Hey y’all, I’m at my wits end here. Bought a 1990 302 recently and while it looks immaculate (zero rust, new paint job), it’s running pretty rough at the moment. I wanted something to wrench on a bit so I got what I asked for. Guy I bought it from swears he’s been through it inside and out, timed it, new distributer cap, new sensors, transmission serviced etc. he’s kind of a hack though and had some seriously janky stuff going on in there.

it has a surging idle, but only when put into drive. I’ve replaced the IAC, ECT, redid some (not all) of the vacuum. It was a mess with open ports and mixed up lines. Mostly fixed it all up with things routing to the right places. MAP is good. Checked the PCV, and this fool had just a breather bolt stuffed in there. Rerouted the PCV to the correct spot and made sure there actually was one in there. This made a huge difference in the idle and I was actually able to adjust it higher using the control screw. (Before it had to be backed all the way out to let it idle at about 800-900 RPM). Still had the same surge from about 400-1000RPM when put into drive. I noticed the Master cylinder was dumping brake fluid into the brake booster so I replaced both and the 30 year old vacuum line to the intake tree. Just finished the job after bleeding all the brakes. Moment of truth and still, bouncing idle in drive. Seems to be running much stronger and steadier, so I’ve made a difference, but still got the main issue.

what am I missing? At this point I am thinking there may be a Transmission issue? Torque converter?

he also installed a brand new throttle body, but not a cheap one, this is the one: Summit Racing Throttle Body it seems to be larger than the stock one. Could that be messing with the fuel air mixture?

 

welcome to FTE

i would look to see if the throttle blade stop screw is set properly, it should JUST start to open the plates at idle back it off turn in till it just starts to open them , this is something i have found on new carbs, efi kits and new tb's
good luck

 

Here are BBK's installation instructions on that throttle body. Follow the instructions from step 7 to end: https://cdn.shopify.com/s/files/1/05...f?v=1703174232 . Did the person that you bought the truck from mention any other modifications to the engine?

 

It's not a control screw, it is a stop screw.

More than likely you still have a vacuum leak, dirty IAC or ECT, and your throttle stop screw is now adjusted incorrectly.

Codes from the EEC?
Does the engine have an after market cam shaft installed?

 

Okay great! That bit about resetting the IAC and TPS I have not tried yet. I’ll try it today!

 

No aftermarket camshaft, IAC and ECT are both brand new I just replaced them. I’m gonna check voltage on TPS today and reset the IAC and then I’ll run codes again.