86 newbie

ok i have a 86 5.0 5-speed. its stock engine wise. Ok ill be just cruising at a constant RPM and all of the sudden it starts bucking, bad. If i floor it it runs great, no hesitation at all. Most of the time when the bucking starts it my idle surges from like 900 to 200 and sometimes dies. I replaced the TPS and that helped for a lil bit but now its back. Do you guys think it can be a baffle issue in the gas tank? Also when i run it hard there will be pressure build up in the tank. I dont want to start spending money on stuff i dont know.

newpony

whens the last time you had a tuneup?

mattdel

EGR valve is very faintly stuck open by carbon deposits, constant cruising RPM opens it further but the position sensor is registering higher opening than commanded by the PCM, so it compensates by reclosing it, causing a bucking.

Edit: Just realized I forgot about the idle issue. Might be related, might not. Need more info. Disregard what I wrote for now, I'll be back in a few hours.

86 newbie

I just replaced the plugs like 15k miles ago, i put a distributor that i got off my buddy in it like 1k miles ago. I just got new 9mm plug wires and new MSD cap and rotor last night.

newpony

Idk much try getting it to a shop if you don't mind paying for a diagnostic , check ignition ,check for air leaks or fuel issue. have fun

86 newbie

come on guys anything, I do have pressure in my tank sometimes but i just have the gas cap loose so pressure wont build up...Im sure with that my vent tube is just clogged. But anything?

mattdel

The vent tube could be causing it. Since you already know about it, repair the issue and see what happens. Vapor lock in the tank could cause a hiccup in the pumps flow.