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    Results 1 to 18 of 18
    1. #1
      Join Date
      Jul 2002
      Location
      Mesquite, TX
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      4,949
      Country Flag: United States

      Why am I idling so high?

      Yes, I know, I'm full of dumb questions.

      To catch folks up who didn't see the previous dumb questions - I had an LS1 in my El Camino. It idled at about 2500 rpm, until a rod let go and then it didn't idle at all. I got a 6.0 from an Escalade, and put it in, and this one also idles at 2500. This is with the Escalade's heads, the only parts that moved over were the valve covers (I put new gaskets on them)

      My first thought was "vacuum leak", so while I was moving everything over I replaced the intake runner seals and the injector o-rings. It still idles high. As the internet suggested, I went around the intake with an (unlit) propane torch and saw no change in anything.

      I took a log of a run (it's EFILive, so maybe nobody can read it, but that's the tool I have). The raw log is at https://www.derekf.com/Log_0004.efi if you can read it.

      The short summary is I can watch as the IAC closes down to 0 steps and as it closes, the RPM goes up, which also makes me think vacuum leak.

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      As it starts, the IAC is at 310 steps.

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      Very quickly the RPM goes up to 1500 with the IAC down to 278 already.

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      IAC down to 0, RPM is up to 2250.



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      Car gets close to a reasonable operating temperature and the RPM is up to 2500. IAC is still at 0.

      Now- at no point during this process did I touch the throttle (I wasn't even in the car) so I assume my TPS should probably be replaced. I've done the TPS/IAC reset more than once and it doesn't help. The previous owner did drill the throttle blade (I think) - the hole is about 13/64ths.

      Does this indeed feel like a vacuum leak to y'all, or is my IAC bad, or the throttle body itself, or any other suggestions? Are there other PIDs I should be monitoring? If it's relevant, it's a vaguely large cam (the SUM-8710R1 that Dual Quad Dave suggested over here, 230/242 on a 113 LSA). Is that MAP value weird? It's still 1.5 psi after the engine has stopped spinning.


    2. #2
      Join Date
      Apr 2015
      Posts
      85
      Country Flag: United States
      Same ECU as before? What MAP sensor is in it? 1.5 PSI is not right for N/A. I wonder if the higher pressure MAP was installed on previous force induction build and now itís causing your high idle.

    3. #3
      Join Date
      Jul 2002
      Location
      Mesquite, TX
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      Same ECU, just changed the field for displacement.

      Label on the MAP says 16212460, which looks to be the regular one. Easy enough to get a replacement though.

    4. #4
      Join Date
      Nov 2006
      Location
      Mountain Springs, Texas
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      4,395
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      Did you try blocking the hole in the throttle blade? Does it make a difference?

      Don
      1969 Camaro - LSA 6L90E AME sub/IRS
      1957 Buick Estate Wagon
      1959 El Camino - Ironworks frame
      1956 Cameo - full C5 suspension/drivetrain
      1959 Apache Fleetside

    5. #5
      Join Date
      Jul 2008
      Posts
      422
      Country Flag: United States
      When the car idles up high and the ecm is reporting IAC is at 0 kill all power to the car. Get it hot, take the belt off, fire it up, make sure IAC is 0 with little run time, then pull the battery cable.

      Pull the IAC and verify it's at 0 or fully seated in the throttle body. Almost sounds like it's wired backwards? I don't know of any other way to check it other than the barbaric way listed above. You just have to start checking off unknowns until you get it figured out.
      *Jeff*
      Project Salty - 1964 4 door Malibu, beaten, neglected, red headed foster child
      Cammed LQ4 / T56 Swap Project Thread <-click to read! 😁

    6. #6
      Join Date
      Sep 2009
      Posts
      2,610
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      I would take a second look at your MAP like what was already suggested. In all of those screenshots it's the same value, and the min, avg, and max are all the same at 1.5 PSI. That to me says that the sensor isn't registering, or is possibly not wired up correctly. Check that you have 12v and ground. Signal wire shouldn't have any voltage unplugged, but if it does, that may be what's causing your issue. If the wiring appears good, I'd swap out the MAP for a new one.

      With the Key off, what value do you see on the MAP sensor?

      I'm pretty sure that EFI live reads 0 to 14.7 PSI scale where zero is full vacuum and 14.7 is atmosphere.


      1955 Nomad project LC9, 4L80e, C5 brakes, Vision wheels
      1968 Camaro 6.2 w/ LSA, TR6060-Magnum hybrid and etc SOLD
      1976 T/A LS1 6 Speed, and etc. SOLD

    7. #7
      Join Date
      Jul 2002
      Location
      Mesquite, TX
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      I bought a new MAP sensor, and it also reported 1.5psi (because that would have been the easiest fix). Checking the wires, I have 5 volts on pin C (which matches the schematic). Pin A - the ground - is ~340 ohms to the negative terminal of the battery. Is that excessive for that wire?

      I changed too many things all at once - replaced the TPS, blocked the hole in the throttle blade, and changed out the MAP sensor. When I tried that, it idled low and eventually died - but I did realize after the fact that I'd also failed to plug the TPS/IAC back in after the relearn so it wasn't a good test. The tablet somehow was at 1% battery so I wasn't able to get a good log.

    8. #8
      Join Date
      Apr 2015
      Posts
      85
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      The ground line does sound like high resistance, but honestly not sure. An easy test with a multi meter is to check continuity on that wire from the sensor terminal to the matching terminal at the ECU bulkhead. It should have have continuity. If not, thereís a short in that wire somewhere.

