So i picked up a 90 Trans Am TPI last week for a good deal (so I thought)

It ran great on the hour drive home. I changed the thermostat to 180 like I have done on all my 3rd gens in the past. Since then:

- Oil pressure was at 45 when starting but dropped to about 15 when engine got hot. Changed the sending unit and now it starts at 30 and never goes over no matter what but will still drops to 15 (ish) when hot. Mechanic checked with gauge, assured me cam bearings are fine.

- Idle dropped to under 500 when in gear, almost stalls. When put back in park/neutral, runs great. Changed the IAC valve and now holds about 500 but still not perfect/where it should be. Idles steady 'ish but kinda rough. Checked vacuum lines and they seem ok.

- Gas mileage has gotten really bad.

Never experienced this before and kinda hard to believe just changing the thermostat could cause all this. Any help would be appreciated.

 

did you bump the wires going to the temp sender when putting the thermostat in?

 

Double checked those. All good there.

 

still listening for any ideas

 

Weak idle in gear could be a sign that it needs to be timed. I've had hit and miss issues with oil pressure sensors. They all seem to read different things, but if it's dropping when it's hot I would consider maybe an oil and filter change or maybe throw some of that engine flush in there to try and clean any residue up. At the same time I would take off the oil sending unit and check it for debris or anything, just to make sure it's clean. Check the trans fluid and maybe consider a fluid and filter change if anything about it seems off.