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Strange, sporadic cold rough idle and tach fluctuation 700

The weather is changing (surprise!) here on the East Coast. This morning, the air temperature was about 52 degrees when I started the car. My vehicle idled surprisingly rough, and for the first 4 or so miles ran like it wanted to stall everytime I came to a stop. The tachometer was right there on 750-800, yet every time I stopped at a light it would idle smooth, and then the tach would drop to maybe 550-600 for a second and then back up to 700 or so. When it dropped you could feel the engine shudder just for the second or so, and then back up to smooth idle at 800. I drove it to public transportation and left it in a covered parking lot. This evening, I started it and it felt similar to how it was this morning. I drove about 20 miles or so, and when I came off the highway it was idling so smooth I thought the engine had cut off. However the tachometer still dropped occasionally to 600 or so for about a second every, say, 15 seconds. The air temperature of course had risen to about 75. I recently cleaned IAC (with brake cleaner, used about a whole can over about a hour of cleaning and shaking the fluid inside the valve), rebuilt distributor, distributor cap is about 6 months old, wires are from PO (I have some to replace them), plugs are about 6 months old. Any ideas why the rough idle in the cold morning, and the engine has this only occasional "miss" in it?








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    Strange, sporadic cold rough idle and tach fluctuation 700

    I think you might have a problem with your IAC. BTW - when cleaning the IAC did you restrict the brake-cleaner to the valve end ? Because theres no internal separation between the valve-section and the armature, commutator & brushes - so cleaning fluid can very easily end up swamping the electrical section.

    To test whether the IAC is the culprit do the following:

    1. Start the car and wait for the short-interval during which it is idling smoothly. If getting to this point is a problem. use the alternate method at the bottom of this post.

    2. Quickly unplug the IAC-connector with the engine running (be careful the belt/fan dosn't get you !)

    3. Now drive it around and see if you have the sporadic jumpy-idle problem. If you dont then you've isolated the problem to the IAC or it's controlling-components (i.e. ECU, TPS, coolant-temp sensor for the ECU). If you are checking continuity for these components, do it at the ECU end (with the ECU-connector-cover off) - since that check ends up testing your wiring-harness as well. Here's another post I wrote for testing the IAC.


    Alternate Method (if your throttle body is equipped with the black idle-adjust screw):

    1. Remove the IAC, and block both hoses going to it (I bridged them with a solid-rod to take the place of the IAC).

    2. Open up (unscrew) the black idle-thumbscrew almost completely - otherwise it will not idle.

    3. Go to step 3 above


    Noel








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      Strange, sporadic cold rough idle and tach fluctuation 700

      [quote]I think you might have a problem with your IAC. BTW - when cleaning the IAC >did you restrict the brake-cleaner to the valve end ? Because theres no internal separation between the valve-section and the armature, commutator & brushes - so cleaning fluid can very easily end up swamping the electrical section.[/quote]

      Oops. I did not think of restricting the fluid inside of the valve. I first used Gunk engine cleaner, a couple of shots of it as a matter of fact, for about a good 10 minutes of rigorous shaking. Then I used brake cleaner to clean out whatever might be left of the engine cleaner until everything was quite clean inside and the little door turned very smooth-like. Then I air-dried the valve overnite before I reinstalled it the next day. I'll keep that in mind when I do similar cleanings on items that have electrical connections to
      one end [smile]. Do you think I might have damaged the electronics inside from the fluids? Maybe another trip to the boneyard this weekend, if so...








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        Strange, sporadic cold rough idle and tach fluctuation 700

        If you read the post I referred to in my previous post, I mentioned that after opening the IAC after attempting an external cleaning, I'd found the armature spindle had dried-up grease in it . This spindle should be oiled lightly for the armature to turn freely. If you've put gunk + brake-cleaner in there everything must be dry by now. Not sure how gunk reacts with copper - not too kindly I suspect...

        If you have easy-access to a junk-yard I would suggest picking a new-looking one from there (in fact I just picked up one for myself this afternoon, for an astronomically-high $2.50 ;-)

        Let me know the results of the bypass-method I suggested.

        Noel







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