454 TBI lean stumble on throttle tip in

[j_cst_10]

Hey guys, I appreciate any advice.

I have a 91 2500 2wd suburban (square body) 454TBI with the 4L80E trans. It intermittently develops what I can describe as a lean condition stumble (although that may not be accurate but almost feels like a carb with an accelerator pump issue). If you hold your foot on the accelerator at this point it stumbles and bucks depending on load. It appears when the truck is up to temperature. During warm up it is fine. And sometimes it doesn't do it at all!

The truck runs pretty rough when it is initially started and burbles and pops, but then smooths out and drives pretty much completely normal. You can power past the stumble if you put more foot into it.

I have done a simple tune up and replaced the temp sensor in the cylinder head and cleaned the TBI the best I could with spray cleaner.

Anybody else run into this with any of the 454's with TBI?

[Corts60]

This just happened to me on my 1991 Chevy van with TBI and it was the EGR valve. Took me forever to find it. The diaphragm failed causing a vacuum leak. Since the vacuum source for the EGR is "ported" it only receives vacuum off idle meaning it stumbles on light throttle.

Take a handheld vacuum pump and put it on the EGR. If it doesn't hold a vacuum when you pump it up, it has failed. If you replace it, be sure to get that gasket surface super clean as well. This can be a source of vacuum leaks if the gasket isn't sealed right.

Hope this helps.

[kwmech]

Not only that, but check the lower water plate under the carb. The egr blow hole in the intake has a tendency to crack the gasket in that area and cause a vacuum leak

[j_cst_10]

This makes perfect sense and I wouldn’t have thought to go in either direction.

It runs like it has a vacuum leak. I did a basic leak check with all the usual culprits years ago, but that’s as far as I went. It’s been running like this for years now. It will be that much more rewarding when it’s fixed!

If anyone else has ideas keep them coming please!

[Bladeboy1998]

I have an original gm tbi service manual if you need any info from it just direct message me and I will send some pictures to you.

[cadillac_al]

any codes?

[j_cst_10]

No codes, and the light is still working haha. I’ve done other diagnostic tasks that have triggered codes before.

It really doesn’t make a lot of sense beyond some sort of vacuum leak. I did a visual yesterday and didn’t come up with anything.

I’ll have to test the egr and spray some carb cleaner around to see if I can come up with something.

I’ll follow up when I can devote a few minutes to it.

[j_cst_10]

Also thanks for the tbi manual offer.i may just need that if I can’t find a smoking gun.

[j_cst_10]

Vacuum leak! I sprayed carb cleaner everywhere and didn’t get anything unusual. Then I pulled ever accessible vacuum fitting off.

The pcv line is a hard plastic line with 2 rubber couplers. When I pulled it and turned it upside down the one coupler had a split down it.

Runs normal so far on my initial test drive. Will test cold start tomorrow.

[Corts60]

That's great! It's always satisfying to fix vacuum leaks!

[j_cst_10]

Yes it is. From my short test drive it appears the drive ability issues are resolved. More driving will confirm this.

It still idles rough when stone cold. Something I thought would be fixed. I may try disconnecting the battery and see if it goes away for an extended time.

It seems to run better when the batt is disconnected but then would eventually return to its issues. I’m assuming it runs off of a set rich open loop parameters and then goes lean with the vacuum leak.

[hatzie]

The 1991 emissions and driveability manual as well as the service, unit, and wiring manuals are in the service manuals post.
There's also several GM Fuel Injection textbooks.

[j_cst_10]

Hatzie- thanks for the info. I was interested to learn more about the tbi set up, I will go through the links shortly.

I did mention the model and year in the first post but probably a good idea to put it in the thread title.