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'99 Coldside Idle Dip

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  • '99 Coldside Idle Dip

    SEE 3/21 for update

    Hey All, long time away (life, kids...) but I have been back to driving the Miata lately and the idle dip is getting worse, often the idle going so low as to cause a CEL or the engine dies. Details - 99 10AE (soon to be for sale), 40k miles, coldside, I have replaced the corrugated tubing with mandrel bent aluminum (likely not a cause) and have a smaller pulley, and paper air filter (only mods)..

    1) If coming to a stop and I depress the clutch and go to neutral I will get the idle dip, it's VERY bad with the A/C on - this is the issue. -- Once FULLY warm and driven, when coming to a stop, if I leave the car in gear while slowing down bringing RPM to near idle speed it will idle just fine at the stop light (A/C on or off),
    2) Even when the engine is not FULLY warm I cannot run the A/C at idle (idle dip issue) - not certain the A/C is the exact cause, but perhaps a condition of the issue.

    I have raised the idle speed but if thinking correctly won't the car somewhat compensate and *try to get it back where it should be? Living in Florida not using A/C at stoplights can get kinda hot.

    -On there are a lot of post saying to clean the intake manifold, but.... I don't have one (Coldside)
    -Miata.Net also states to go the the driver's fender and use the electrical block (not sure what to call it) and ground the GND/10 PIN) and that will give you a 900 RPM idle so I need to match my car to it.
    -They also say to take out the idle air bleed screw and run some TB cleaner through it. - So far the one item that *may be a solution - going to get some TB cleaner.
    -Miata.Net also had a post stating to clean (I'll have to look up the name of it) a control valve of some sort on the passenger fender (that has a vacuum line running to it). I have so many threads open I can locate the one with the info.

    I suppose it could be something to do with whatever controls the idle with the A/C on, but since this is such a common issue w/ Miata, that's not my first thought.

    Has anyone experienced this and does anyone have any ideas? My thoughts are that allowing the car to coast to a stop in gear pulls any remaining fuel/air or creates a vacuum somewhere perhaps 'cleaning out' whatever may be left in there.

    Thanks, Bruce
    Last edited by blueroadztr; 03-21-2021, 07:14 AM.

  • #2
    You need to clean the Idle Air Control (IAC) and make sure its connected. On the FFS install, this is no longer attached to the intake. When you installed the FFS, you should have found a location to mount this.

    Check that its operational electrically. You may need to locate a factory service manual for proper testing procedures.
    96 Montego - FFS ColdSide, 97 Black & Tan, 90 White


    • #3
      Originally posted by bogey View Post
      You need to clean the Idle Air Control (IAC) and make sure its connected. On the FFS install, this is no longer attached to the intake. When you installed the FFS, you should have found a location to mount this.

      Check that its operational electrically. You may need to locate a factory service manual for proper testing procedures.
      bogey you seem to be knowledgeable about this - help me understand and go along with me here for a minute...

      On a stock car the IAC is bolted directly under the butterfly and when it receives a signal (e.g. A/C compressor turning on) it opens to allow additional air to immediately bypass the butterfly, this happens instantly as there is little to no air flow delay as the passages are <1 inch long.

      On a Coldside the the IAC intake port is routed about 2.5 feet to just behind the MAF (a lot of volume and restriction of air). As well, would not the intake tube also be under a small amount of vacuum. The outlet port is routed over to a "T" fitting connected to the PVC on one end and a small vacuum manifold block under and just behind the butterfly, this manifold block appears to be connected to a second (main) manifold block along with several other vacuum lines (mine's on the firewall). I can't tell about the intake manifold block connections for certain w/o taking off the intake tube which I can do later.

      So.... based upon the operation of the factory unit vs. the way it is now routed, it could never work as intended- regardless if the solenoid opens/closes correctly. My theory - the IAC inlet will always have some sort of vacuum pulling air away from it as it's connected to the intake tube (maybe doesn't matter as the stock unit is also supplied by the intake air supply).

      As well, if there is an inlet into the bottom of the intake, would not the PVC valve be constantly pushing air into it? Besides since the vacuum manifold located under the intake is plumed into the main vacuum manifold, would this not also create a constant vacuum to the port under the intake.

      Do I have my vacuum lines incorrect?

      In theory.... I could relocate the IAC much closer to the butterfly valve, disconnect/reroute all the other vacuum lines and PVC valve currently connected onto the intake-vacuum manifold, thereby allowing a similar function to the stock unit. One question - if I leave the IAC port open to the atmosphere it will be pulling in unmetered air - do you think this is an issue?


      • #4
        OK - having re-read the installation instructions it appears that mine is not 'plumbed' like the instructions read, maybe there has been an update? edit - It appears there has been a change or mine was never even close to the plans. If I am thinking correct there is little to no vacuum at idle so having the IAC draw air from the vacuum block *should not be an issue. I will re-route the lines, test the IAC, and see where that leaves me.

        NOTE: I did look into the butterfly with a bore scope and noted an oil film, possibly blow by from the PVC? Would this indicate anything particular?
        Last edited by blueroadztr; 03-20-2021, 07:41 AM.


        • #5
          UPDATE: 3/21

          I spoke w/ Tom yesterday, I am 99% sure I had my vacuum hoses routed correctly, unfortunately I had removed them all prior to speaking w/ Tom, but based upon memory they were correct. I possibly* had the aluminum reducing orifice that is inserted into the exhaust side cam breather hose in the PVC valve side. I confirmed I had the updated routing instructions and he also confirmed there is PLENTY of air flowing through the hoses and IAC regardless of the hose length ((he gave me a tutorial: engines require 1.65 SCFM per HP and at idle maybe 5-8 HP is being used and the IAC flows 3 something if I recall - that kind of info) that man is SMART).

          I will test the IAC, reassemble, and see how it does. One piece of advice he gave is to only adjust the idle stop screw 1/4 turn at a time and them drive for a few hours to allow the computer to learn then re-adjust as necessary to the point the idle is stable; if you have idle hang up you have gone to far.

          He also stated that moving from a colder state to a much warmer state the air density changed requiring re-adjustment.

          HE also told me a few stories of his car, the evolution on the kit and design. H also promised to share the story of horsepower wars in jet engines in the 60's and 70's, I believe he helped design them.

          Once I get a day to re-route and test everything I will report back, thanks for reading!

          FINAL UPDATE for those following. I tested the IAC against 2 others and it was the best of the three. I relocated the IAC closer to the throttle body (it was up near the passenger headlight), then reconnected all the vacuum hoses and it's... PERFECT. Idle actually goes UP when the A/C turns vs. down. I'm certain drivability will be enhanced as well. There were 2 possibilities where I went wrong installing the kit.

          1) I had the intake/outflow vacuum hose from the IAC backwards (doubtful).
          2) The small aluminum restrictor that belongs in the exhaust side cam cover breather was on the PVC side (most likely).

          Anyway - I'll be driving the car tomorrow and take it on a nice test run.
          Last edited by blueroadztr; 03-21-2021, 05:11 PM.