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Double trace RPM signal and unexpected high idle.

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  • Double trace RPM signal and unexpected high idle.

    Hoping that someone with some tuning experience would be willing to take a look at two datalogs I recorded on the way home yesterday. Been having trouble with a couple "bugs" that I can't find and they are throwing off the consistency as I try to work on the tune of this Sniper unit. SBC ZZ4 crate engine with MSD box and MSD Billet Distributor. Sniper EFI software only controlling the fuel. Distributor has mechanical advance with springs & weights.
    The first datalog show the injector pulse width going crazy at one point. The second datalog (I hit Stop Datalogging and Start Datalogging to create another consecutive file) actually shows two RPM traces. During the second half of the second datalog the RPM would not return to idle. It stayed at 1500 and I have no idea why. Any feedback would be greatly appreciated.​
    Attached Files

  • #2
    The first datalog shows somewhat normal operating with the exception of your IAC signal. At warm idle the IAC is about 35% which indicates that your idle screw needs to be turned in 1/2 or so. To avoid having the TPS go above 2% and invoke the IAC Hold Position, turn the screw in 1 turn, start the engine (warm) and turn out the idle screw until you get the IAC Position to 2 to 5%. This has affects on idle performance overall and could be related to your high idle in the second datalog EMI/RFI is the more likely culprit.

    The second datalog shows a significant ECU reset (maybe 2). This is almost always due to EMI/RFI. The key is first, negative time values in the datalog, and second, the appearance of what appears to be a double trace. There are actually two separate RPM traces with a different starting point. It records the first trace, experiences a reset which will usually go to an even larger negative time value, and then continues the trace. It's hard to read these exactly because the datalog viewer doesn't like negative time values but visually you can see this happen.

    It's possible that you could have had a reset during the start. For example, if you have some starter drag that pulls the voltage too low, the ECU may have reset at that time. When the engine is fully warm, turn on the ignition, start a datalog, and then start the engine. This will show how much the voltage drops during the start. Note that in some cases, EMI/RFI is more likely when the engine bay is warm/hot which is probably related to how efficiently EMI/RFI travels and how much humidity is present at temperature.

    If there's not a big voltage drop/reset, then you'll need to start troubleshooting EMI/RFI problems. Usually this can be traced to Sniper wiring running near/over other engine wiring such as the alternator, but more often the ignition wires and coil. There's a lot of information on EMI/RFI if you search on this forum. It would be helpful if you could take a few pictures of your engine bay so we can see your basic wiring positions.

    Although you're not controlling timing, rotor phasing can still be a source of EMI/RFI. Inspect your rotor and cap to see if the system is firing on the edge or at the center of the cap posts. Overall poor rotor and cap condition can also lead to arcing that may interfere with the Sniper.


    • #3 (Holley Sniper EFI RFI/EMI Issues)​
      May God's grace bless you in the Lord Jesus Christ.
      '92 Ford Mustang GT: 385"/6.3L SBF, Dart SHP 8.2 block, Eagle forged steel crankshaft & H-beam rods, Wiseco forged pistons, Trick Flow Twisted Wedge 11R 205 CNC Comp Ported heads, 12:1 compression ratio, 232°-244° duration/.623" lift/114° LSA H/R camshaft, TFS R-Series FTI Comp Ported intake, BBK 80mm throttle body, Holley Dominator MPFI & DIS, Holley 36-1 crank trigger, MSD 1x cam sync, PA PMGR starter, PA 200A 3G alternator, Optima 34/78 Red battery, 100HP progressive dry direct-port NOS, R134a A/C, Spal Dual 12" HP 3168 CFM fans, Frostbite 3-core aluminum radiator, Pypes SS dual 2.5" exhaust, SS off-road X-pipe, SS shorty headers, Earl's -6AN fuel system plumbing, Walbro 255 LPH in-tank pump & Pro-M -6AN hanger, S&W subframe connectors, BMR upper & lower torque box reinforcements, LenTech Strip Terminator wide-ratio Ford AOD, 10" 3000 RPM C6 billet converter, B&M Hammer shifter, Stifflers transmission crossmember & driveshaft safety loop, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 M/T Street Comp tires.