Thread: Bucking under cruise conditions, misfire?

My NX just started doing this a few weeks ago. It has gotten worse now, and it's time to get it fixed.

Symptom: SR20VET, Bucking/misfire at 2300-3000rpm. Very noticeable during cruising. At 50mph some bucking can be felt thru the steering wheel. Over 60mph the car bucks alot, you can feel it miss thru the whole body. Car idles great and pulls very strong WOT. AFR during 60mph pendles between 14.0->15.5 during narrowband closed loop. Seems abit excessive, with 256cc injectors it swings between 14.5->14.9. Car bucks in open loop aswell, when running steady state 60mph@14.7AFR. Mechanical issue therefore suspected.

Things recently changed:
-550cc STI injectors->750cc JECS injectors
-Flywheel, 12kg stock -> 6kg lightweight (50% weight reduction)[FX500 clutch]
-Retuned it, it's not perfect, but it didn't behave like this the first week.

So, what to check? Ignition issue? Lean/rich miss?

Ignition component status
Sparkplugs, NGK BKR7EY, 0.9mm plug gap (1½years old)
Ignition wires, stock OEM, resistance checks out according to FSM (24 years old)
Rotor and cap, aftermarket noname, no cracking, looks good with ocular inspection (3 years old)
Ignition coil, resistance good according to FSM, (24 years old)
Distributor, no oil when cap was removed. (24 years old)

Sparkplugs look even, like this


Advice?

Scared me with that 0.9 gap but then I realized that's millimeters so ~0.035"

Have you tried tightening the gap at all? What's the timing set at? Bump timing?

Timing table


At 60mph, i'm at ca 600mbar load, 2800rpm, so the final ignition is 20ish degrees BTDC

That gap has worked perfectly before :/

Played around some more this evening. Started by swapping over sparkplugs to a know good set. No difference

Then I went bonkers with the tune. Smoothed out the fuel map at the place where it bucks, but unfortunately no difference.

I am now almost absolutely sure that this is an mechanical issue.

If anyones interested, heres the lates config that I tried to do alot of small tweaks to. Removed many correction factors, and tried to make it as smooth as possible.
https://www.dropbox.com/s/67k1t0615whs3fq/v1.50%20Smoother%20map.tcd?dl=0

https://www.dropbox.com/s/1a1otodroisbovt/300rpmLog.TCL?dl=0

Now, i think it's time to swap out the injectors

Those spark plug wires need replacing. Holy crap. Resistance check means nothing if the insulation is worn out. Replace now!
Same goes for distributor cap and rotor. Oil means nothing. Them shits are toast.


You're using Speed-Density to tune with, yes? If so, I'll need to convert mbar to kPa and get back to you about your ignition map. Right now it seems WAY retarded.

Edit: As I thought, 1,000 mbar = 100.0 kPa

Your ignition timing is WAY too retarded!

To give you an example, an ultra-conservative ignition timing at 1,000 mbar from 4,000 RPM to 6,000 RPM is around 27 degrees. That's for a 9.5:1 compression DE+T with stock DE cams, so a VET might use a little less timing, but 25 degrees would probably still be a very safe place to start from.

But you have 18 - 21 degrees on your 1,060 mbar column. You could use another 4 degrees at least. And I would carry that change throughout almost the entire rest of the tune.

I know you say this just started, and the tune hasn't changed, but once you get your plug wires and other ignition components replaced I would seriously look into fixing that ignition map.
The ignition map seems to assume a torque peak of 4,700 RPM. Do you have a dyno chart for this engine?

Thanks for your input Ben,

I find it hard to believe the spark plug wires went bad instantly. Maybe there is some other way to test them? Like spraying water mist on them? I'd really not want to throw new parts at it until something fixes it I'll try to yank some of another engine, and see if that helps.

Back to the ignition map, I've been using flex fuel sensor to add more ignition advance across the whole map. That's only 4 degrees extra at most, so maybe it could be increased even more. If I'm honest, I have always been hesitant to touch ignition maps. When I initially created this tune, I started from a SR20DET ignition map, and globally removed a few degs of timing to make it more VET friendly. I've been using the same ignition map for 1½ years now, only adding ignition via flexign.

You are right, after looking at a stock DE map,

it cruises at around 27-31degs at 60mph. 10 degrees more than mine ;P.

Unfortunately, I've never dynoed this car. Another thing I ruled out was bad gas. I switched to 98 from e85 in preparation for winter storage. Exact same behaviour...

You could maybe wrap the plug wires in a few layers of electrical tape? I think your plan of swapping wires temporarily is probably better.
Seriously though. They are 24 years old? Replace them!
Same goes for the distributor cap and rotor...

I would take that DE map, remove 2 degrees everywhere, and use that as a basis for your new ignition map. And fill out the boost sections accordingly... Then have the flex fuel sensor add up to 4 degrees on top of that.
However you can save all that for after you fix the bucking issue. It isn't going to be your cause.

Do you have logs of this bucking? How does the RPM and CAS/trigger signal look? How about your MAP signal readings?

Is that a real VET engine, with 9.0:1 compression?
If so, shit, just use a DE ignition map with no adjustments as your base. It would be super safe given the VET's lower compression and better combustion quench characteristics.

Okay, this whole time I kept thinking your cap and rotor were 24 years old, but that is your distributor! My bad.
You're right, checking for oil makes sense for the distributor.

So the ignition coil, distributor, and spark plug wires are the items that are 24 years old? I would replace the wires, no questions asked.

Replace every ignition components with genuine OEM parts. Can't be cheap with regular maintenance items on a high performance engine. Think about it, you spent all that money on a turbo, exhaust, maybe cams, injectors, intake, to get the most fuel air mixture in the cylinder but that mixture don't get lit because of some 24 year old ignition parts.

Also go to a warmer plugs for daily driving, use the BKR6E for daily driving. Use the 7 on dyno or race day.