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Thread: RPM signal erratic, car undriveable, pro help requested please.

  1. #1

    Unhappy RPM signal erratic, car undriveable, pro help requested please.

    Hello, I'd really appreciate some help. Holley Avenger 550-200, on a rebuilt dodge slant six 225 engine in a restored 1969 Dodge Dart. It’s been running great for the past year, 7000 miles driven. I’ve learned so much from Danny Cabral’s posts to this forum to get it tuned so great.

    My EFI problem began a few days ago, when the Bosch O2 sensor died (suddenly always super-lean), so I limped it around in Open Loop mode by disabling closed loop (though the ECU’s Learn Table was certainly corrupted during sensor troubleshooting, as it would cough and spit once in a while). Unrelated, the Holley Handheld unit showed blanks on the 'Gauges' view, but actual values on the 'Monitor' view. I thought that was odd, but ignored it.

    After 3 days driving it somewhat mildly sputtering in Open Loop (waiting for the new O2 sensor to arrive), it was driving perfectly on the freeway and very suddenly lost power (2700 RPM, constant speed cruising), and the engine began running awful. Undriveable, in fact, so I exited the freeway on momentum. I got it to barely start up, and there was very erratic IAC behavior, with a massive sucking sound coming from it. I simply couldn’t get it to run, wow did it idle horribly, any throttle setting over ~20% made it stall. I even tried setting a brand new base calibration using the handheld, as a hail-Mary to drive it home on that, in open-loop mode. That made the engine run even worse. So I towed the car home 70 miles.

    I then tested the IAC motor response using the Handheld unit, marching up the ‘parked IAC’ settings, and the IAC responded seemingly fine, though it would only be commanded between 1% and 52%. The handheld would not even allow me to select anything higher than 52%, which seemed odd.

    Then I checked out the engine mechanically, the car’s electrical systems, the EFI wiring harness and all the connectors. It all checked out. No vacuum leaks. Good spark. Excellent cylinder compression on all six (only 1% variance). 22 psi fuel pressure. I have a factory distributor running the Pertronix II electronic conversion inside and paired with an Accel canister-style ignition coil (therefore the Avenger EFI is reading RPM from its ‘loose yellow wire’ to the ignition coil’s negative post), and therefore the EFI does not have any timing control; that has always stayed mechanical/vacuum. I saw the forum 'Sticky' regarding the RPM input error, but I am not running any CDI boxes or anything, and the RPM has always been perfect.

    MAP reads fine. Air temperature sensor reads fine. Coolant sensor reads fine. Pulled out those massive 82 PPH fuel injectors even, their screens were squeaky clean. Pulled out the IAC and found no debris in the throttle body passages. Checked every fuse.

    Then I installed a new correct Bosch O2 sensor & a new Holley TPS sensor (as I had one on hand, even though the old one seemed to be reading fine). Starting from scratch, using the handheld Wizard, I made brand new Global base calibration, then hooked it up to laptop with the latest 2.2.0.4 EFI software and setting all parameters to what they should be (6 cylinder, 225 CID, no timing control, Bosch O2, etc). Synced to ECU, then did a TPS reset. (It’s weird that you can’t set number of cylinders nor engine displacement with the handheld, by the way!)

    The engine started right up, but with IAC still behaving intermittently erratic, giving poor/high, RPM idle, something around 1200 rpm according to my ear. It was running MUCH more solidly than when it died on the freeway, so I could tell that things were already improved. But something was clearly off, as evidenced by the IAC behavior. Then I noticed RPM indication on the Handheld was erratic. I hadn’t looked at that previously. It showed around 400-600 RPM immediately bouncing to around 800-1200 RPM (factor of 2) back and forth, back and forth. Holding the throttle open a bit and raising engine speed stopped the factor of 2 bouncing value (in both ‘Monitor’ and ‘Gauges’ view), and instead it just indicated half of the real-world engine RPM. Not good!

    At least the new O2 sensor was working fine, showing decent convergence to the 14.0 target AFR. I fiddled with IAC settings for an hour, including disconnecting it after setting its parked position to something small (4%) and cycling the ignition key power. And the bad/2X bouncing RPM reading persevered. The engine definitely running ‘off’, I’m afraid to drive it. If the ECU thinks the RPM is only half what it really is, I'm sure bad things can happen. I combed this Forum for hours looking for similar problems, but it looks like this is unique.

    I updated the ECU firmware to version 1589 that I downloaded today (techlibrary_hefi_1589.eep), thinking it was corrupted, even though it had that version originally. Re-set everything all over again, 225 CID 6-cylinder, no computer-controlled timing, etc, and synced it up. Same rough-running / high idle with erratic factor-of-two RPM signal.

    I then reverted to a prior firmware file I had last year, HEFI_00001589.eep. Started from the top all over again. But still have the same problem.

    I even put a new ignition coil on, and it did nothing helpful. I have traced the yellow wire that’s connected to the ignition coil’s negative post all the way back to the ECU, it’s fine. Its ring connector on the coil's negative post has zero corrosion. The Pertronix 1361A electronic ignition module is fairly new, only 3 months old (the original 1361A crapped out after 9 months or service).

    Would a bad IAC kill the RPM signal like this? I ordered a new IAC motor and MAP sensor from Holley’s web site today. Fingers crossed. Is my ECU dead (installed benignly inside the car), after only 12 months of usage? This is a daily driver, so I hope to get it back on the road soon, luckily I have a friend's car to borrow for a few days.
    Last edited by DartDave1969; 05-30-2017 at 04:19 PM. Reason: added a detail

  2. #2

    Thumbs up Follow-up with good news:

    So I was able to get in touch with a Holley Tech today, and he quickly deduced that the behavior I described was likely RF interference from the aftermarket electronic ignition system being wonky (not his words; I'm paraphrasing).

    That was it! The magnetic pickup retrofit ring inside the stock distributor had vibrated loose over the past three months, and the 0.030" air gap that is supposed to be maintained had gone to zero. So there was rubbing going on, which wore through the high-strength wrap that keeps the 6 neodymium magnets in place. One of those magnets popped out of its recess. In other words, the electronic ignition module was partially damaged and acting wonky, sending an erratic signal to the negative post of the ignition coil, and generating RF interference (throwing the IAC into confusion). Luckily I had a backup of the Pertronix distributor internals on hand. I took out the old, installed the new, set the air gap - tightly! - and fired the car up. And it works! The RPM signal is robust and proper, and it quickly found a fairly steady idle. Time to learn a good tune on this clean slate. Thank you Holley, for quick troubleshooting. I should have dug deeper into the distributor, but I guess I had reached a point where my frustration clouded my judgment.

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