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?

 

[Doubeleive]

What throttle body did you replace the factory one with?

Mine is the 6.2l so a ls7 throttle body was a direct bolt on

 

[PG01]

Mine is the 6.2l so a ls7 throttle body was a direct bolt on

Like you really needed anything from an ls7

 

[Doubeleive]

Like you really needed anything from an ls7

I like the ls7 throttle body it's a all aluminum design and obviously better quality than what they throw on the SUV's so it was a no brainer
$150 well spent. Did not require any tune or calibration, plug-n-play. I do have a cai installed so it is possible a stock intake housing may not fit.
it's been several years if someone wanted to retain the stock intake housing you would need to do some research the ls7 tb sticks out on one side a bit

 

[Doubeleive]

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.

doesn't sound like he is getting any codes. a scanner could be connected to check live data for any misfire counts while driving. Not sure how many it takes to throw a code but normal operation should show 0/0/0/0/0/0/0/0

 

[992dr]

I like the ls7 throttle body it's a all aluminum design and obviously better quality than what they throw on the SUV's so it was a no brainer
$150 well spent. Did not require any tune or calibration, plug-n-play. I do have a cai installed so it is possible a stock intake housing may not fit.
it's been several years if someone wanted to retain the stock intake housing you would need to do some research the ls7 tb sticks out on one side a bit
View attachment 427230

Hmmmay have to that route especially for 150 smackers. I can figure the intake out that should be easy.
Thanks

 

[Doubeleive]

Hmmmay have to that route especially for 150 smackers. I can figure the intake out that should be easy.
Thanks

prices seem to have gone down on them a little bit, but I bought mine new, in the hitachi box, Hitachi makes them for GM.
pretty sure I bought the 90mm the 6.2l intake I believe is 88mm (outside, circumference), the rear mating surface is the same size.
there are like 107mm one's out there so just be sure to get the right size, generally the larger size are more like $250.

 

[992dr]

prices seem to have gone down on them a little bit, but I bought mine new, in the hitachi box, Hitachi makes them for GM.
pretty sure I bought the 90mm the 6.2l intake I believe is 88mm (outside, circumference), the rear mating surface is the same size.
there are like 107mm one's out there so just be sure to get the right size, generally the larger size are more like $250.

Gotcha man, Thanks I appreciate the info

 

[houstontaylor]

I tried a Hitachi throttle body but it went bad after a few months. So I went back to OEM and no problems. Also, I had air leaks where the pcv hose hooks to the rear of the valve cover and the fuel regulator rubber boots/hose. Like someone said, check the fuel trims with a scanner that shows live data.

 

[tgwhit12]

On Doubleeive's post, I agree; at least from my experince. I mentioned before that I put in a new factory GM 6.0 in my 212K mile 13 Tahoe. When that was done, the intake was cleaned and most of the sensors replaced. I had a stumble / high idle during braking and it was bugging me. I'd run across a GM mechanic trying to help me with my reverse lights and while he was plugged in with his laptop, I mentioned the characteristics above. He reset the mileage (not on the odometer) but somewhere. He told me the computer was compensating for what it thought was build up in the throttle body of a 212K mile vehicle. Once he did that, it all went away and never came back.
I don't know if that helps, but it may be worth a look.

​

 

[Geotrash]

FWIW, I've been running a Dorman throttle body on my '07 for the past 40K with no problems. Runs smooth and strong.