1990 Dodge Caravan transmission issue?

[Lee'sdaytona]

Hello everyone!
Last year, I sold my 89 5 spd turbo caravan. I bought a 1990 with low miles, but a 3.3 V6. I hope you forgive me as I know this isn't a turbo, but I hope someone here is willing to offer advise. Most of the past year I drove it around trouble free. Back in September, the transmission went into limp mode, and I took it right to a privately owned Aamco, who I have had rebuild other transmissions and I have been fairly happy with them. The van has the Ultradrive automatic transmission, one of Chrysler's worst transmissions ever built. So I expected to have transmission troubles when I bought the van.
I took it to Aamco and they said it needs a rebuild. $3800 later, I figured I'd have a great running van for quite awhile. When I got it back, I noticed while cruising on the highway--either with or without cruise control, the tachometer would move from 2200 RPM to 2000 rpm, and you could feel the engine power loss. I thought maybe the trans was shifting in and out of overdrive. Brought it back to Aamco, and they thought maybe it was a bad valve body, even though it was brand new. They changed the valve body, and after driving it for a day, the problem was still there. Since the Aamco is a half hour away, I took it to my regular mechanic. Ran the codes, and no transmission codes, but there was a lean condition code on the computer. Odd, I thought since the exhaust smelled so pig rich. They discovered a bad o2 sensor. They replaced the sensor and the problem seemed to disappear. About two weeks later, I notice the problem is back, but not as bad. Now, if I drive like I usually do, like a grandpa, it runs fairly well. But if I am merging on a highway or I floor it to pass someone, it seems to fall on its face and lose power. I'm thinking this is more of an engine issue rather than a transmission problem.
That said, the transmission does down shift kind of hard, and one time last week, I pulled out of work, not fast or anything, and when the transmission up shifted from 1-2, around 15mph, it shifted so hard, I thought I was rear ended at first.
I did have the plugs/wires changed. I am wondering what would cause these two issues, which Im not convinced are related issues, probably two separate issues. If the clutches in the transmission were slipping or something, wouldn't the engine race higher RPMs than drop lower? Thank you in advance for your help!!

[4 l-bodies]

So in other words, when the vehicle sees a high load the car bogs down or loses power? I agree with you, it doesn't sound like tranny issues. I'd be looking at a fuel system related or exhaust (cat) issue. Fuel pump could be on it's way out, or converter could have easily melted internally while running in a suspected rich condition you spoke of. This can create huge back pressure in exhaust system. Idling or low speed, the exhaust system can get by, but place higher loads on the system and it will effect engine performance.
BTW- a super rich (eye watering) condition certainly can trip a lean condition fault code. The o2 sensor reads oxygen content not fuel content, so drowning in fuel could be interpreted as oxygen starved. So my point is trust your nose and eyes rather than what the fault code says. The fault code is saying it is reading lack of oxygen in exhaust system. Lack of oxygen because its drowning in fuel? Sometimes you have to read between the lines interpreting fault codes, because it is reading the end result of a condition. Basically the SBEC can identify CEL RESULTS, but not mechanical failures.
Being a SBEC, it will give you feedback on long and short trim fuel cut/add, and if o2 sensor is working (switching) properly. It also shows many other sensors outputs. Is it showing fuel cut/add on long or short fuel trim? It can add or cut like 25% from it's speed density based program. A scan OBD1 tool that reads real time or close to it (like DRB II or OTC 4000E) will provide you this info. You just need to find a mechanic that is familiar with older OBD1 systems and can interpret what it is conveying. That could be used to determine if it is truly running rich as you suspect. Dropping the exhaust and test driving the vehicle at higher engine loads can determine if exhaust system has been compromised requiring replacement.
Good luck,
Todd