350 tbi strange stumble at idle

[Big RED Burban]

I have a 1994 suburban 350tbi with 332000 miles and counting it recently broke a main bolt on the number 1 main cap so I pulled the pan and fixed that while I was in there a put in a new melling m55 oil pump. now its back on the road and at idle it has a strange stumble as the engine warms up it gets a little worse and if you don't stop it eventually it will die. at anything but idle it runs great and the stumble is gone by 1100 rpm ive changed the fuel filter and checked two or the spark plugs and they were fine so I'm assuming the rest are too. it is not throwing any codes. it sat up a month during repairs I think its bad gas but my dad doesn't, do any of yall have any ideas thanks

 

[TylerZ281500]

timing wuold be the only thing somewhat relevant to the work you did

 

[Big RED Burban]

well ive got the timing set pretty good I don't think its related I'm starting to think the fuel pump is going out would this cause it

 

[michael hurd]

Diagnose, diagnose, diagnose... don't start tossing parts at it.

Borrow a timing light... doesn't matter what kind / type it is. Remove air cleaner, put the inductive pickup on the coil wire ( so it flashes 8 x faster, and then start the engine. Shine the light at the injectors, notice the spray pattern. Is it nice and smooth, conical pattern?

If yes, then start checking other things.

While running, reach under the top of the EGR valve with two fingers, and pull up on the diaphragm. Does it stumble more / less? No change? Careful not to burn yourself.

Pull the distributor cap, inspect the contacts inside and the rotor. If excessively worn or lots of build up, you might need to replace it.

Also, while in there, check the reluctor magnet for cracks, this will cause erratic signals to be sent to the pcm.

You might want to check and clean grounds to the engine as well, they can wreak all sorts of havoc.