'99 5.3 Silverado no code misfire

[Rotiart]

Kind of a strange one to me. I inherited a low mileage truck, a '99 silverado, long story short gapped all the plugs for farts nd laughter, and now I have an intermittant misfire. Only adjusted the gap on 2, the one was mashed, had a gap of about 10 thou, the other was was .40, which i gapped up to .60 to match the rest, all except the .40 are original from factory. What really throws me, is the truck will not throw a misfire code. Can literally run it with a wire off and it wont throw a code, makes trying to find the bad one a pain. Any advice? Dont want to buy a bunch of new plugs if I dont have to.

 

Misfire only occurs low speed high load.

[txab]

Use plugs designed to be set at .040. That's the recommendation per GM.

 

https://www.gm-trucks.com/forums/topic/121181-did-i-ruin-my-new-iridium-plugs/?do=findComment&comment=1079941

[Rotiart]

Should I regap them? I set to .060 because thats what the old spec was, and what the other 7 are.

[txab]

What plug number do you have now? If you're having misfire issues, I would replace with plugs that are designed to be set at .040. This is why GM changed back to .040. Use platinum or double platinum. No need for iridium on this age vehicle. AC Delco # is 41962 for double p. I think # is 6 for plats. Check parts sites for yourself. If you do replace plugs and  buy AC Delco, there is a rebate.... $3 iridium. $2 for double p., $1 for plats.

 

Your misfire may be caused by leaky intake gaskets. Check them for vacuum leaks

[Rotiart]

7 of them are originals from the factory, and are numbered 1256370. However, when I got the truck I did a compression test for future reference and my dumb arse broke one off, the repalcement i got from the dealership has the number 41-962. 

[txab]

How many miles on this truck? Considering it's 20 years old, you might want to replace plugs and wires. Search around, either Amazon or Rockauto.com have the best price on plugs probably. Don't forget the rebate too.