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Thread: SBC 24x tune Backfire and Engine Stall at WOT from cruise

  1. #1
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    SBC 24x tune Backfire and Engine Stall at WOT from cruise

    Hi All,

    I'm having a strange issue that I can't seem to find anyone who has had the same issues and was wondering if anyone can point me in the direction of possible causes.

    When I try to go 100% WOT from maybe 30-50mph the engine goes to spool up and then backfires like crazy and the engine stalls. The backfiring continues for a bit, but no combination of throttle positions can bring it back to normal operation, I need to put the car in neutral while rolling and then turn the key again to get it running again.
    The transmission downshifts from 3rd/4th to 2nd when all this occurs as it should and the revs start to shoot up but suddenly ends with the backfiring.
    Cruising and mid throttle acceleration is all good, idle is fine but a little high (950-1100rpm)

    I have a couple of ideas of potential causes but was wondering if anyone can either help me out with the cause, or at least understand what would cause the backfiring and stalling.

    1st idea is that the engine is just getting completely flooded with fuel, enough to blow out the spark and stall the motor. But I don't think the tune would be out that much, and I can rev the engine to those same ranges when staying in a particular gear and the issue doesn't occur.

    2nd idea is valve springs, I know the ones I have are a bit soft for my motor, but I would suspect that would cause some valve float up high, but haven't ever heard of someone experiencing these symptoms from it. They're only slightly out of recommended specs anyway, but had been shimmed to get the pressures close.

    3rd idea is in a lazy effort, I hardwired my neutral/park switch to ground to bypass my broken safety switch. Is there any chance this is telling the PCM it's in neutral and kicking in some limiter that would shut off the engine indefinitely?

    4th is that possibly the PE or transient fueling is not set up for SBC and is still using the LS1 settings?

    5th is that the motor is pulling that much fuel that the fuel pump can't keep up and it starves it.

    Any ideas?

    I've attached the tune. A log is also included, but I hadn't induced the issue on this run.
    Attached Files Attached Files

  2. #2
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    1. I dont see that youre logging a wideband. Are you? If not, get one and log it.

    2. All short term and long term fuel trims are showing ZERO activity. Looks like youre running open loop.

    Install some O2s and a wideband. Enable closed loop. Scan. That should point you in the right direction. Of if you dont want to run closed loop, you definitely need to log a wideband...
    Daily Driver= 1998 Pontiac Grand Prix GTP Coupe (3.4 pulley, SD Headers, SLP catback, FWI, 180 t-stat, hotwired FP, Autolite 103's, poly dogbone bushings, solid rubber engine and trans mounts, self tuned with HP Tuners)
    Weekend Cruiser= 2009 Pontiac G8 GT (Vararam, TSP LS3 N/A Stage 1, OBX, CTS-V converter, MagnaFlows w/ J-Pipes, 160 t-stat, self tuned with HP Tuners)
    Project Car= 1991 Chevrolet Camaro RS (On3 7875, LM7 build on hold)
    Truck= 1995 K1500 ECSB 5.0 5-speed (Dusty...)

  3. #3
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    Hey Jaydubb71,
    Yes I'm logging Wideband through the EGR sensor method.
    So (EGR volts / 0.5) +10 = AFR

    I think the tune was set up to not use closed loop, but left bank o2 is not set up properly. Because I'm using the wideband in place of the stock o2, I wired the 1v narrowband output from the wideband o2 into the PCM, but forgot to attach the reference low(gnd) back from wideband to PCM.

    I'll fix it up today and take a log and post it.

    It would be funny if that was the reason for the pop fizzle bang because the car can't read the narrow o2 when it gets loaded with fuel.

  4. #4
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    Quote Originally Posted by evilstuie View Post
    Hey Jaydubb71,
    Yes I'm logging Wideband through the EGR sensor method.
    So (EGR volts / 0.5) +10 = AFR

    I think the tune was set up to not use closed loop, but left bank o2 is not set up properly. Because I'm using the wideband in place of the stock o2, I wired the 1v narrowband output from the wideband o2 into the PCM, but forgot to attach the reference low(gnd) back from wideband to PCM.

