454 TBI 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.

 

[Can Can]

Did you test your wires when they were cold? Resistance really builds up in a bad wire when it heats up.....

It sure sounds like a bad wire to me.

 

[4xcrazy]

Similar issue awhile back with the 4.3 in the pick up, couldn't find it initially, but then one morning as it was leaving, I heard that familiar sound of an arching spark plug wire.

Turned out the original wires were still on it from '94 and it was arching out behind the motor.

 

[crawlink5]

I think a lot of stuff is bypassed while cold. Sounds like your timing is not advancing correctly. It should be reading rpms to determine timing along with temp and tps. Is your TPS good? I'd change the TPS first.

 

[diesel4me]

It sounds like the coil is weak once it warms up,a classic symptom of one crapping out is it'll work fine cold,but crap out as it gets hot after several minutes of run time...--if you have indivudual coils they fail fairly often and cause intermittent misfitring and dont always act up 100% of the time...individual coils that have a long boot where they go onto the spark plug can have those boots get a puncture in them and allow the spark to jump to ground instead of firing the spark plug too..

 

[nightmare73]

OK the results are in. Plug wires!
Thanks for all your input. It was very helpful. I didn't know that plug wires had to be tested when warm. I just bought a cheap set and put them on this morning. No more missing. I put MSD Street fire wires on last year, pretty sh!tty perfomance from $70 plug wires.

Thanks again for everyone's help. This is a great forum!

 

[Can Can]

Yay for tech!!!!

Glad you got your issue fixed, brother.