Sierra misfire issue

[MunsterZ28]

looking for some help here guys, i have a 2000 gmc sierra with a 4.8l. it has a misfire on cylinder four. i replaced the coil, wire, and plug. it still has a misfire on cy #4 under 8% load (idle or cruising down the road at a constant speed). anything above 8% load the misfire goes away and the truck runs fine. (i know the exact load % because i have a tech-2 at my disposal.) after some idea bouncing i am thinking it could be an injector issue or a intake gasket leak. the only thing that makes me lean away from the injector is unplugging the injector makes it idle like even more shit. so whats y'alls opinion?

thanks mifbody!

[hoogiesngrinderz]

What are your fuel trims at idle?

[NullHead]

Watch your misfires at idle then run a garden hose of water and soak the intake area where the gaskets are. If you see misfires every time you apply water then your intake gaskets are bad.

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[hoogiesngrinderz]

Don't forget to check the PCV lines for cracks

[Boost Addict]

Check for shorted o2 sensor if it doesn't miss in open loop

[MunsterZ28]

At idle the iac count is 60 the fuel trim numbers at idle are as follows

Short term ft avg. bn1 0%
Short term ft avg. bn2 0%
Long terms are the same
Short term ft bank 1 10%
Short term ft bank 2 11%
Long term ft banks 1&2 -1%

and it's is missing in open loop

[98blackbeauty]

Swap injectors, that's easy. Injector might be partially clogged and under more load, longer duty cycle its sprays better. I would do that before attacking gaskets etc etc.

[Boost Addict]

Take a can of brake or carb clean and spray near the intake gaskets and watch the the o2 voltage. If its staying high (acts like its not switching) then you can condem the gaskets.

[1BADAIR]

Pull the vacuum line off the fuel regulator and see if its wet

[hoogiesngrinderz]

Take a can of brake or carb clean and spray near the intake gaskets and watch the the o2 voltage. If its staying high (acts like its not switching) then you can condem the gaskets.

I like to avoid carb cleaners and brake clean with these newer engines, they don't play well with rubber. Propane gas is another option.

And good call on the regulator, I forgot they had return lines up until 2005(I think?).