Rep Power: 0

Hokay, so, I've got an issue with my car's engine idling roughly, and the cause is...mysterious. I've looked up a lot of stuff on it and nothing adds up...

I don't think it can be a bad motor mount because the problem is not constant. It only happens sometimes.
I have changed the air filters, spark plugs, and even run Cataclean through the system and the issue still persists.
Every other potential cause I find in my research(bad fuel pump, cat. converter too clogged for Cataclean to work, timing belt issue) should be throwing up a Check Engine light, but I got nothing.

So here's what I know about the issue: 99% of the time, the car idles perfectly normally. But once in a while it runs noticeably rough - it's usually not terrible, just obviously not right, but once in while it almost feels like it wants to stall. It is sometimes paired with hesitance to accelerate. I also on occasion have trouble starting the car(it takes about 4-5 times longer than normal for the engine to turn over - the starter is working fine, the engine just doesn't turn over easily) It is more common when the temperature is cold - the near-stalling level of rumbling has only ever happened when the temperature is around or below 20F. I have never had any issue at all if the outside temperature is above freezing.

So given I've ruled out the obvious stuff, what should I be looking at? I've got to be careful with my money so I can't blindly throw money at trying to figure it out, I need to try and find the specific problem.

The car has about 94,000 miles on it.

 
Rep Power: 222

Cleaned the intake sensors yet?
any check engine light?

 
Rep Power: 0

I do not know how to do that(or even tell if they need it).

EDIT: Wait, do you mean the Mass Airflow sensor? I checked it when I replaced the air filter and it looked clean. I don't have any MAF cleaner on hand and I can't get to the parts store until tomorrow, but I can take it out and sue some canned air to blow it out before I head into work tonight if that might help(and it's going to a COLD night, so I'll likely know if that had any effect).

As I already said, no.

 
Rep Power: 222

Yeah give the maf a clean, and sorry I skimmed

 
Rep Power: 0

If cleaning it does not fix the problem, should I go straight to replacing the sensor or is there something else I should look into before I spend that kind of money? (I already checked prices in my area, I know what to expect)

 
Rep Power: 222

Tried disconnecting the battery yet?
Got a scanner that will do live data?

 
Rep Power: 0

Yes and yes. I got one of those Bluetooth ones that sync to an app on your phone shortly after I replaced the air filters and spark plugs started SPECIFICALLY in the hopes of tracking data for future incidents(at the time I had thought I'd fixed the problem with that). Sadly I don't know what to look for for it to help me on this one.

 
Rep Power: 0

Okay, so I used a canned air duster to blow out any dust from the MAF sensor before heading to work(newspaper delivery, for the record, so I get plenty of chances to feel out what the engine's doing). The super cold temperatures that were predicted did not come, so the conditions that the rough starts and most severely rough idles occurred in was not experienced. Therefore I cannot tell if dusting out the sensor had any effect at all, but I CAN say it did not FIX the issue as I still had some rough idles from time to time. I had no discernible acceleration hesitation, but roads were still slick and snowy which required me to be very cautious on the pedal which could have covered up some instances of hesitation.

I will pick up MAF sensor cleaner at the auto parts store later today to try that, but I have tonight off from work so will not get a full test in until the night after. Any suggestions for what to do if the MAF cleaner fails to fix it(as well as anything specific I should be made aware of about using the MAF cleaner) are welcome in the meantime.

Something I forgot to mention earlier is that the rough idles only happen if the engine drops below about 800/850 RPM(it usually idles around there but sometimes it goes down to around 650-750 and that's the range where the rough idles pop up when they decide to come along). It isn't a constant issue even at such levels, but I figured noting that would be a good idea nonetheless.

 
Rep Power: 367

vacuum leaks?
(easy check for smokers )

 
Rep Power: 222

If you can record data with it I would drive around with it acting up and record the data. Or get someone to take screenshots while you're driving.
post the screenshots here and we'll see if we spot anything out of the ordinary

 
Rep Power: 0

I've been using Torque Lite OBD-II reader because the thing came with info on how to sync to it, but it's been...weirdly uncooeprative in letting me see all data at once.

 
Rep Power: 0

Okay, I got the MAF cleaner and thoroughly cleaned the MAF with it. I'll work on getting a better app for my OBD-II reader set up so I can take some shots of the readings if the rough idle resumes.

When I got home from picking up the cleaner, the engine did it's rough idle for the first time ever at such warm(39F) temperatures.

 
Rep Power: 0

No dice. The rough idle issues were not only present again tonight, but they were WORSE than on the night after I just did the air duster cleaning despite identical weather conditions. It wasn't the worst it's ever been - no rough starts or idling so bad it feels like it's about to stall, but it WAS A lot rougher and had a LOT of hesitation to accelerate.

Where do I go from here? Replace the MAF sensor? Is there something else I can look at before dropping $50+ on a part?

