LS6 Misfire

[pormgb]

I have a LS6 in a Porsche 944 but I have been experiencing a vibration between 2000 and 3000 rpm, after 3000 rpm there is no sigh of the vibration. I have changed plugs, coils and wires, the motor also has a set of fresh 243 heads.

Initially, I thought I had a balancing problem but I think the vibration is a misfire, its more pronounced when decelerating and the vibration is in sync with a muffled sound from the exhaust. I have been doing some logging and have found positive fuel trims and misfires around that rpm range so I ran a CALC VET and the vibration significantly reduced. The vibration is still there so I am trying to determine if I am missing something in my tune, I have attached some log files with my current tune, looking for any advice.

Specs,

- LS6 Motor
- 226, 229 - 575,585 116 LSA Cam
- 30lb SVO Injectors
- Professional Products 85mm TB

[joecar]

I put you on my queue of things to look at...;

make sure all the ground connections are intact (including the ground point at the rear of the LHS cylinder head);

make sure there are no aireaks between MAF and heads, and between heads and O2 sensors.

[pormgb]

Thanks,

I do have all the correct ground straps, I will check intake manifold for leaks

[Highlander]

Clutch vibration....

[pormgb]

Clutch vibration....

It may be but the vibration is very light when cold the gets heavier as the car warms up, as I tune the car the vibration gets less and I am seeing misfires.

[joecar]

VE table is incorrect.

PE table is lean... go Edit->Properties and set Fueling units to EQR, restart tunetool and redo B3618 PE table (set it to 1.175 EQR).

[joecar]

Start with those, and then there's a few other things to fix.

[Highlander]

if the missfire is off PE there is nothing he can do to PE that fixes it... If its not a balance issue (it has happened to me like that before) then he should start checking out spark plugs and wires.

LEAN or Rich will not cause missfire and vibration. Loss of spark on one cylinder will.

[joecar]

Agreed, he should first test for spark at each cylinder.

His tune is setup lean, so it needs work anyway.

[joecar]

pormgb,

Take the stock LS6 VE table and add 8% from 3200 rpm to 5400 rpm, and start tuning from there.

Post pic of MAF plumbing.

I'm still looking thru rest of tune.