    I'll fix it up today and take a log and post it.

    It would be funny if that was the reason for the pop fizzle bang because the car can't read the narrow o2 when it gets loaded with fuel.
    Youre definitely having some fuel delivery or tune issues. Your EGR voltage tanks as you increase throttle and your MAP climbs towards 100 KPA. Lean and high KPA = lean pop.

    Here are my suggestions...

    Set your MAF Frequency Fail High to 1. Set your Throttle PE tables to 30 on hot and cold sides. Set your PE Enable MAP to 30 KPA. PE Wont enable until 30% throttle. You can raise the enable TPS once you get the car dialed in. Set youre PE EQ Ratio to 1.142. They are kinda rich now. Then scan/log.

    Question. If youre running O2 sensors, why run open loop?
    Daily Driver= 1998 Pontiac Grand Prix GTP Coupe (3.4 pulley, SD Headers, SLP catback, FWI, 180 t-stat, hotwired FP, Autolite 103's, poly dogbone bushings, solid rubber engine and trans mounts, self tuned with HP Tuners)
    Weekend Cruiser= 2009 Pontiac G8 GT (Vararam, TSP LS3 N/A Stage 1, OBX, CTS-V converter, MagnaFlows w/ J-Pipes, 160 t-stat, self tuned with HP Tuners)
    Project Car= 1991 Chevrolet Camaro RS (On3 7875, LM7 build on hold)
    Truck= 1995 K1500 ECSB 5.0 5-speed (Dusty...)

  5. #5
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    So I wired up the narrowband but I don't think it worked correctly as now it just registers 3mv and only seems to spike when the wideband max's out at 4.9v then it blips.

    On the way to work this morning I logged it and everything was going ok... then i decided to shift it into 2nd and drive at part throttle to warm it up and also get some consistent readings at higher rpms.

    That's when it all went to shit, the car was spluttering away and sluggish as hell for the latter part of the trip.

    Looking at the logs, the ST and LT fuel trims kicked in at that point, so I'm assuming it was a massive imbalance between left and right banks due to the o2 sensor not working.

    To fix the issue, I'm just going to get a 2nd o2 bung welded in on the left bank today and run the narrowband independent of the wideband, seems easier than trying to figure out what ground to use for the narrow band low reference signal to the PCM.

    I've attached the logs to show the craziness.

    Also, just to add, it's a MAFless setup, and the o2's were in there from the start, I figured I'd get better milage once it was all diealed in, Ben who did the base tune for me said it's better to run without them, so the tune might not be setup completely for them.
    Attached Files Attached Files

  6. #6
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    Quote Originally Posted by evilstuie View Post
    That's when it all went to shit, the car was spluttering away and sluggish as hell for the latter part of the trip.
    Yeah, you got something funky going on with your wiring.

    See if youre getting P0101, P0102 and P0103 DTC's in the scanner? If not, change your MAF Frequency Fail High to 1 and try again...
    Daily Driver= 1998 Pontiac Grand Prix GTP Coupe (3.4 pulley, SD Headers, SLP catback, FWI, 180 t-stat, hotwired FP, Autolite 103's, poly dogbone bushings, solid rubber engine and trans mounts, self tuned with HP Tuners)
    Weekend Cruiser= 2009 Pontiac G8 GT (Vararam, TSP LS3 N/A Stage 1, OBX, CTS-V converter, MagnaFlows w/ J-Pipes, 160 t-stat, self tuned with HP Tuners)
    Project Car= 1991 Chevrolet Camaro RS (On3 7875, LM7 build on hold)
    Truck= 1995 K1500 ECSB 5.0 5-speed (Dusty...)

  7. #7
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    Yeah I have the 3 DTCs so the MAF delete is working as intended.
    We'll put the 2nd o2 bung in today and start from there.

