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HT4100 4.1, 4.5, 4.9 Discussion, last problem with HT4100 (I hope) in Cadillac Engine Technical Discussion; I'm a little confused? The timing is set to 14 BTC? The +4 means four degrees above 10? When I ...
  1. #31
    carnut is online now Cadillac Owners Master
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    Re: last problem with HT4100 (I hope)

    I'm a little confused? The timing is set to 14 BTC? The +4 means four degrees above 10? When I mentioned plug wires, I was referring to routing correctly, not replacement. Every factory ser. manual I have says 0.060 on the ISC gap? Wouldn't hurt to set it to that to see. I assume the TPS reads zero at min idle at 450? Have you looked at the dist gear? If the engine stalls with the throttle lever resting on the ISC plunger its either incorrectly set or not extending nor retracting. The min idle setting is not an exact RPM. because the throttle never rests on it in normal use. I set mine to where the engine warm just barely idles maybe 400 RPM.

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    Re: last problem with HT4100 (I hope)

    I asked the mechanic to advance the timing +4 using the on-board timing mode (the +10 thing), but I don't know if he did it correctly. I have not pulled the distributor to look at the gear. I am going back there next week sometime to get all that square.

    In the meantime... I have the stop screw set to idle at about 650 RPM. I then set the ISC plunger to the screw per the FSM specs. I also set the TPS to the screw per the FSM specs so that it is 0* when the throttle is closed against the screw. Everything is set to the screw, but the screw is set to 650 RPM instead of ~500 RPM (this is needed to keep it from bogging down to ~300 RPM when it starts churning at a stop light). The throttle switch inside the ISC works correctly, and the stuff that uses the throttle angle (ie the EGR control routines) also work correctly, however the car has a minimum idle under load of about ~500 RPM because the stop screw keeps the throttle open after the plunger retracts.

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    Re: last problem with HT4100 (I hope)

    Quote Originally Posted by carnut View Post
    Every factory ser. manual I have says 0.060 on the ISC gap? Wouldn't hurt to set it to that to see. I assume the TPS reads zero at min idle at 450?
    Couple of things worth arguing about here.

    Throttle position is indicated by TPS according to voltage output--steeper angle produces more voltage. As long as you set the TPS to reference value then you can make 0* anywhere you want it. In my case I bumped the stop screw, which keeps the throttle from closing, and then set the TPS reference voltage so that it was showing 0* there. If I did not do that then the computer would think I was always at ~2* or whatever, which would cause it do things like open the EGR too soon.

    The ISC plunger kind of follows the same principle. When you set the gap from the stop screw, you are setting the plunger's maximum retraction, and since the screw can only extend so far (it will fall out if you keep going) then you are also setting it's maximum extraction as well. By making the gap small, you are really just reducing the maximum retraction distance, and extending the maximum extraction distance. With a small gap the stop screw would not be used since the plunger would rarely retract all the way, but a slightly larger gap allows the plunger to retract and allows the stop screw to come into play.

    BTW I just checked the FSM again and it says exactly the same thing about .150-.160 gap.

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    Re: last problem with HT4100 (I hope)

    I went back to the mechanic this morning and we set the timing according to spec. The problem with the loping RPM at a stop light is gone, and the car no longer dies when the AC or power steering kicks in. I have to roll back the stop screw on the throttle body but will probably keep it a little high since it seems to give me a little more power.

    Good call on the timing, guys

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    Re: last problem with HT4100 (I hope)

    Please understand that the idle stop screw is never contacted unless the ISC is fully retracted to set air gap. In normal use the idle speed is totally controled by the ISC. As the engine is shut off the ISC actually should extend slightly to anticipate the next restart.

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    Re: last problem with HT4100 (I hope)

    eh, the stop screw provides minimum throttle so the engine won't conk out when it goes under sudden load. Also the TPS and ISC are synchronized to the screw, and they both affect throttle position when the engine is operating under computer control.

    Anyhoo, I just set the base idle to 500 and put the TPS a little aggressive, and the car is running great. It's a little bit bubble at light throttle but nothing bad. I should probably turn the TPS down a bit.

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    Re: last problem with HT4100 (I hope)

    Yes, I agree the ISC and TPS are adjusted once min idle is set, but the whole reason for the ISC is to maintain a controlled idle regardless of load, sudden or otherwise. An ISC will never fully retract in normal operation. The ECM must be overridden to command retraction. The theory of keeping the engine from stalling is flawed. If a sudden load occurs, a min. air setting of 450 to 500 RPM would not be enough to keep the engine running. The most obvious sudden load would be the compressor engaging on a very hot day ( high head pressure). The ISC if working correctly, will anticipate this load and a 1/2 second before it engages, will extend to maintain idle. If both the ISC and TPS are set to spec and both are fully functional no "turning down a bit" should be necc. My car will idle between 625 to 675 at fully warmed up in gear, A/C on. I get the feeling the ISC is either slow to respond or maladjusted.

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    Re: last problem with HT4100 (I hope)

    With the car in gear and the brake applied (ie, the engine under load) then the engine should idle around 525, although 600 is good like I said it produces a bit more power off the line, but that is because you basically have the engine pressing the gas for you a bit

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    Re: last problem with HT4100 (I hope)

    The computer controls the idle speed. It should be around 530 rpms when the engine is fully warm with the A/C off. Timing is everything on the 4.1. Too advanced and A/C engagement or fast parking lot turns will make the car stall. As I've said before, this is because the ISC is commanded to step out a certain amount when it senses either condition. Once the plunger extends, the computer senses the idle increase too much (because the timing is advanced), it will then slightly retract the plunger. This all occurs in the 1/2 second before the A/C compressor engages. So, once it finally engages, the car stalls, or nearly stalls.

    I have my timing set to about 11.5 degrees base timing. I find this is the best compromise between pep and drivability. I don't find that I have any more pep at 13 degrees of timing than at around 11.5.

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    Re: last problem with HT4100 (I hope)

    Just an update, I rolled the TPS back a hair and the bubbly acceleration is gone. Also has smoother power off the line, but lost a little bit of oomph. I'm going to turn the ISC up a little and see if reducing the minimum throttle gives me any more power off the line.

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