KR81

I've had this strange issue before and after a lot of recent work on my car. When accelerating, especially at low speeds, I lift my foot off the accelerator, the rpm's jump up, not much but maybe a couple hundred rpm, enough that it feels weird and I don't like it.
Recent work involves new valve seals, intake manifold gaskets, valve cover gaskets, new iac, tps, cts, o2 sensor, fan switch, distributor, rotor and cap, new oil pressure sender, injectors cleaned and serviced, new fuel rail o rings, fuel pressure regulator, fuel filter, fuel pump, sender and strainer, oil and filter, coolant flush, a few vacuum hoses replaced but not all, transmission fluid changed may 2020... none of this may relate to this issue but thought it worth mentioning..
Iac and tps both adjusted correctly, tps at exactly .54v, timing set at 6° btdc with est wire disconnected.
Bearing in mind the car did this before and after all this work and with new parts, what could cause this strange jump up in rpm ?

BobWolf

Need more details on the car. Year, make, model, engine.
You might drawing air from somewhere other than the throttle body. My first suspect your vacuum lines, throttle body connections, could be a bad throttle body or intake gasket, It doesn't matter if they are new. Check the EGR valve operation it replaces fresh air with exhaust when the throttle plate is closed to make up for the excess vacuum when the throttle plate is closed. If its sticking shut or if its been removed the ECM can be opening the throttle plate; clean it up or replace it. I would also check the wiring harness to the throttle body, distributor, crank/cam, MAP/MAF, sensors its possible you have a loose connection causing the ECM to go to a default.

KR81

1988 305TPI TransAm
I took the egr valve off during all this work and cleaned it out with throttle body cleaner and tested it by depressing the inside part of it and holding my finger over the bit where the vacuum line connects and it held until I took my finger off, from what I've been told about egr valves, that says it is still functioning properly but maybe you can confirm that, I did have a new egr anyway but didn't install it cause I figured the original was still good.
If even after new intake gaskets, in that case, would that not imply something other than the gaskets like maybe the intake was warped or something causing a bad seal?
I can go over the harness and double check all connections, I had the harness all over the place doing all this work cause it constantly got in the way but when reconnecting everything I did make sure they were all connected properly. Where do I check the harness in regards to crank/cam ?
I was also planning to replace as many vacuum lines as I could, I have already replaced some that looked dry rotted in the pic..

KR81

I'm just a bit baffled cause it was there before and after which to me says it probably isn't related to anything I cleaned or fixed which leaves me having to check here for help where to go next.
Edit: I forgot to add in the first post, at these lower speeds it also feels like the transmission isn't shifting at the correct time..

Reddragon88gta

Maybe check your TV cable adjustment? That will affect the shifting timing. IIRC with engine off you should be able to fully open throttle and the tv cable should be tight. Also do a search and see what proper shift speeds are and compare to yours.

Your other issue sounds like vacuum lines.

KR81

Thanks for that, I dont know anything about the tv cable so upon checking it, there was a bit of slack in it still at wot so I've made a few adjustments and test driven and seems a lot better.

In regards to a possible vac issue, I've replaced a few including the brake booster line and seems like there's a slight improvement already.