Intermittent ISC? TPS? Problem still
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HT4100 4.1, 4.5, 4.9 Discussion, Intermittent ISC? TPS? Problem still in Cadillac Engine Technical Discussion; Well after resetting everything to awadecki's specs it seems to be working pretty well but...internittently it still will accelerate in ...
  1. #1
    rstewart65 is offline Cadillac Owners Member
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    Intermittent ISC? TPS? Problem still

    Well after resetting everything to awadecki's specs it seems to be working pretty well but...internittently it still will accelerate in the 30-50mph range and then just as quickly let up. I suspect it is the throttle closed switch in the ISC, even though I've cleaned it up and on the bench it works perfect.

    I'm now thinking that the backside of the gear that contacts the ring that activates the switch could be somewhat worn. Think I'm wasting my time on this? I hate to pony up the $ for a new ISC, I have 4 used ones and think one should be workable. Any tips on cleaning, lubing. bending, etc. that might help? Any input appreciated!

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  3. #2
    rstewart65 is offline Cadillac Owners Member
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    Re: Intermittent ISC? TPS? Problem still

    bump....

  4. #3
    awadecki is offline Cadillac Owners Enthusiast
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    Re: Intermittent ISC? TPS? Problem still

    Any new results? Did you try a new ISC?

  5. #4
    rstewart65 is offline Cadillac Owners Member
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    Re: Intermittent ISC? TPS? Problem still

    Tried 3 ISC's, all of which have been cleaned, re-greased, points cleaned for the switch and test A-OK on the bench and still get the trouble...occasionally the ISC stays out too long and you are idling along at 45 mph. If you slow it down, the ISC will retract eventually. The "fix" is the unplug the ISC, apply power to extend it to get a decent idle with the AC on and drive.

    Still looking for info as to what tells the ISC what to do or where the ECM reads rpm, etc. I suspect that my next step is to pull the ECM and clean all of the connections to be sure I don't have a dirty one. I may even grab another ECM next time I'm at the U-PULL-IT just to try it.

  6. #5
    peteski is offline Cadillac Owners Enthusiast
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    Re: Intermittent ISC? TPS? Problem still

    ECM reads engine rpms from the distributor's TACH output. It also takes many other things into consideration. One of them is the Idle switch. Another is the TPS which tells ECM what is the throttle position. It also monitors engine temperature, manifold and ambient pressure, manifold temperature. And the O2 sensors...

    All of those inputs go into the "black box" (ECM), get "digested" and then ECM sends out signals to control the engine.

    If you think one of those is not correct, you can visually monitor each of those sensor's values on your Climate Control panel while in the diag mode. In real time.

    I don't think anybody here knows exacly how the ECM actually regulates the idle rpms. You woudl have to dissassemble the ECM's firmware to understand how it is done.

    It might be possible that the current drivers which control the ISC are weak and they have difficulty in driving the ISC motor.

    Did you do the output cycling test to see how well the ISC extends and retracts? That too is available within the diagnostics.

    Peteski

  7. #6
    rstewart65 is offline Cadillac Owners Member
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    Re: Intermittent ISC? TPS? Problem still

    I've observed and/or adjusted everything. I have the Factory Shop Manual and well as a Haynes, 3 extra ISC & 3 extra TPS, all of which test out OK. I've checked everything you can check in the P.0... sytem. ISC extends and retracts fine using the system and jumper wires. The car functions fine most of the time, but occasionally you'll be cruising along and when you go to slow down, the engine is idling way high. You pull off and the ISC is extended. Given some time, from a few seconds to a minute, it'll settle back down. If all of my ISC's didn't behave the same way I'd suspect them. Same for the TPS's.

    It's as if the ECM "forgets" to idle it down or is retarded. I was thinking maybe a dirty connection at the ECM, I've cleaned and dielectric greased everything under the hood, even though they all looked fine. I may pull the glove box and clean each connector on the ECM next. It shouldn't take more than an hour to pull it all, clean it, grease it and put it back together and I can rule out a bad connection.

    At the moment I have the ISC nudged out a bit and unplugged and the car is fine. You have to let it warm up a minute, otherwise it runs and drives fine. (Idle is about 600rpm) My GF drove it on a 1000 mile trip recently and it did great, 25 mpg with the AC on. She didn't have a bit of trouble. I'd just like to get it "right" for once.

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    cadillac_tech is offline Cadillac Owners Fanatic
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    Re: Intermittent ISC? TPS? Problem still

    rstewart65,
    I'm not sure what spec's awadecki gave you or how you set them. Did you perform the TPS learn and Idle learn? I don't know your knowledge level so bear with me. It is critical to reset the PCM memory and do the learn procedures.

  9. #8
    rstewart65 is offline Cadillac Owners Member
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    Re: Intermittent ISC? TPS? Problem still

    Yes, did the relearn. Set everything to factory manual spec, then reset the TPS based on the internal display instead of using the volt meter. I don't recall the setting, I'd have to go back and check his instructions.

  10. #9
    awadecki is offline Cadillac Owners Enthusiast
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    Re: Intermittent ISC? TPS? Problem still

    It was to set the throttle position to 0.7 at min air instead of 0 to prevent cruise "sailing". This wouldn't cause the issue he's experiencing. It, unforutnatelym sounds like you may have a wiring issue, but I'm not sure. Try watching for the "Off" light on the climate panel in diagnositcs when driving. It should ALWAYS be lit when you're off the gas pedal.

  11. #10
    kdcing is offline Cadillac Owners Fanatic
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    Re: Intermittent ISC? TPS? Problem still

    Had the same exact problem w/ the ISC on my '93 Eldorado w/ a 4.9L....the ISC appeared to function correctly (i.e. it would extend and retract), however, often the idle speed would be elevated @ 900-1500rpm. My short term solution for 3-4 months was to disconnect the ISC at its connector.

    I finally was able to get the ISC to act correctly only after finding 1 article on the net that indicated that the Idle speed must be less than 450rpm when doing the Idle Speed relearn procedure. This suggestion worked like magic and immediately fixed my high idle speed problem.

    To get the idle speed down to 450, it was necessary to:
    1. Run the engine until operating temperature is achieved.

    2. Get the ISC to retract fully by increasing the engine speed to 2k rpm (i.e. hold the throttle open by hand) while simultaneously holding the ISC's plunger closed (i.e. close the switch inside the ISC). Once the ISC is fully retracted, unplug the ISC @ the connector.

    3. If the Idle speed is higher than 450rpm, turn the idle speed screw counterclockwise (the idle speed screw is near the ISC). Note that there is a fine line here....we need to get the idle speed down to 425, but the engine will stall just below 400.

    So, give this a try on your engine and send a note back to this post as I'm sure it will help others out too. I had researched the ISC procedure quite a bit and was lucky to find the 1 article that mentioned the need an Idle Speed of less than 450rpm when doing the ISC procedure. Most articles do not mention the requirement and that is probably why there are quite a few of us that have had problems w/ getting the ISC to act correctly.

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