06 Buick Lacrosse 3.8L Surging/Bouncing RPMs when accelerating lightly

[th0mathy]

I've got an issue with my 2006 Buick LaCrosse where the RPM's surge anywhere between 1800 and 2200 RPMs. Sometimes the bounce will be +/- 100, while other times the engine will bounce a bit more aggressively, likely +/- 200 to 300 RPMs. As soon as I let off or put more pressure on the pedal, the issue disappears. This issue began almost immediately after I replaced my Air Filter, but I've checked it 1000 times to make sure it's the right size and that it's seated properly. I've even replaced it with another filter, but it didn't stop.

Now, I'm aware similar posts have been made, but the solutions I've found haven't worked. I cleaned my MAF, cleaned my EGR, replaced my Throttle Body, bought new spark plugs and wires, and tried a vacuum leak test with a can of Carb Cleaner, and nothing has shown up to be a problem.

Another thing that makes my problem unique is that this issue will stop sometimes, mostly after I disconnect the battery for a few hours. However, it'll start back up again after a few weeks of it driving completely normal. This is the part that has really stumped me.

As for data on my scan tool, the only thing I've identified that may be an issue is that my Fuel Trim seems to always be negative. It seems to be -5 at idle, and between -10 to -15 when driving. I'm not sure what's causing this, but I have no check engine lights indicating that there's any type of fuel problem. I've also checked my mass airflow sensor data, and it seems to be reading as you'd expect, but the data does bounce up and down when the engine and RPMs do. Otherwise, the data seems completely normal. I'm no mechanic, so there could be valuable data I'm missing, and I'd happily upload my data if someone here is willing to look at it.

I've been left completely stumped by this issue, and it seems to be becoming slowly more frequent as time does on. I'm not sure what my next step should be, and I'd greatly appreciate some help here

[Black92GS]

Check the live data with your scanner and look for a Data PID reporting the fuel alcohol content. It should be reading 10% or less, (3-4% is the general default value) assuming you're running fairly standard fuel, ethanol containing or otherwise.  Mis-reporting of the alcohol content in fuel of Flex Fuel GMs of this era is relatively common, and leads to rich conditions when running normal fuel. I forget the exact critera...but a long term fuel trim around -25 is typically where the CEL is finally illuminated for a fuel system rich condition. I believe there are also some cases of non Flex Fuel equipped vehicles reporting this data in the PCM and experiencing the same alcohol content issues, despite the vehicle not having flex fuel capability. Normal scan tools should be able to read this data, however it generally takes a higher end scanner capable of special functions to be able to reset the fuel alcohol composition. 

Another thing to check is the MAP sensor.  These engines use both the MAP and MAF sensors, but they can run properly with only one of them connected.  A quick and dirty test is to check the MAP sensor data with a scan tool with the engine not running.  It should be reading very close to what you'd expect atmospheric pressure to be in your area.  If it appears to be skewed one way or another, unplug it and see if the car runs better. I ran into this on similar era buick allure (canadian LaCrosse). The car would idle rough, the RPMs would fluctuate....but it would also fall flat on its face when driving and would lose all power.  The MAP sensor reading was not correct with the engine off.  Unplugging the MAP sensor instantly eliminated all of the issues.  Replacement MAP and the vehicle was fixed. Unfortunately, I can't recall if the fuel trim levels were off due to the MAP sensor readings being skewed.  

[Schurkey]

On 5/31/2024 at 3:50 PM, th0mathy said:

As for data on my scan tool, the only thing I've identified that may be an issue is that my Fuel Trim seems to always be negative. It seems to be -5 at idle, and between -10 to -15 when driving. I'm not sure what's causing this, but I have no check engine lights indicating that there's any type of fuel problem.

How ancient are the O2 sensors?  O2 sensors get screwy with age/mileage.  USUALLY they report too lean, causing the computer to go into rich-command.  But I suppose anything is possible.