Misfire on cylinder 4 at idle only

[Jfriday]

Hey everybody, new to the forum and looking for some help...

I recently purchased a 99 Tahoe with a 350 vortec, great shape body wise for the year, 124k miles. I purchased it knowing there was a misfire on cylinder 4. It only occurs at idle - as soon as you touch the throttle, it's smooth as silk and performs perfectly. The dealer I bought it from said they replaced an injector and it didn't solve the problem.

Took it to my mechanic, performed a compression check, inspected the valves and thought maybe the injector that was replaced was faulty. He replaced the injector, and still the same issue exists. He also looked at plugs/wires, and a handful of other basic possibilities that may be causing this. He's determined to figure it out, but I don't need to be putting more money into this thing than what I paid for it. Does anyone have an idea of what this issue could possibly be? The dealer had said they believed it to be possibly a wiring issue, maybe from a rodent or something chewing through...they had said one of the owners was using it as a personal vehicle before the pandemic and it was operating fine at that time. When the lockdowns/business closures happened, they let it sit for a few months. When they returned to it after a few months, it was missing on cylinder 4, only at idle.

Thanks in advance for the help, I really like this thing and want to get it on the road...

 

[drakon543]

one cylinder could be an intake manifold leak. i believe to do the injectors only the top half comes off. if the lower is leaking right at cylinder 4 it would cause issues just with that cylinder. other possibilities is just a simple distributor cap replacement. 5.7 distributor caps are known to potentially burn through internally which can cause problems with just 1 cylinder. its not a common issue but it is a known potential. other possible would be a lifter of valve issue brought about from it sitting. i would think it would be noticed during a compression test but can be easily missed if its just a single mechanic doing the work.

 

[Jfriday]

Drakon, thanks for your response! I know the dealer said he also did a compression check too, so I'm really hoping that's not the issue. I'll question the intake manifold gasket with my guy, is there a way to test that for failure other than what it's currently doing?

 

[east302]

I’d second the intake gasket leak. With 124k miles, it’s entirely possible that it’s still running the factory gaskets. Coolant leaks usually give it away (any coolant loss?) but a pressure test may show something.

 

[exp500]

I would try the eliminate all other causes before intake gaskets, And would update spider while intake off. Try spark plug, wire, distributor cap, and valve adjustment first.