Missfire between 2000 and 3000 RPM

[pormgb]

I've been chasing a problem for months related to a suspected missfire/shudder between 2000 and 3000 RPM, I don't have any codes but I know its fueling related, this problem disappears after 3000rpm. I've also noticed that when the car is cold the problem is not noticeable, by the time I finish my drive and the car is warm I can hear a difference in exhaust tone and can feel the vibration. So far I've done multiple Calc Vets (VET-VEN are very close)and recently done a RAFIG. I have two wide bands installed that show around 14.5 when driving, the car does run a bit rich in open loop but once in closed loop (warm) AF's look OK, this may be due to my Cam overlap and am working on this issue.

Based on my observations this problem has to be heat related, I'm waiting for a new set of plug wires but am trying to figure out if there are other temperature related fueling parameters the need to be adjusted.


Parameters adjusted so far;

IFR
Calc VET
RAFIG (desired airflow)
Spark Tables


Motor specs;
LS6 Block
Camshaft 226/229 .575/.578 116 LSA
LS6 Intake
85mm Professional Products Throttle Body
Hooker Headers
No Cats
Denso Iridium plugs gaped at 40

[pormgb]

12009Done some AF logging today and here are the results, the car does seem to be rich at high RPM but I don't see any thing out of the ordinary around 3K RPM.

[joecar]

Looks ok at in rpm range where you said there was misfire...

[joecar]

Are you A4 or M6...?

Is coolant level same every day...?

Are plugs/wires ok...?

[pormgb]

Are you A4 or M6...?

Is coolant level same every day...?

Are plugs/wires ok...?

The motor is hooked up to a Porsche 968 6 speed manual.

Coolant level hasn't changed since my last fill (2-3 months ago)

Plugs look good, I have brand new wires but I'm expecting a better quality set this week.

The motor's performance is quite flat until I reach 3k, then its a completely different motor, I didn't think my cam was that large. The reason why I mention this is the vibration stops when the motor hits its that 3k spot, I can also hear the exhaust tone sounding like the motors plugged between 2k and 3K.

[pormgb]

Are you A4 or M6...?

Is coolant level same every day...?

Are plugs/wires ok...?

Got new plug wires but the problem is still there, I'm going to take a look at my exhaust to see if I'm experiencing some sort of resonance.

I'm still having a rich cold start problem, when the car is started form cold it runs around 12.5 AFR until it goes into closed look. I looked around and even modified B3605 but still have the issue.

Are there any logs that can help me understand whats causing this rich cold start situation? is cold start AFR only controlled by B3605?

[Ira]

While this almost certainly has nothing to do with your problem, I point it out as a reminder that things sometimes happen for reasons that seem to have nothing to do with the problem. My Triumph at one time had a mis-fire between 3000 and 3500. After much gnashing of teeth and trying everything that made any sense, it turned out that the gap between the pickup and reluctor in the distributor was set very wide. The pickup was flexible plastic and in that RPM band it vibrated enough to cause the gap to get big enough to cause a miss. Closed the gap back to where it belonged and all was well again. Had no reason to look there as it was a brand new warranty replacement distributor set wrong from the factory.

Ira

[pormgb]

While this almost certainly has nothing to do with your problem, I point it out as a reminder that things sometimes happen for reasons that seem to have nothing to do with the problem. My Triumph at one time had a mis-fire between 3000 and 3500. After much gnashing of teeth and trying everything that made any sense, it turned out that the gap between the pickup and reluctor in the distributor was set very wide. The pickup was flexible plastic and in that RPM band it vibrated enough to cause the gap to get big enough to cause a miss. Closed the gap back to where it belonged and all was well again. Had no reason to look there as it was a brand new warranty replacement distributor set wrong from the factory.

Ira

Yep, at this point ill look at anything, it does seem to be tune related because as I change my tune it gets better and sometime worst, its also more pronounced as I come of the gas and past 3000rpm.

[joecar]

I think maybe you could go a little richer between 2000 and 3000 (or even 4000 rpm)... see if this makes any difference.

[pormgb]

I think maybe you could go a little richer between 2000 and 3000 (or even 4000 rpm)... see if this makes any difference.

OK, ill increase the VE table between 2k and 4k and see if that makes a difference.

Thanks!!

[pormgb]

I think maybe you could go a little richer between 2000 and 3000 (or even 4000 rpm)... see if this makes any difference.

Joe,

what about my rich cold start, could this be B3905 being incorectly set? i've tried editing it but nothing changes.