99 Camaro 408 - breaking up 2500 to 3000 rpm range

[Chevy406]

I'm working with a guy on the "tune" for his new build. He recently installed a 408 with a 24x/25x cam, LS3 intake, big tube headers, 6-speed manual car. He also bought 50 lb/hr FAST injectors

So we worked through a full VE and MAF tune and have pretty decent manners - all except for pulls through the 2,500 to 3,000 rpm range where it always spits and pops. Once above 3,000 it will pull cleanly (and strong) to 6,600.

Here is what we have experimented with, in an effort to eliminate the problem area:

• Using VCM Controls I manually added and subtracted timing - no change
• Using VCM Controls I manually added and subtracted AFR Control - no change
• Adjusted PE Throttle higher and lower - no change
• Disabled closed loop and all fuel trims - no change
• Adjusted transient "Fuel Boiling Time" and "Fuel to Wall Impact Factor" both up and down - want to say it may have affected some but the problem was still there
• Tried a crank sensor re-learn
• Used injector data for 50 lb/hr Deatsch Werks injectors - no change
• The owner replaced spark plugs and wires - no change

With stock misfire table settings I see lots of random misfire at idle and cruise but no smoking gun. We are fairly stumped. The car idles and cruises well and WOT feels crisp and strong. I personally don't fell like the problem area is a fuel or ignition timing issue. So I've about decided it is a fuel injector transition issue. The FAST injectors are supposedly made by Bosch but I have never found decent data for them. Unless someone comes up with a new great idea to try, my next step is to pick up some stock LS7 or LS9 injectors and swap them out.

I've attached the current tune, but I don't have a scan right now. There really isn't much to see in the scan though. Ignition timing is steady in the upper 20s and fuel trims <5 or 6%.. I know it's very difficult to remote trouble shoot, but figured I would at least toss this out and see what ya'll can come up with.

Clyde

[POWERZONE]

Probably would be best to see the log so we can see what else is going on when this problem occurs.

[Chevy406]

I got back together with Jay this evening and we swapped the FAST 50 lb/hr injectors for a set of stock LS3 42 lb/hr injectors. I used flow values that DSteck provided in another thread for using LS3 injectors with an LS1 OS. Car has the exact same issue with breaking up / popping as it pulls from ~2,500 to 3,000 rpm. So far nothing we have done has made a noticeable improvement for this condition. I at least saved a short data log this time. The two pulls at :38 to :45 seconds in the scan had some heavy popping. The owner's description of it feeling like an over-carb'd SBC seems accurate. It feels like it gets too much fuel in that range but the fuel trims and a WB sensor don't indicate excess rich condition. However it's a short duration and maybe there isn't enough time to see a quick transient issue?

I'm going to reach out to a couple local guys for ideas. Then the next step would be trying a different PCM just to try something...street scan 1.hpl

[Chevy406]

I'm not seeing my scan attachment in the previous post so I'll attach it again

[nkautz]

Looks like you're going lean in the 75 - 90 kpa range around 2600-2800rpm. I think its your VE or your injector settings. Would be nice if your wideband was working. I don't like your offset, short pulse adder, or pulsewidth tables. If it was me , first thing I'd do is put all those back to stock and retune.

[Johnny408]

I know this is an Old thread but did you ever get that 408 sorted out I built the same setup and having the exact same problem in the exact same areas I'm tried the same things LOL I'm stumped

[lukearmstrong1990]

I would have suggested few thing with a cam that big in a manual car.

1. I personally would tune 400+cubes 240/250+ cam in a manual in full open loop. I wouldn't trust the o2 sensors, especially below 3000rpm. that cam would have heaps of overlap and the o2 data would be false and inconsistent. I would suggest trying full open loop tune. I would also disable LTFT

2. despite his relatively low TPS, its beyond 80KPA MAP. that combo would be appreciating a near stoich condition at that RPM. I would be lowering the PE enable TPS to have it enter power enrichment at that MAP. the o2 mV and STFT both indicate lean. I would be trying to use PE to richen that point up to 12.8-13.0 AFR and see how it behaves. I would be trying from 11.5-13.0AFR to monitor the behaviour.

3. its likely there is inlet manifold reverb in that RPM range. I wouldn't be surprised if it takes a rich condition with excessive timing to make it pull through cleanly. that cam should be used with a 4000rpm + converter. it will have poor drivability in a manual car. if the LSA is tight it will be worse.

[Chevy406]

Thanks for the detailed response Luke. It's always so helpful to hear input from highly experienced engine builders & tuners. Especially love hearing from Australia friends.

Johnny, you should try what Luke has recommended. I lost touch with the Camaro that started this thread. Looking back on it I never got paid for the 3 or 4 evenings I spent on his car, but that's part of learning I suppose. Wow, my last post was 4 years ago today. From what I remember he still had a small window of stumble the last time we street drove it.

