Need Advice on Engine Stumble

[Kwing]

Got a stumper I need some advice on...

2007 Tahoe LT, 5.3 FFV, 287k on the clock, longblock was replaced at 200k due to AFM lifter failure, AFM disable dongle was put in with the new engine.

A couple weeks ago she developed a stumble at idle. Just a quick little stutter and then normal. Registers as about a 50 rpm drop on the tach. It's getting progressively worse as time goes on (meaning freequency between stumbles is getting higher), but there is no check-engine light or OBDII codes.

It's to the point that it stumbles every 2-3 seconds sitting at a light, and if I just ease off the brake and let it start to roll, it increases to about 0.8 seconds between stumbles (sat there in traffic with a stopwatch recording a lap at each stumble).

As soon as I give it any gas pedal, it smooths right out and does not feel low on power at any point. Occasionally, it will stumble at steady-state cruising on the freeway, but just a one-and-done kind of thing.

It feels like a missfire, but I would think the computer would recognize that?

Any advice on where to look?

 

[Fless]

I'd probably start looking for a small vacuum leak or a dirty throttle body / IAC. It might be helpful to document the short-term fuel trims at idle, then at 1500 or so rpm to see how much they change. Try this with the engine under load (as in Drive, but be careful when you raise the rpms).

If you have a scanner that can show individual cylinder misfires, you could monitor them to see if something like that is happening at idle.

 

[Doubeleive]

Got a stumper I need some advice on...

2007 Tahoe LT, 5.3 FFV, 287k on the clock, longblock was replaced at 200k due to AFM lifter failure, AFM disable dongle was put in with the new engine.

A couple weeks ago she developed a stumble at idle. Just a quick little stutter and then normal. Registers as about a 50 rpm drop on the tach. It's getting progressively worse as time goes on (meaning freequency between stumbles is getting higher), but there is no check-engine light or OBDII codes.

It's to the point that it stumbles every 2-3 seconds sitting at a light, and if I just ease off the brake and let it start to roll, it increases to about 0.8 seconds between stumbles (sat there in traffic with a stopwatch recording a lap at each stumble).

As soon as I give it any gas pedal, it smooths right out and does not feel low on power at any point. Occasionally, it will stumble at steady-state cruising on the freeway, but just a one-and-done kind of thing.

It feels like a missfire, but I would think the computer would recognize that?

Any advice on where to look?

as noted already probably the throttle body, cleaning might help maybe. Otherwise it might be time for a new one
computer won't pick that up unless it completely errors out
not saying run out and buy one, but it is the most likely cause.
mine did it when I first bought it, tried cleaning it and didn't help so slapped a new one on it and no problem since, that's been like 5 years now

 

[Doubeleive]

also if it is still the original maf you might take a look at that too

 

[Kwing]

Appreciate the suggestions!

It does have the original MAF and throttle body.

I checked for obvious air leaks down stream of the MAF, but didn't find anything. Similarly with the vacuum lines.

Maybe I should hit it with a smoke machine....

For the throttle body and MAF, I've sprayed them their respective cleaners over the years, but nothing more than that.

Are either one of them serviceable for a full teardown and cleaning?

 

[Foggy]

Also could just be the standard old items:
bad/worn spark plug(s) or wires, coil pack getting weak, etc

 

[Kwing]

Also could just be the standard old items:
bad/worn spark plug(s) or wires, coil pack getting weak, etc

I did the plugs and wires back at the end of January, but the coils are another story.

Wouldn't coils trigger a missfire code though?

 

[Doubeleive]

I did the plugs and wires back at the end of January, but the coils are another story.

Wouldn't coils trigger a missfire code though?

Depends, sometimes yes and sometimes no.
and no the maf & tb are not serviceable

 

[petethepug]

Cruise down to the local big brand auto parts store and let them try to pull some codes.

You’re on track with the gen maintenance. Now you need to diagnostics to dig a little deeper.

 

[992dr]

as noted already probably the throttle body, cleaning might help maybe. Otherwise it might be time for a new one
computer won't pick that up unless it completely errors out
not saying run out and buy one, but it is the most likely cause.
mine did it when I first bought it, tried cleaning it and didn't help so slapped a new one on it and no problem since, that's been like 5 years now

What throttle body did you replace the factory one with?