Loss of engine power between 2100 and 2500 RPM on humid days - 2011 Malibu

Question

I drive a 2011 Chevy Malibu LT (4 cyl, 2.4L, 6 spd. automatic) and have been experiencing this problem for the past year and a half. Basically, whenver its hot (90f+) and humid out, my car exhibits loss of engine power between 2100 and 2500 RPM. Its a very strange problem that is hard to reproduce so I haven't taken it to a mechanic, I'd like to tackle it.

If I'm gently accelerating, I'll hit somewhere between 2100 and 2500 RPM and the RPMs will drop to 2100 and bounce back up to 2500, repeatedly, as if I'm tapping the gas in ~1 second intervals. If I continuously but gently apply more throttle, it seems to have no effect until the point where the car actually downshifts to accellerate hard. Once the vehicle downshifts, I'm up above 2500 RPM and running fine. Once I hit that same RPM range again, the problem reoccurs.

Another thing to note, my average MPG reads 21.5 on a vehicle that should get 26/33 MPG. Then again, I drive pretty hard.

The gist:

• This only occurs between 2100 and 2500 RPM
• It must be over 90 degrees f. outside
• It must be humid
• It occurs in all gears
• No apparent sputtering
• RPMs bounce from 2100 to 2500 as if I'm tapping the gas
• No check engine light
• This problem never shows up in the winter

I think the first thing Im going to do is clean my MAF. I also think some other possible causes may be my O2 sensor and ECM.

What do you guys think? Has anyone encountered a problem like this?

Update: Cleaned MAF sensor, issue still persists.

Update 2: Today it was raining and 80 degrees and the problem happened again. I saw that my long-term fuel trim was actually at about -12 at idle and between -10 and -20 while driving. Normally my LTFT stays between -5 and 5. Any insights?

Update 3: Problem reoccurred today, it was only 64 degrees and had recently rained.

Update 4: I forgot to update this 3 years ago, but I sold the vehicle to a dealership and got a truck about a month after this post.

Answer 1

You may want to check with the dealer, I have 2011 cruz and have had similar issues with it. I used my scan tool and oscilloscope to find that the ECM had my coils out of time. I suspected something else must be causing this. I eventually got tired of complaining to them. They called me up and said they would put an ECM in it and see if that would fix since I'd called Chevy and even emailed them my data. ECM was replaced and the car runs fine now.

Your fuel trims are the first thing you need to look at. Those are negative. Which means the ECM is retarding the flow of fuel because there is too much in the fuel mixture. A rich mixture could definitely lead to the symptoms you're describing. What is causing your fuel trims to act like that is a whole different diagnosis. I would check that out first, because if you have one problem you can't fix all others and gain a good understanding of what is going on. Your short term fuel trims should fluctuate quite a bit, but not in huge margins. -5 to +5 is a bit off for a 2011. I usually see those in older vehicles. If you'd like to see a good video on diagnosis of fuel trims, there is a YouTube channel called Shrodingers Box. He does an excellent job of explaining the science and methodology of how it works and how to use it to figure things out. You'd be extremely surprised how many times I've diagnosed a vehicle just based on fuel trim and o2 data. You need to do injector pulse testing, timing testing, fuel pressure testing as well as using testing your coils to make sure they are firing hot enough.

You can achieve this by using an oscilloscope. You can measure your injector pulse timing, crank position, and coil firing with it to make sure it's working cohesively.

I would start with the easy stuff. Check your vacuum with a vacuum gauge. Check fuel pressure for drops and spikes. You could use a low amp probe to check your coils to see if they are delivering proper amperage.

Check your spark plugs and see how they look. They can be a good visual representation of how the engine is running.

This is a bit of a difficult question to answer over the internet, but hopefully I've pointed you in the right direction.

Answer 2

The ideology of "I better clean the MAF Sensor" and "My O2 sensor is faulty!" is the product of "blanket fixes" for the slightest bump in the road (bad pun not intended). From what you described, those would be the very bottom of what I would be looking at in the case of your vehicle (unless I found something in a scan tool guiding me in that direction). There are three pieces of information that lends me to think it's something else. 1) It's under heavy load (you said it was during acceleration between 2.1 and 2.5k rpms) 2) It's humid out 3) You mentioned replacing plugs, but not the wires. Especially if you don't keep up with maintenance, the plug wires could've degraded over time and with the high KVs traveling through the wire, there could be arcing happening to the engine block.

Answer 3

I also had this problem and believe I have resolved it. It happens when the humidity restricts the air flow thru the filter. I put some slits in the air filter with a sharp razor . The filter looks like it will still keep out dirt because the filter material buts right up against itself after the slit is made.

Seems to have resolved the problem the car is running at lower RPM's now so its not hitting that 2100 number. My son has the same car and had the same problem. Its defiantly not the transmission because we put a brand new one in for another problem and the car still did it. So its restrictive air flow at hi humidity.