[thebeewantsboost]

I have the EXACT same problem! i just had the guy get good injectors with data, but it doesn't look like thats the answer. did you have any luck Johnny408? did either of you adjust the injection timing?

[anniversaryss]

I have a similar issue where my STFT would jump on moderate throttle between 2 and 3k rpm. It almost sounds like a misfire but seems to act normal on harder throttle and upper rpms. Smaller motor, smaller cam in sig.

[Jgarretts86]

Same problem going on with my build. Ls3 headed 403 with 239/254 .624/.595 114+3 cam. 2k to 3k its cutting up bad, you can feel it surging or missing whatever you want to call it, and sometimes I get a good pop or two. After 3k it cleans up perfect and pulls nice and clean. The car is speed density open loop tuned with a wideband. The wideband is not indicating a lean or rich condition at all when this is happening in that 2-3k rpm range... I'm wondering if injection timing is causing it. Also might try and lower the PE throttle tip in and see if getting it into PE sooner will help.

[thebeewantsboost]

Please let me know what you try and figure out. I plan to log timing advance and retard. Disable burst kr. Enable pe earlier. Use injector's with good data. Max out my PE enrichment rate. injector timing.

[Jgarretts86]

Problem solved! Car was wanting more fuel in that particular rpm range. I basically just added 15% to my VE table from 1600 to 3000rpm and it cleared right up. It likes to be in the 12.7 afr range at that rpm and map. I’m going to start leaning it back down as much as I can before the misfire/hesitation comes back. 14.68 stoich just wasn’t cutting it in that rpm and load range for my particular setup. Hope this helps someone else. I was for sure it was a timing issue, never even considered fueling since I had it dialed in at 14.68 afr. I’ve read something about intake reverberation with a large overlap cam. Maybe that’s what I have going on. Anyways the motor is way happier and feels way stronger in that rpm range now.

[anniversaryss]

Problem solved! Car was wanting more fuel in that particular rpm range. I basically just added 15% to my VE table from 1600 to 3000rpm and it cleared right up. It likes to be in the 12.7 afr range at that rpm and map. I’m going to start leaning it back down as much as I can before the misfire/hesitation comes back. 14.68 stoich just wasn’t cutting it in that rpm and load range for my particular setup. Hope this helps someone else. I was for sure it was a timing issue, never even considered fueling since I had it dialed in at 14.68 afr. I’ve read something about intake reverberation with a large overlap cam. Maybe that’s what I have going on. Anyways the motor is way happier and feels way stronger in that rpm range now.

Thanks for the updats this is what im going to try. Right now im dialing in the maf so ill add some fueling to that area.

[anniversaryss]

Thebeewantsboost
What are you planning on doing. Are you running full open ooop?
Are you gonna richen it up and deal with the error vs stoich?
Are you gonna bring PE down that low in the rpm range to even out the error?

If bringing the PE down that low whats the point of running oxygen sensors at all?

Thanks! Lol

[anniversaryss]

Actually come to think of it. If your running open loop you can modify your tables to match your command and bring the error in line? Also using open loop fuel trims?

[thebeewantsboost]

sorry, i haven't replied.i should be getting the car back tomorrow or the next day. Jgarretts86 i'm so glad you found a resolution. i'm sure that will fix mine, i believe its fueling but was confused about how when it was dialed in to stoichometric. but it wants what it wants... I'm not sure just yet how i'm going to go about fixing it. theres basically two options. open loop tune. which im not crazy about doing or enable pe lower.
anniversaryss - the point to running oxygen sensors to me, would be to help keep the idle stable. keep the vehicle running good in changing weather conditions and locations. the customer is about to move to virginia. maybe its not necessary but i've never done an open loop tune.

[anniversaryss]

sorry, i haven't replied.i should be getting the car back tomorrow or the next day. Jgarretts86 i'm so glad you found a resolution. i'm sure that will fix mine, i believe its fueling but was confused about how when it was dialed in to stoichometric. but it wants what it wants... I'm not sure just yet how i'm going to go about fixing it. theres basically two options. open loop tune. which im not crazy about doing or enable pe lower.
anniversaryss - the point to running oxygen sensors to me, would be to help keep the idle stable. keep the vehicle running good in changing weather conditions and locations. the customer is about to move to virginia. maybe its not necessary but i've never done an open loop tune.

Keep us updated. I plan on testing this on my car in a few weeks
I hear you on being confused. Car is tuned to stoich but still not doing well, why? Eveyone says tune to stoich. But as you said it wants what it wants.

[thebeewantsboost]

So my problem is resolved. I believe mine was just caused by injectors with bad data. but i can't say definitively because i also changed the PE enable and ramp rate. However without PE enabling i could rev through 3000 smoothly under moderate load i also leaned it out from 11.0 to 12.5. idk what the original tuner was thinking on that.

[anniversaryss]

Running high 12s low 13s in that rpm range made that rpm range feel better but seems my real issue is 4500 to red line