  8. #8
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    Could we get some info on the intake, heads and cam this engine has. I've had Similar issues with a 5.7 vortec after an Intake swap. Edelbrock super victor EFI part # 29135. I was having eccessive lean tip in and backfire upon stabbing the throttle. Considerably worse lean tip in before complete engine warmup. Mine issue was a combination of transients and just plain old inexperience. I tend to agree with checking the wiring for scuffs breaks and bare spots. confirming the wiring is a non issue is imparitive. I can't check the tune at the moment. But have been checking on the thread for a couple days. I'm interested in the engine combo because I'm partial to the old sbc dinosaurs.
    Last edited by Oleblu; 01-17-2019 at 08:12 PM.
    97 ext cab short bed silverado 5.7 411 swap, edelbrock 29135 intake, factory unported 906 heads milled .035 thou. factory press in rocker studs & rockers, comp 787 retainers & stock locks, ls6 springs. Comp cams 08-503-8 4l80e swap & Fti 3800 hard hit stall.

    86 SWB chevy crate vortec engine from scoggin dickey Chevrolet, mild unknown cam The wife's truck

  9. #9
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    Quote Originally Posted by Oleblu View Post
    Could we get some info on the intake, heads and cam this engine has. I've had Similar issues with a 5.7 vortec after an Intake swap. Edelbrock super victor EFI part # 29135. I was having eccessive lean tip in and backfire upon stabbing the throttle. Considerably worse lean tip in before complete engine warmup. Mine issue was a combination of transients and just plain old inexperience. I tend to agree with checking the wiring for scuffs breaks and bare spots. confirming the wiring is a non issue is imparitive. I can't check the tune at the moment. But have been checking on the thread for a couple days. I'm interested in the engine combo because I'm partial to the old sbc dinosaurs.

    Ok, so complete engine setup is:
    400SBC, stock crank and rods, hypereutectic pistons 8cc relief
    AFR 220cc ported and polished heads
    XFI280 Hydraulic Roller cam with 1.6 roller rockers
    Holley Stealth Ram EFI intake
    36lb ford evo injectors
    LS1 coils, vortec dizzy 0411PCM
    4L60E transmission with stock converter, and 2.77 open end diff
    __________________________________________________ _______

    So after putting the new o2 bung in and going through the wiring and some other checks I found a few things:

    1. The O2 heater wire was not getting any power. After almost a day of searching I found the wire and attached it to an ignition source.
    2. Both narrowband o2 sensors had crapped out either from constant sooting/excess fuel, or because they were cheapies. When heated up with a butane torch they took a long time to reach 1volt, and then proceeded to go to 1.2 volts and then when the heat was removed, stayed at 1.2 volts for at least 20 seconds. So I replaced both with ones that passed the butane test.

    3. Checked HPTuner scanner and low and behold both o2 banks are now registering and logging with fluctuations on both sides.

    Now the fun part, as far as I'm aware my tune should be ignoring the o2 sensors anyway, or at least until the engine warms up yes?

    Not sure if I've stuffed my wideband or not (I'm assuming not because there's other symptoms) but it's now showing around 16-17:1 AFR at cruise/ 10% throttle, and the car idle and sound is rougher with a bit of a miss almost, it sounds similar to when the engine has run lean when i was having fuel pump issues before it would cut out.

    Another strange occurrence is that when i disable the injectors one by one, there's an instant drop in rpm and noticable miss, except when i do inj#3 and it either doesn't change, or takes about 10 seconds to register.
    Is this a sign of a leaking/clogged injector or something else?

    I'll post the log file tonight.
    Last edited by evilstuie; 01-20-2019 at 03:34 PM.

