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Intermittent high RPM miss

1112 Views 6 Replies 3 Participants Last post by  V6 pony
At times my engine misses/ surges at high speed (9300 and up). Seems to be pretty specific to at or just above 9300=9400, log shows int. very high engine speed (35,000) then shows very retarded timing trying to control engine speed. Resulting in a surge or misfire feeling. I'm running a VR sensor gapped at .050 in. thinking I need to either make the gap larger or smaller to prevent this issue but unsure which to do. Anyone know the answer other than simply guessing? Again, engine doesn't quit it feels more like it's on rev limiter, however I believe the crank signal is getting "unhappy" at the higher RPM range. Thanks for any insight. Joe
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What does your WDBG do when the RPM spike occurs? What crank trigger wheel code are you using?
Doesn't have WDbug number in data log, may have wrong size log selected. I'm using the "logman" internal logger. Using 60 tooth code 60-2 wheel 034 universal VR sensor, less than .002 runout on wheel. Seems to me the ECU calculates engine speed off crank wheel speed and the fact that it's at a fairly specific RPM would indicate that maybe it's at times losing track of wheel... Have used plotter function at times and have seen some WDbug number changes but it seems I'm having other issues running with laptop connected to ECU, during that test engine would do a similar "surge" but in the 8000 rpm range. Joe
Or maybe you are hitting a crank resonant frequency and that is causing excessive runout? It's hard to say. Weird things can happen at high speed.
Yes that is possible, figuring out if this is the issue would seem to be the difficult part. My trigger wheel is mounted to the front of my balancer, so it's possible. Would changing the gap lead to any sort of answer? Obviously the crank itself can move around as it has bearing clearence. I believe I'm logging at 100 frames per second and the erronous engine speed will show up in 1 frame then it's gone then 10 or more frames and it's back timing of course is going negative so engine slows down then speeds back up causing the surge. I'm running my oil pump off the front of the crank with a drive mandrel so that may be contributing to the problem a bit, I'm just trying to come up with a "test" to determine the actual issue. I'm pretty much stuck with my trigger wheel arrangment so working with what I have is my only option at this point. Joe
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Yea, where it's such a high speed occurrence it's going to be tough to log, also try wheel code 160, if you are on the latest firmware. This will prevent the ecu from firing coils and injectors if it does lose track of the trigger wheel timing. Then a little trial and error with the sensor gap is probably your next step. I typically find problems like this improve by pulling the sensor away, but it's impossible to say for sure which way your is failing, or if an airgap adjustment will even fix it without recording the event on a good scope.
Sorry I am using 160 wheel code (strict) thought I remembered reading in the tuning manual somewhere that a miss may occur if the gap was too small but don't recall if that was the case. Trial and error seem to be the order of the day, since it would appear to be physically impossible to maintain any sort of absolute gap on anything but a roller bearing motor anyone have any idea if larger or smaller gap is better suited for high speed operation? Guess for now I'm more inclined to widen the gap but if someone has any first hand knowledge they would like to share that would be great. BTW can you even log the WDbug number using the internal logger, I don't recall seeing it before.
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