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Old 11-30-2019, 10:01 PM   #1
subydude
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Default Cylinder 3/4 roughness only in gear at idle?

I'm chasing a cylinder roughness issue in cylinder 3 and 4 on a fresh build.

It only counts up at idle (1k rpm) when in gear or coasting. So coming off the freeway if I clutch in and go to neutral it will count up in 4 and some in 3 until I stop, then it goes back to zero. Car doesn't feel like it's misfiring at all.

While stopped if I put it in gear it immediately starts counting up in 4, doesn't matter what gear. And when I go back to neutral it goes back to zero. I can listen to the exhaust and tell there's no misfire going on.

As stated, fresh build, fresh valves, known good coil packs, have swapped injectors around, running speed density, fresh fuel pump and filter, new spark plugs.

Is it possible it's from mounts or something else? The car has Cusco engine mounts, a kartboy dog bone, window weld trans mounts, and home brew very stiff cross member bushings. I also have a light weight flywheel in there, but OEM crank pulley.

Any advice or thoughts?
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Old 11-30-2019, 10:23 PM   #2
gaby
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Lighter flywheel and Forge pistons will increase idle roughness. What is the count?
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Old 11-30-2019, 10:27 PM   #3
subydude
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4 counts up to 50-60 or more and then resets. As soon as I pull it out of gear at idle it goes to and stays at 0. It's only when the car is in gear below about 1,500 rpm, and pretty much at idle 99% of the time.

Car does have CP pistons at 3-3.5 thou PTW. Forged rods and OEM crank with stock heads/cams/valves. Nothing special, and it runs fine in boost or otherwise. It's only at idle in gear that it seems to really advance the count.
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Old 11-30-2019, 10:31 PM   #4
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Cp pistons are very hard, they are loud. I personally have CP pistons. I lowered the timing to not more than 40 before 90gsload. That helped a bit. As long as your compression is good and the ECU is not pulling timing, you should be ok.
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Old 11-30-2019, 10:37 PM   #5
subydude
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It's running roughly 17 degrees timing during, not pulling any for it that I can see in the logs. Engine has maybe 50 miles on it so things are still good.
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Old 11-30-2019, 10:39 PM   #6
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how is the IAM and the learning values?
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Old 11-30-2019, 10:44 PM   #7
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Fine so far.
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Old 12-01-2019, 01:58 PM   #8
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Have you tried adding timing in those areas?
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Old 12-01-2019, 02:37 PM   #9
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Maybe one of those two cylinders is just a tad more lean because of injector variance and exhibiting it only at idle because of idle afr?
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Old 12-01-2019, 02:43 PM   #10
subydude
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Quote:
Originally Posted by 2slofouru View Post
Maybe one of those two cylinders is just a tad more lean because of injector variance and exhibiting it only at idle because of idle afr?
Unfortunately no. I forced idle in OL as a test and even with it idling at 11.5-12:1 it would count up. I have also switched injectors around and it does not follow them.

It's only when the car is in gear or in neutral rolling to a stop around 1k-1.4k rpm.

I did try adding a little timing at idle just for fun and will test today.
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Old 12-01-2019, 08:10 PM   #11
subydude
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So I think I have it narrowed down to the neutral switch. I've swapped everything else and finally unplugged harness stuff until I narrowed it down to the neutral switch. With the switch unplugged it counted right up, with it plugged in it stopped.

Since the switch should be 12v in gear and grounded when in neutral is there any way to test besides swapping sensors?
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Old 12-01-2019, 09:21 PM   #12
gaby
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Quote:
Originally Posted by subydude View Post
So I think I have it narrowed down to the neutral switch. I've swapped everything else and finally unplugged harness stuff until I narrowed it down to the neutral switch. With the switch unplugged it counted right up, with it plugged in it stopped.

Since the switch should be 12v in gear and grounded when in neutral is there any way to test besides swapping sensors?
Romraider logger will tell you if is on or off.
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Old 12-01-2019, 09:38 PM   #13
subydude
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The Cobb I have shows it turning on and off.
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Old 12-01-2019, 11:03 PM   #14
gaby
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Make sure it doesn't stay on or off between gears. mine is inverted polarity do jdm transmission switch.
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Old 12-02-2019, 10:40 AM   #15
subydude
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I have a spare from another case that I'm going to plug in and test tonight to see if the issue is the same. If so then maybe a wiring issue. If it solves it then I'll have to swap the switch.
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Old 12-05-2019, 12:57 PM   #16
subydude
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Following up on this, I tested with another neutral switch and the issue remains. Only counts up misfires in gear at a stop, or when in neutral coasting to a stop, so basically below 1,500 or so rpm at 0 mph in gear or below that rpm coasting in neutral.

Still only on cylinder 4 and 3 really, even after swapping injectors, coils, etc.

I have the factory grounds in place, but did put phenolic spacers in so I was thinking of running a few extra just to test. I also was wondering if an alternator could cause the issue at idle, or the fuel damper at the feed/return. It all seems to be low airflow/fuel flow (sub 1,500 rpm) and seems related to the neutral switch.

Again, still not actually misfiring, so it seems to be a false reading.
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