Engine 91 hatch hesitation at low rpm

What’s up all….. New this forum…… recently purchased a 91 lx hatch, with minor mods……. Cobra upper and lower, cam, msd, exhaust etc……there’s a surge and hesitation at low rpm but when you get on it it’s strong. There’s also a intermittent CEL which seems to only come on after getting on it, if just cruising and being easy on it, it won’t come on. I pulled 2 codes, 81 and 85……. I ordered a AID solenoid and purge canister solenoid…..also ordered a new tps……. Car has no cats……I’m unsure of vacuum routing and what needs to be there and doesn’t need to be there, as there’s a few things capped and rerouted etc. there’s so much info on here that it’s tough to wrap my head around where to start with this issue….. any input ?
  • Sponsors (?)

Hard to wade through the amount of info around here. And then there is the BS in between (mostly from me :() that takes up space.
I would go over to the 'technical thread/how to index'
and look for a head/intake install thread and look through it, IIRC they show a couple intake vacuum line routing pics.
Or....... Wait a little while and one of the fine members here at :SN: will post a pic or maybe a hand drawing showing you vacuum lines routing.
I have not had enough coffee yet for info that requires thought :nono:
Are you running the engine codes with the engine running as well, or just with the engine off?

If not, i suggest doing it both ways as you'll get a different set of codes with it running
Pull codes and fix those issues. I would suggest making a list from no money out of your pocket to costing money. Throwing parts at it usually ends up in the forehead slap and “WTF” when the issue is solved.

You need to read through this and see if it solves the problem after you have cleared codes.

Thanks for the input……. I swapped out the tps, it’s set at .96……. Car is still surging idle, from 900-1200rpm……. When IAC is unplugged the car idles perfect at 750-800 rpm, soon as I plug it back in it surges again…… gotta pull it out and take it for a ride to see if miraculously hesitation eased but I’m not holding my breath
Msutang5L5 explained the 0.999V myth really well here if you would like to read up on this:

Back in the day all the Mustang magazines beat that dead horse unmercifully and unfortunately information was not at our fingertips like it is today so we bit and started oblonging the holes in the TPS to "adjust" it to make our B cams make more power...could not help myself. All hail the mighty B cam.
  • Like
Reactions: 1 user