View Full Version : Which is right?

October 7th, 2009, 08:30 AM
Hey guys,

Having a problem with my TT VS Commodore.

Im getting a bad miss under boost.

The data log is picking up knock aswell as a DTC Crank angle sensor intermittant.

Now when i read the power trans light on the dash it doesnt bring that code up.

I dont know what to believe, EFILive or the ECU itself?

The miss seems to feel real harsh more like an ignition miss but i dont know what it would feel like if it was retarding. Would it surge if retarding or jerk around harshly?

It seems to be a heat related issue. Its not as bad cold as it is hot which makes me think its the sensor.

Would the knock sensors pick up an ignition miss?

October 7th, 2009, 09:39 AM

A misfire event is when the spark event doesn't properly burn/ignite the air/fuel charge leading to low combustion chamber pressure/temperature... misfire events are not picked up by knock sensor, can cause some amount of unburnt fuel in the exhaust (smells like fuel), can damage catalytic converters, foul spark plugs and O2 sensors.

A knock event is when the air/fuel charge explodes/detonates regardless of spark event leading to excessively high combustion chamber pressure/temperature... knock events are usually picked by knock sensor, can damage the motor (pistons, rods, rod/main bearings, head gasket, spark plugs), can cause high levels NOx in the exhaust (smells ~bad~).

Knock and misfire are the "opposites" or each other.

Causes of misfire:
- insufficient spark,
- insufficient fuel,
- gross overfueling (flooding),
- loss of compression,
- other;
you find which one(s) of those, and work back to find the cause.

If you have boost and your plug gap is too big, then the spark can be "blown" out...

It may be good to check your ignition components, check that you can get good spark to all plugs.

OBD-I ECM's don't necessarily trigger DTC's for misfires.

The log data is what the ECM is saying it saw... if it says it saw knock (and it is real knock and not due to mechanical noise), then you may have a lean condition which is causing both knocking and misfiring as separate events... i.e. in any cycle it either knocks or misfires, but not both together.

If you feel it is a sensor problem then check your sensors by logging them and seeing if the values they produce are sensible.

October 7th, 2009, 10:30 AM
Ok thanks mate.

The problem has progressively got worse over time so im guessing its sensory.

I suppose i can disconnect the knock sensors for 1 short test run and see what happens.

Im going to replace all my coils, DFI module and crank angle sensor and see what happens. Got to start the process of elimination unfortunately.

October 7th, 2009, 10:36 AM
Also check your plugs and leads...

see if you can eyeball spark using a spark tester (looks like a spark plug but has a earth clamp) --> unplug the injectors whenyou do this...

and check all the usual stuff like fuel filter, etc.

October 7th, 2009, 10:55 AM
Plugs and leads are new.

I have been mucking around with coils lately. I had those MSD coil packs which are crap. Car felt doughy and miss was worse, but when i put the factory coils back on it was also more responsive the miss would be better but still there.

Ive lowered the timing down to around 10 deg and its still spluttering. I cant see how 10deg is making it knock.

October 7th, 2009, 11:53 AM
I cant see how 10deg is making it knock.You mean misfire...

If ignition is good (have you eyeballed spark...?) then there may be a fuel problem or a compression problem (including coolant leaking into CC's).

October 7th, 2009, 12:14 PM
No its knock. EFILive is showing knock and retarding the timing.

Just went and got a new DFI module and coils. I'll throw them on. Then i'll try a new crank sensor.

I do believe the knock sensors would pickup a BIG ignition miss as they can pickup any kind of vibration.

October 7th, 2009, 03:43 PM
Maybe they will...

October 7th, 2009, 07:45 PM
So this arvo i put the new DFI and coils on. Made a slight difference but still not good enough.

Disconnected the Knock sensors and its alot better but still has the miss.

Definately an ignition miss somewhere and the knock sensors are picking it up.

Im currently running 1.3mm plug gap (Stock is 1.5) but i might close it down to say 0.9mm and see. Alot of people are running 1.3 even 1.5mm gap with the same boost im running.

October 8th, 2009, 04:34 AM
The GM revised the plug gap from 0.060"(1.52mm) to 0.040"(1.02mm)... for stock non-boost application.

For boost, many people say to gap to 0.035"(0.89mm) or even 0.030"(0.76mm)...

Sorry, I'm old-school, I do know Metric quite well, but plug gaps are in units of thou's...;)...they don't seem right in mm...:grin:

Edit: I corrected this.

October 8th, 2009, 08:01 AM
Yeah im definately going to have to close them up.

October 19th, 2009, 04:11 PM
Closed the plug gaps down to 0.8mm allgood! No more missing. Cheers guys.

October 19th, 2009, 07:20 PM
Cool good job...:cheers:...thanks for posting back.

October 19th, 2009, 09:41 PM
Nothing worse than trying to troubleshoot on the internet, you find the perfect thread and no-one posts their fix lol.

January 25th, 2011, 06:46 AM
Amen to that mate.



January 26th, 2011, 05:32 PM
yeah I run 28" thou on mine boosted.

January 27th, 2011, 12:42 AM
yeah I run 28" thou on mine boosted. wow 0.7mm is really closing them up. But if it works for you then its all good. :)



January 27th, 2011, 07:37 AM
Take a look at the forced induction forum on LS1 Tech.
most boosted cars are running 25-28" thou to prevent spark blowout under boost.
The higher the boost you run the tighter the gap you'll need.

I'm running 17-18 psi boost