What can cause small misfires on all cylinders

Motor swap done, can runs pretty well. I do have very slight misfires on all cylinders. Not enough to throw a CEL but my Accessport reads cylinder roughness on all cylinders, usually at idle, and they don't go above 5.

 

Only happens at low rpms (under 3k) and at light load. When cruising you can sometimes feel the miss, feels as if you were quickly and lightly tapping the brakes. The car does have Group N motor mounts which are probably making the issues feel worse than it is.

 

Car has 176k miles, 400 miles since motor swap

-Cleaned MAF

-New Bosch 02 Sensor (P#15011)

-Original Coils

-Original Injectors, but were cleaned/tested by DW

-DW Fuel Pump (have been fighting P0230 FP Circuit Malfunction but *appears* to have been fixed with a new Relay)

-No Boost Leak

-New NGK Iridium Plugs

-A/F ration just about perfect while driving, a bit rich at idle (~14.2)

-Car is on a Cobb OTS Stage 2 tune until the motor is ready for a pro tune

 

Any ideas as to what might cause the minor roughness/misfire? I read on a thread it could possibly be the seals between in the intake manifold and TGVs. Any other ideas? Your help is greatly appreciated!

bump, any ideas? Last night I cleaned MAF for a second time, and checked airbox to make sure its sealed up nicely. Checked cylinder roughness at idle and I had a 9 on cylinder two and hardly any others. Figure something is up on that cylinder and planned to swap injector/coil with another cylinder tonight. Check roughness again this morning and all cylinders were at 1 or 2. Cylinder 2 was not an outlier this time.

 

Thoughts?

I had a good friend with a similar problem. He was tuning the car for a customer and was having the same issues. They threw part after part into it with zero change. Ended up posting on a local board and about 8 folks told him it was the coils. Even though they were 'working,' they weren't working correctly. That ended up being the cause.

Give it a shot!

I had a good friend with a similar problem. He was tuning the car for a customer and was having the same issues. They threw part after part into it with zero change. Ended up posting on a local board and about 8 folks told him it was the coils. Even though they were 'working,' they weren't working correctly. That ended up being the cause.

Give it a shot!

 

Thanks the suggestion! I was hoping to avoid new coils since they aren't cheap, but at 175k miles I'm sure its getting to be about time. They are definitely on the list.

Remove the coils and use a set of needlenose pliers and twist the three pins slightly on the coil so that when you click the connector into place they are scraping a tiny bit on the male plug. The plugs become loose over time and when you twist the pins, it helps them to make good contact again and should alleviate the problem providing the coils are, in fact, good.

Great suggestion, thanks! I'll do that. After a little more investigation I'm leaning towards injector issues. The last couple days I've been reviewing cylinder roughness at idle as monitored by the AccessPort. Cylinder one had zero pretty much all of the time. Cylinders three and four had usually maybe 2-3. Cylinder two would have much higher rates, usually around 9 or 10.

 

Last night I swapped injectors on one and two. Now, Cylinder one reads high numbers and two reads next to nothing. Doubt its a coincidence but will double check again today.

 

Assuming I get the same results, than I'll start by contacting DW who did the clearning/flow test. I've attached the results the report they sent me. I see a 3.7% variance among injectors at low IDC. Would this support my suspicions?

 

Should I just replace the one causing the most issue? Three of them? All four of them? Not sure which route to take at this point.

IMG_20140920_0001.compressed.pdf

You might try sending all 4 to Deautchwerks for cleaning and see how they match up before spending money on a single injector. The best thing is to have them all with a couple of percentage points as this will give the most performance. They can tell you how they all look and it's only a few bucks to have them cleaned and checked. In the IDEA sticky above, there is a post about a couple of different companies that do FI cleaning and hopefully that helps.

You might try sending all 4 to Deautchwerks for cleaning and see how they match up before spending money on a single injector. The best thing is to have them all with a couple of percentage points as this will give the most performance. They can tell you how they all look and it's only a few bucks to have them cleaned and checked. In the IDEA sticky above, there is a post about a couple of different companies that do FI cleaning and hopefully that helps.

 

Thanks for the reply. All four have already been cleaned and tested by DW so not sure if sending them back will improve anything. I did email them though so we'll see what they say.

I would be suspicious of the Bosch O2 sensor. OEM or Densor (oem manufacturer) or I wouldn't trust anything else. I got the Denso a few months ago and I'm already having some symptoms of a busted one. Also maybe a failing fuel pump but that would be cutting out at higher rpm.

I would be suspicious of the Bosch O2 sensor. OEM or Densor (oem manufacturer) or I wouldn't trust anything else. I got the Denso a few months ago and I'm already having some symptoms of a busted one. Also maybe a failing fuel pump but that would be cutting out at higher rpm.

 

You could be right, though I'd had pretty good luck with Bosch 02s in the past. Shouldn't be the pump, its a brand new DW 65c. I'll see if I get anywhere with the injectors and than take a closer look at the 02.

Or maybe related to this topic: DIY: Rough idle in cold start/cold temps LGT 07-09 - Subaru Legacy Forums

Subarus are very picky about O2 sensors. I had a Bosch in my Accord that worked great but I would avoid the brand for a Subaru.

Thanks for all the help guys. I'm still leaning towards a faulty injector since the cylinder roughness/misfire moved from cylinder two to one when I swapped injectors. Will definitely keep the 02 in mind.