Also, my Bluetooth OBD-II reader decided to stop working. I have access to another reader(that cannot see live data, only check/clear codes) so I will be able to find out if the OBD-II port is going bad or if I jsut need to return the Bluetooth gadget(it's still well within the return window), but checking live data is a no-go for now.

 
Rep Power: 0

Okay... Went to test the OBD-II port with the other reader but decided to test the Bluetooth unit one more time beforehand and it decided to start working. No idea why.

Still need advice on next step. I will try cleaning the throttle body if I don't hear any other recommendations, and I've heard about also possibly cleaning the MAP sensor, but I've heard conflicting info on whether or not the 8th gen Civic EX even HAS a MAP sensor. I certainly haven't been able to find any tutorials on how to clean it if it does - everything I do find in that regard is for older models.

I'm considering getting a new MAF sensor anyway just to be sure. While I'm not keen on dropping $58 if I don't need to, it doesn't seem like the worst idea to change this as a precaution.

 
Rep Power: 0

Okay, my information is still limited because I have not yet been able to figure out ow to get a full data log(Torque Lite keeps giving me an error when I tell it to start logging data), but I tracked a few things while I was idling at stops tonight and based on what I saw... I'm thinking O2 sensor.

I monitored vacuum, manifold pressure, and MAF. Manifold pressure at idle bounced from 5.9 to 7.1 PSI, MAF bounced from 2.9 to 4.3 g/s, and I can't remember the number for vacuum. But these ranges stayed consistent whether the engine was idling smooth, rough, or EXTREMELY rough(as it did as I arrived home). I would think these numbers would go above or below the range of a smooth idle if the MAF sensor was the issue causing rough idle.

Am I on the right track to think O2 sensor based on this?

 
Rep Power: 222

It's a possibility, if you have torque lite that might be part of the problem of seeing data, full app is only $5.

 
Rep Power: 0

I'm thinking I might go ahead and replace the O2 sensors regardless. I'm seeing a lot of stuff about 8th gen Civic O2 sensors needing to be replaced from around 100K-150K miles, and the car is rapidly approaching 100K. (I put 135 miles on it five nights a week for work)

I found a cheap set of O2 sensors on Amazon, but I'm wary of going outside of major parts stores for this stuff. On the other hand, I have Amazon Prime and returns are pretty painless....

 
Rep Power: 222

Factory is Denso or ntk, do not buy any other brand. Get the part number from densos website, then check Amazon/rockauto, be careful with Amazon if the price is significantly cheaper than other sellers for the same price it's probably a fake part

 
Rep Power: 0

Denso gave me two results for the upstream sensor. How do I know which one to go with?

EDIT: Well, several parts sites say one of them doesn't fit, though others do, while they all say the other one fits. I think that's a pretty clear indication of which one I should get.

 
Rep Power: 224

Look at the "NOTE:". One is for a gas engine and the other is for CNG (compressed natural gas).

 
Rep Power: 0

Yeah, I figured that out after my last edit. I got the right one on Amazon - quite a bit cheaper than in the local parts stores(that had to order it anyway) but not suspiciously so - price was in line with what it came up at on RockAuto.
But I only got the upstream sensor, which seems to be the one most likely to cause the problem.

The big issue now is that the socket I have for removing it is too thick-sided to fit and remove the sensor.

 
Rep Power: 222

Downstream only checks that the cat is working, doesn't serve any other purpose

 
Rep Power: 0

O2 sensor due to arrive Monday. Then it's just a matter of figuring out how to deal with the socket problem...

 
Rep Power: 222

Amazon again, or local parts stores they're not too expensive

 
Rep Power: 0

The problem with Amazon is that I can't be certain the socket will be thin-walled enough.
The problem with local parts store is none have ANY in stock so I have to order anyway leading to the same issue as with Amazon...

 
Rep Power: 125

My dad had the same problem a few years ago changing the timing belt on his Ducati. He used a belt sander and shaved the socket down. They’re pretty cheap and super handy to have.

 
Rep Power: 222

If you buy an o2 sensor specific socket that shouldn't be a problem. That or sand it down, all mechanics have at least 1 custom tool

 
Rep Power: 0

I have an O2 sensor socket. We already checked the fit to make sure it'd work, and it doesn't. We(as in, my dad and I) aren't exactly keen on the notion of sanding it down.

 
Rep Power: 0

Okay, thanks to O'Reilly Auto Parts loaner tools, I have the new O2 sensor installed. I gave the car a short drive around the block - no error codes, no hesitation to accelerate. There was a slight roughness in the idle when I stopped to park it afterwards, but it was very mild - I would never have noticed it if I wasn't expressly paying attention. Between that and some cars needing to re-learn how to idle after changing such a part, I'm not exactly worried QUITE yet.

There was a smell of metal, but I'm sure that's the copper anti-seize burning off the excess.

 
Rep Power: 222

Glad to see that it seems like you fixed it
Can always perform an idle relearn yourself