2017 Tahoe with very rough idle

[dksharp]

We have a 2017 tahoe with 155K miles on it. It began idling very rough a few weeks ago. It had bank 2 sensor 1 O2 sensor bad so I replaced it. Still not better. I ended up replacing all the spark plugs and wires. No change. Cleaned the MAS air sensor. No change. 3 mechanics have looked at it. Bank 1 sensor 1 O2 indicator was delayed so replaced that one. Chevrolet recommends changing motor mounts which are worn out. We will but not at this time. 3rd mechanic says there is a technical service bulletin that mentions a lifter issue and possible camshaft issues. This mechanic thinks one of the lobes is rounded on the cam and causing this issue. His recommendation is to trade it in. I personally think it is a vacuum leak. Does anyone have any feedback on this top or know the vacuum system well?

 

[GranPrix]

Might want to do an AFM DOD delete

 

[Joseph Garcia]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

I cannot personally assist you with your issue, but other members of this Forum much more knowledgeable than me in this area will chime in.

 

[BUCKSATH]

The injector seals go bad on the L83/L86. The let air by causing a rough idle. This can feel worse if you have a bad driver side engine mount. The miles on your vehicle would tell me that both are possible. You’ll feel a clunk in the pedal from a stop that would tell you if you have a bad motor mount. The engine jiggle is most likely the injector seals.

 

[dksharp]

Might want to do an AFM DOD delete

Is this something a professional should do? How much does it cost?

 

[dksharp]

The injector seals go bad on the L83/L86. The let air by causing a rough idle. This can feel worse if you have a bad driver side engine mount. The miles on your vehicle would tell me that both are possible. You’ll feel a clunk in the pedal from a stop that would tell you if you have a bad motor mount. The engine jiggle is most likely the injector seals.

I will check these seals. You can check it like you are checking for a vacuum leak? The engine mounts are on our list to repair $1250. Would you recommend
replacing all the injectors?

 

[Fless]

One thing you might notice with a vacuum leak is that things smooth out at higher rpms. Get a scanner on it showing live data, and monitor the short- and long-term fuel trims. Do this test to rule out a simple issue with the fuel mix.

Document the trims at idle, then again at 1500-2000 rpm. A vacuum leak will show somewhat higher (positive) fuel trims at idle since the unmetered air is diluting the fuel delivery, and the ECM is trying to catch up by adding fuel. That's a lean condition. Raising the rpm will overcome the vacuum leak and the trims should come down.

This guy does a somewhat reasonable job of showing how it works:

Great fuel trims are within ±5% total (between -5 & +5) when the LT and ST trims for a given bank are added together. I would say that ±10% is a threshold that cries out for some attention. Of course there will be some variation but overall they need to be low for proper fueling.

You could also look at the MAF reading at idle, in g/sec (not lb/min); that should be close to the engine displacement. Consider an OE MAF replacement if it's somewhat off.

 

[Doubeleive]

sounds like you just need a better diagnostic, like a misfire graph and/or compression test that would probably tell you a little more
the fact that the mechanic says to "trade it in" means either it's something they could probably fix easily and flip it or they don't even want to deal with it.
injectors on some of these trucks have a extended warranty period under special coverage which you could check at the dealer with your vin.
in any case you are on the hook for diagnostic cost. It could be beneficial to invest in a decent scanner, for this scenario and any future ones.

 

[GranPrix]

Is this something a professional should do? How much does it cost?

Depends on how mechanically inclined you are. Just in parts you're looking at $1-$2k. But this would be kinda of a worst case scenario. It gets rid of your AFM system. But like doubeleive said, sounds like you need a better diagnostic

 

[dksharp]

One thing you might notice with a vacuum leak is that things smooth out at higher rpms. Get a scanner on it showing live data, and monitor the short- and long-term fuel trims. Do this test to rule out a simple issue with the fuel mix.

Document the trims at idle, then again at 1500-2000 rpm. A vacuum leak will show somewhat higher (positive) fuel trims at idle since the unmetered air is diluting the fuel delivery, and the ECM is trying to catch up by adding fuel. That's a lean condition. Raising the rpm will overcome the vacuum leak and the trims should come down.

This guy does a somewhat reasonable job of showing how it works:

Great fuel trims are within ±5% total (between -5 & +5) when the LT and ST trims for a given bank are added together. I would say that ±10% is a threshold that cries out for some attention. Of course there will be some variation but overall they need to be low for proper fueling.

You could also look at the MAF reading at idle, in g/sec (not lb/min); that should be close to the engine displacement. Consider an OE MAF replacement if it's somewhat off.

Thanks for the good detail and video. I will get me a live data reader and try this. The video is very helpful. Will report back on my finding soon. I have to get the vehicle here soon to troubleshoot further. It is currently 3 hours away.