of crazy behavior, namely delayed and/or hard shifts.

Definitely try to get that TPS reading closer to the 0.5 volts reading, but 0.534 volts might be close enough. But I'd personally work it a little more and try to get it around 0.5 volts.

Get your catch pan out again and loosen the bolts on the pan a little bit so that the fluid will leak down into it. Each half inch you are over the full line is roughly roughly roughly one quart of fluid. So drain an according amount, then re-tighten the bolts and check it again. Do the whole thing where you start the truck, drive it around the block, and then recheck the fluid, with the engine still idling.

As always, keep us posted and feel free to come back here any time.