  10. #10
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    I would tune idle and cruise in closed loop. Cam overlap will throw the wideband off at idle, depending on how far down the exhaust you have it installed. Enable closed loop and use your STFT to get idle and cruise. Disable LTFT while you're adjusting STFT.
    Daily Driver= 1998 Pontiac Grand Prix GTP Coupe (3.4 pulley, SD Headers, SLP catback, FWI, 180 t-stat, hotwired FP, Autolite 103's, poly dogbone bushings, solid rubber engine and trans mounts, self tuned with HP Tuners)
    Weekend Cruiser= 2009 Pontiac G8 GT (Vararam, TSP LS3 N/A Stage 1, OBX, CTS-V converter, MagnaFlows w/ J-Pipes, 160 t-stat, self tuned with HP Tuners)
    Project Car= 1991 Chevrolet Camaro RS (On3 7875, LM7 build on hold)
    Truck= 1995 K1500 ECSB 5.0 5-speed (Dusty...)

  11. #11
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    Quote Originally Posted by jaydubb71 View Post
    I would tune idle and cruise in closed loop. Cam overlap will throw the wideband off at idle, depending on how far down the exhaust you have it installed. Enable closed loop and use your STFT to get idle and cruise. Disable LTFT while you're adjusting STFT.
    Ok, will do.

    Just to get an idea of the final product, once I've adjusted everything, what sort of fluctuation should I see in the wideband reading overall?
    Like if I dial it in to get 14.27 AFR and get everything sorted, will it still fluctuate and richen up when PE kicks in?

  12. #12
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    Yes. As long as you input PE equivalent of 12.5 (in the area) and reach the conditions to enable PE, the computer will go into open loop. Then you will need to adjust using your wideband.
    Daily Driver= 1998 Pontiac Grand Prix GTP Coupe (3.4 pulley, SD Headers, SLP catback, FWI, 180 t-stat, hotwired FP, Autolite 103's, poly dogbone bushings, solid rubber engine and trans mounts, self tuned with HP Tuners)
    Weekend Cruiser= 2009 Pontiac G8 GT (Vararam, TSP LS3 N/A Stage 1, OBX, CTS-V converter, MagnaFlows w/ J-Pipes, 160 t-stat, self tuned with HP Tuners)
    Project Car= 1991 Chevrolet Camaro RS (On3 7875, LM7 build on hold)
    Truck= 1995 K1500 ECSB 5.0 5-speed (Dusty...)

  13. #13
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    Here's the log file from the short run i did.
    Attached Files Attached Files

  14. #14
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    Ok here's this morning's run. The wideband was showing it running lean, around 16:1 for the most part after warm up.
    At around 20:12 in the log was when I decided to try WOT and the car cuts out almost immediately after kicking back through the gears from 4th to 1st.

    The o2 sensors seem to peak out at 900mv indicating its running rich, and the EGR voltage which is the Wideband voltage goes low at 1volt backing that up.
    So is the car dumping so much fuel in that it's cutting out the car? Is that a thing?


    Any help someone can offer would be greatly appreciated.

    It appears my car has gone from running rich to lean, but then still dumping heaps of fuel in at WOT.

    Also the AC pressure is the fuel pressure and you can see it drops out, but I'm not sure if thats because the car has stalled and the PCM is no longer sending signal for the fuel pump to run, or if its just running out of fuel.

    Pump is a LS1 in tank from a 2001 firebird.
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    Any ideas guys?

    I'm gonna put a walboro 255 in this weekend and see if there's any difference.

  16. #16
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    Yep, it was the pump.
    The walbro i went to put in was actually a 450, and had to make some mods to the assembly but it all worked.
    Fuel pressure is now constant at WOT and readjusted the system and tune to run at 58psi.
    Still need to add some fuel here and there, but WOT now pulls hard and changes into next gear without issues.
    It's a little lagged but that's because of the stock convertor and 2.77 gears I'm guessing.

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    Quote Originally Posted by Ben Charles View Post
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  19. #19
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    So I pulled the motor to fix an oil leak and I'd been having some issues still with the engine cutting out after WOT and found the issue. 24 tooth reluctor had slid off the keyway and gouged its way about 5 degrees into the reluctor. So that I believe was the root cause. Replaced the keyways and had a mate repair the reluctor wheel by welding in the spot and then regrind and put everything back together.
    Will test to confirm after the transmission is rebuilt and put every thing back together.