Rough Idle Diagnosis

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RoGuE

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My 2007 Malibu has 49k miles with the 2.2l Ecotec has developed a bit of a rough idle lately. It can only be felt in gear but it is rough enough to the point that I can feel vibrations through the steering wheel. It runs fine otherwise. I hooked up my bluetooth OBD2 scanner and there are no codes. However I did notices this:

In park, the engine RPMs vary from 720-750 RPM.

In gear they vary from 715-760.

(I can't remember if these are the exact numbers, but variation in RPM is right)

I checked the throttle body to see if it was dirty and getting stuck. It looked clean but had a film of oil/gas on the back of the plate.

Also, from what I understand this motor doesn't have a traditional PCV, instead it is built in to it. I've read a clogged PCV could cause a rough idle. Could that built in PCV be part of this hose that goes from the valve cover to the throttle body air intake? On the end that goes in to air intake it is covered in a bit of oil. Also, at my last oil change this engine used about 1/2 a qt of oil Pic:


Are there any other things to check for the rough idle?
 
That engine is not known for burning oil, could be a defective PCV valve. Is it integrated into the valve cover, or a replaceable part? Historically a bouncing idle is a faulty TPS. But nowadays thats all part of the electronic TB. So I'm guessing vacuum somewhere. I would take the air tube off and inspect the MAF and air filter. Pull a plug and see what they look like.
 
Originally Posted By: ejes
From what you describe, you might check for bad motor mounts.

I didn't even consider motor mounts with just 49k miles on it. Could an indication of bad mounts be a bad vibration I get at about 25-30mph?

Originally Posted By: LeakySeals
That engine is not known for burning oil, could be a defective PCV valve. Is it integrated into the valve cover, or a replaceable part? Historically a bouncing idle is a faulty TPS. But nowadays thats all part of the electronic TB. So I'm guessing vacuum somewhere. I would take the air tube off and inspect the MAF and air filter. Pull a plug and see what they look like.


To be fair though, it was 1/2 a quart used in nearly 6k miles over a course of a year, though it never happened before. Wouldn't a faulty TPS trigger a code though? And I think the PCV is integrated into the valve cover and that hose that lead into the big plastic intake part is some sort of a baffle. It just seems odd that there is oil build up on the end of the hose.

The air filter was changed within the last year. I could try some MAF cleaner spray I have on the MAF. I'll check the plugs some time this week then.
 
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A faulty TPS won't throw a code because it says to the computer that the car is greater than 0% throttle and to not bother fighting the idle. When the car is restarted the "new zero" is set at 0.35 volts or whatever the current, flaky reading is.

But this car should have an electric throttle which has redundant sensors, in the pedal at least, and a feedback loop to confirm it is where it should be.

I'd look for stuff like a stuck open EGR (if equipped), normal tune up stuff, vacuum leaks, and slow o2 sensors.
 
That engine does not have any kind of replaceable egr valve or PCV valve. Plugs on mine went 150,000 mile so at 49,000 miles the plugs should be good. Blow by past the rings will contribute to oil on the throttle plate. I would clean the MAF sensor. It's held in with tamper proof 5 point torx screws.
 
Not sure my situation is similar, but my trusty ol' 2004 Buick Rainier SUV with 5.3 V8 was idling a little rough a couple of years ago, so I changed the spark plugs, installed a new air filter, and cleaned the MAF. It didn't make much -- if any -- difference. I decided to just ignore the slightly rough idle and keep driving.

Around that time, I learned about Top Tier gasoline, and I decided to start using Mobil-brand fuel rather than the bargain-priced gasoline I had always used. That was about two years ago, and I can tell you that my ol' Buick now runs better -- and idles smoother -- than ever.

I haven't touched the engine (other than oil and filter changes, of course) since I replaced the spark plugs, so I have to credit the Top Tier gas for the performance improvement. BTW, the ol' Buick has 182K miles on it now and is still going strong.
 
It could be a partially clogged fuel injector. I used to have that problem when I was buying Arco gas. Switched to Costco Top Tier gas and after about 5 tank fulls, the idle returned to smooth.
 
Originally Posted By: WobblyElvis
That engine does not have any kind of replaceable egr valve or PCV valve. Plugs on mine went 150,000 mile so at 49,000 miles the plugs should be good. Blow by past the rings will contribute to oil on the throttle plate. I would clean the MAF sensor. It's held in with tamper proof 5 point torx screws.

I can try cleaning off the MAF sensor. Would a can of MAF sensor spray from 10 years ago still be okay to use or should I get a fresh can? It's been kept indoors and not in the garage so it wasn't subject to large temperature changes. I know the MAF sensor is supposed to be really delicate so I don't want to damage it.

Originally Posted By: Merkava_4
It could be a partially clogged fuel injector. I used to have that problem when I was buying Arco gas. Switched to Costco Top Tier gas and after about 5 tank fulls, the idle returned to smooth.


I have always used Top Tier gas in this car so that hopefully shouldn't be the issue.
 
Originally Posted By: RoGuE
Originally Posted By: ejes
From what you describe, you might check for bad motor mounts.

I didn't even consider motor mounts with just 49k miles on it. Could an indication of bad mounts be a bad vibration I get at about 25-30mph?



I would not have suggested that except for the fact that you haven't thrown any codes. When all else fails, I look for a vacuum problem, which was also mentioned early on above by LeakySeals.
 
Originally Posted By: LeakySeals
Any status on this issue?


Not really. I took off the air intake by the throttle body again and could not find much wrong in there except for a small amount of oil in the hose coming from the valve cover. I was going to pull the plugs this weekend and inspect (I don't want to do it on a hot engine after work). The MAF looks okay, but I can try spraying it with the MAF cleaner. I have a can that is about ten years old that has been kept indoors. Should that be okay to use on the MAF still?

Also, I read elsewhere that the purge solenoid (releases fuel vapors in to the intake) can cause rough idle if it is defective. That may be why my hands smelled like a gas/oil mixture when I wiped off the throttle plate.
 
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I ran a test yesterday ont he toqure app and it came back with these results. Notice the O2 sensor, EVAP, and misfire count. Are these accurate? I'd think if it was serious it would turn on the warning light. I'll still try cleaning the MAF and checking the spark plugs some time today.



 
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What are your fuel trims at idle, 1500 rpm and 2500 rpm.

I'm working on the explorer and found high fuel trims at idle (but not code setting high) and the culprit is aged brittle plenum gaskets.
 
Okay, I have an update. First I cleaned the MAF sensor. There are before and after pics in the link. It helped with the rough maybe a little, if at all. Then I pulled one of the spark plugs. I'll reserve judgement on that until I hear what you guys think. Something I did notice was the hole forming on the underside of the ground electrode...is that normal?

Here is the link to the photos:
http://imgur.com/a/6IjQs

Originally Posted By: ford46guy
What are your fuel trims at idle, 1500 rpm and 2500 rpm.

I'm working on the explorer and found high fuel trims at idle (but not code setting high) and the culprit is aged brittle plenum gaskets.


How do I check fuel trims?
 
Originally Posted By: Phishin
Those plugs look fine and normal.


Thanks, just dirty looking I guess?

Originally Posted By: Ramblejam
Maybe it's just the pictures, but that gap looks huge; what does it measure-out at? OE is .040?


I think it is the photos, they didn't seem that large. It is possible they need to be re-gapped. Perhaps that is the cause of the rough idle? I'll have to check this weekend when I have time.
 
For me, go through the trouble of removing the plugs, I just change them out.
 
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