    9. #9
      Join Date
      Jul 2002
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      Mesquite, TX
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      Agreed; I'm trying to avoid that though -- the ECU is mounted on the outside of the battery between the battery and the pass fender; access there is limited and I definitely don't want to pull the pass fender off again at this point. Hopefully I'll be able to get the bulkheads off the ECU without major surgery.

    10. #10
      Join Date
      Jul 2002
      Location
      Mesquite, TX
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      A tale in three pictures:

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      Yep, no signal coming through the signal wire

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      This is what access to the computer looks like. Heavy light green wire is the new wire I'm running.

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      And this is what I found after running a new wire to the computer.

      After replacing the connector in the connector, the engine is happier with the MAP sensor:

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      I also found a PID that's the desired IAC position - if it's wired backward, what it wanted and what it got wouldn't line up?

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      Does anything look out of line here, or do I just need to tune it like it's getting way too much fuel right now? If that's the case, wouldn't the LTFTs/STFTs be negative?

      (oh - and these logs are with the hole in the TB covered with duct tape)

    11. #11
      Join Date
      Sep 2009
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      2,610
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      Quote Originally Posted by derekf View Post
      After replacing the connector in the connector, the engine is happier with the MAP sensor:

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      Those values look normal to me for the MAP sensor. What I'd do now is a TPS reset, confirm 0-100% TPS values and uncover the hole in the TB. I think that will get you in the right direction.


      1955 Nomad project LC9, 4L80e, C5 brakes, Vision wheels
      1968 Camaro 6.2 w/ LSA, TR6060-Magnum hybrid and etc SOLD
      1976 T/A LS1 6 Speed, and etc. SOLD

    12. #12
      Join Date
      Jul 2002
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      Mesquite, TX
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      Really? The more I've thought about it overnight the more sure I was that I still have a vacuum leak - IAC is closing off the intake (plus the closed up hole) and the trims are still indicating not enough fuel for the air getting in

    13. #13
      Join Date
      Jul 2008
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      422
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      You still don't know what the IAC is doing.. You have your TB idle hole plugged, throttle is at 0, and are still hitting 2300 rpm? Why did you not track down the 340 ohms on the GND of the IAC? That is a problem. You should have traced the IAC ground back to the ECM before putting it all back together.

      The IAC ground goes to the ground rail inside the ECM. If there's resistance between that ground and battery ground, you will definitely have biased readings on EVERYTHING. If it's just the connection between IAC and ECM, then that will be your idling issue.

      Your commanded IAC PID, does it change with throttle input? If it remains at 0, then no, it's not the correct PID to be monitoring. Some PIDs will display, but are inactive.
      *Jeff*
      Project Salty - 1964 4 door Malibu, beaten, neglected, red headed foster child
      Cammed LQ4 / T56 Swap Project Thread <-click to read! 😁

    14. #14
      Join Date
      Apr 2015
      Posts
      85
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      Weíll nice work fixing the MAP, but yeah looks like itís time to figure out the IAC.

      As much as I like fixing things, electrical issues arenít my idea of a good time. Since this appears to be tampered with by a previous owner, or at the very least has signs of wear and tear, have you considered buying a pre made swap harness? It may be money well spent in this situation.

    15. #15
      Join Date
      Jul 2002
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      Mesquite, TX
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      I may have misspoken. The 340 ohm reading was MAP sensor ground to bat negative. I stopped after fixing the one wire because we knew the MAP was a problem and the unconnected wire was a very obvious smoking gun.

      Tomorrow's plan includes verifying resistance on all four ECU grounds, the four IAC wires, and the MAP sensor power and ground (and the MAF while I'm in there), plus verifying that I've got a good ground between the block (the PCM grounds go to G110, the back of the driver's side cylinder head) and the battery.

      I am relatively sure that I have seen commanded IAC at non-zero but will verify before taking everything apart.

      Everything here has been tampered. The MAF is home-ported, wouldn't surprise me if the TB was too. If this gets much worse I may need to look into a swap harness. I'd rather not though.

    16. #16
      Join Date
      Jul 2002
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      Once I got past the honey-do list I was clear to get out there and work.

      First, verified that that PID actually reflects anything. It does, at least when the engine is cold:

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      Checked resistances. The four ECU grounds (Red 1 and 40, Blue 1 and 40) all have resistances between 2.3 and 2.8 ohm to the bolt in the back of the driver's side head, and that bolt has less than half an ohm to the battery negative.

      The MAP ground pin has 0.3 ohm resistance to pin Blue 54.

      Map signal now has 0.9 ohm resistance to Red 32.

      The MAF signal has 0.8 ohm to Red 31, and then I went to check the IAC where things got interesting:

      Pins 77, 78, and 79 were between 0.7 and 0.9 ohms. Pin 76 (IAC Coil B High) didn't have a wire in it. Found the missing wire in pin 74 for some reason. Put it back in 76 (resistance 0.9 ohm), removed the tape over the hole in the TB blade, did an IAC/TPS relearn, and the car idles right around where it should.

      Still running pig rich, but it idles.

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      Thank you for the guidance - I probably wouldn't have thought to verify the wires were in the right place for quite a while yet.

    17. #17
      Join Date
      Jul 2008
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      422
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      Get r dun!!
      *Jeff*
      Project Salty - 1964 4 door Malibu, beaten, neglected, red headed foster child
      Cammed LQ4 / T56 Swap Project Thread <-click to read! 😁

    18. #18
      Join Date
      Apr 2015
      Posts
      85
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      Nice job!