Hey everyone, back again to pick your brains.

383ci
LT1 Intake - Converted
AFR195cc Heads
CC306 Cam ~74* overlap
Small Disy and External Coil
LT Headers
LC1 is simulating the O2 sensor
730 $8D
6 Spd Manual

I seem to have a misfire under light load that I can't track down. It seems most audiable in the 2000-3000 rpm range. Its not bad enough that the engine stubles, it just looses its smoothness and I can hear a very sublte, resonent popping/thudding from the exhaust (rear).

Smooth throttle progression seems to clear it (especially as you get into high tps) but a quick "snap" seems to worsen the stumble.

BLMs and INT are always within ~ 5-6 of 128 and the wideband doesn't register a "lean" misfire of any kind.

It seems to get worse when the engine warms up.

I've changed the plugs even though the old ones all showed clean with a good mixture, leads, cap and rotor are only 3,000 miles old - the coil is an "unknown" though.

I do have an exhast leak on the LT Header collector at the moment but I can't see this causing too much of an issue?

Could it be too much timing? BLM and INT looks good but should I try adjusting VE?

This engine idles at around 50kpa for the most part it seems "happy" its just light throttle/load and off load conditions that are giving me the run around somewhat.

Current Bin - http://www.abm-vettes.co.uk/dan_t/383_cc306.bin

 

Coils do go out at operating temperature, as do ignition control modules...does it have an MSD box? and what size injectors do you have?

 

I'm running a standard external camaro coil. Injectors are 30lb Bosch II's

I would have thought the coil would have show under WOT as well?

 

try doing a compression test. And then a vaccuum test to see if you have a late closing valve or something. I see you pulled the plugs already.

Ive got a similar issue with mine but its happening all the time, I just dont hear it at the higher RPMs. Im leading toward a blown head gasket between two cylinders... hope not but thats the only other thing i can think of. I am going to try to put my brothers known good battery in my car and put her back on the 305 VE tables and see if its still mis-shitting.

 

Yeah I guess I should do that.

I must also note that if I richen things up it seems to go away - but the car runs noticibly rich then.

Could I just be hearing a RPM related resonence in the exhaust?

 

Just a thought,
That's about the point that cam starts to wake up, some others who run that one may be able to shed some light on that. Could try moving your BLM vs RPM boundarycurrently at 2K (and possibly the MAP boundary too) up a bit and try to tune for light throttle use there. You may have just different tuning requirements at 2200-2500, and a different requirement as the cam gets into its working range. You "may" be able to split the area to get it to smooth out in both instances. Play with some throttle following too and see if just a bit more/less air in that region could help.

Did you correct the extended table address in your xdf? (see thread on it)
I see your using it and wanted to be sure to comment on that.

 

I'm 99.9% sure I did correct the address (the graph looked WAY off before iirc?) but I will double check that.

Sorry but you lost me on the boundrys? have not come across this before.

The throttle follower is a good idea - I belive this also helps with uneven air distrubustion thought the intake?

Its so hard to describe this - when out in the car the GF doesn't hear or notice anything so it can't be a FULL missfire, more like just a weak fire maybe. Hopefully I am getting some slip fit connectors made for my headers as the fitment that comes with the standard headmans keep leaking! I know this won't be helping matters.

Thanks for you input

 

Well you could try a hotter ignition coil and use a aftermarket ignition box if you dont already have one.

Definately helps spark output in the 2000-3000 rpm range.

I have a very similar cam but on a tighter LSA with more overlap on my 89 MAF system and never noticed any slight missfires. Its was an auto car with big converter tho so i never really noticed below 2800rpm under light acceleration and such. I cant comment on 8D setups tho as i never used one

For what its worth, around 2300-2500 rpms the exhaust note really changes so it may be the cam causing a difference in exhaust tone too, however the stumbling and such is not because of exhaust note changing

You say its worse with a quicker snap of the throttle? But light smooth transition seems to eliminate the problem? That sounds like you need more acceleration enrichment. That may help alittle

Whats your plug gap set at? and make sure your wires are not arching/burnt.

 

I would like to try an aftermarket coil but I don't just have or want to throw money at this

Maybe the stuble (AE?) and the "misfire" sound are unrelated?

Plug gap is at 0.35 - I have checked the leads and all look OK.

The first time I noticed this was when I swapped from using my normal narrowband 02 to using my LC-1. I didn't trust the NB02 because the LC1 was showing a MUCH richer mixture. These lamda's are in different banks.

Is it possible that its now running lean on that left back (where the NB02 showed correct) even though the LC-1 now shows good on the right bank? Either way the LC-1 is ignorant to this missfire/stuble - which makes me think its on the left bank.

 

Could be a bank to bank problem. I believe miniram/LT1's are known for that. You can be on the safe side and add more fuel to it to make sure its not lean and causing stumbles/misfires.

Have you changed any of the o2 milivolt constants to account for the large overlap?

So are you using the LC1 to send the NB signal to the ecm?

 

I've not messed with the O2 constants but have disabled closed loop below 1200rpm to it blindly reads from my tables. That seems to work well but is a bit rich at startup.

At the moment yes the LC1 is giving me an output the the ECM as a simulated NB02 sensor