454 TBI engine missing under load

[nightmare73]

I have a 1990 GMC TBI 454. About 6 weeks ago it started missing under load. If I accelerate easy it doesn't miss. If I accelerate normally or if I have a heavy load and have to give it more throttle it misses. Once I get the truck moving though it runs smooth, I can even kick it down into passing gear and it doesn't miss. It's not throwing any codes. I changed out the ICM, no change and now I have a spare. Last year it got new plugs, wires, cap and rotor. Checked the plugs, they were all nice and tan but I broke one so replaced them all, no change. Tested all the plug wires, pretty sure they all tested good. 1.3 - 1.7 ohms, each 2 of approx the same length tested the same. Checked the cap and rotor, very little wear, no cracks, cleaned it and replaced. Found a vacuum line from back of TBI to MAP or MAF sensor(pass side rear of engine) and thought yes this is it. Replace it with a rubber line and any other line I could find with the smallest crack in it. No change. Injectors were also rebuilt last year and still seem to be working fine. The coolant temp and TPS sensor were also replaced last year. I've counted anything electrical out though due to there being no codes. Changed the fuel filter thinking I had overlooked the easiest thing. No change.
So what the hell am I missing?

EDIT: It only does this after it has warmed up. It runs great while it's still cool.
Also changed out the EGR last year with new hose. I can still compress the valve so I'm assuming it's still good.

 

[dudeiwin86]

checked all grounds?

my late model LS2 would start getting random misfires.
traced it to a loose ground on the passenger side of the block.

dont know much about TBI architecture (even tho i have one lol)
we have an O2 sensor, right?
do they have closed/open loop tuning operation like late model cars?

could be a bad O2 sensor. typically once cars warm up, they go into closed loop and start controlling based off the O2 sensor, if its sending bad voltage readings you could be over or underfueling.

 

[nightmare73]

Checked the grounds, all good. The O2 sensor was replaced 2 years ago, when the old one went bad though it was throwing a code. I'll try checking this one though.

 

[nightmare73]

Problem solved. Plug wires!
I didn't know that they needed to be tested when warm. Not to mention that plug wires should last longer than a year.