HT4100 Driveability Question/Problem
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HT4100 4.1, 4.5, 4.9 Discussion, HT4100 Driveability Question/Problem in Cadillac Engine Technical Discussion; Hi everyone-- Well my HT4100 (in my 85 Coupe Deville) has been running pretty decently, but I'm still battling a ...
  1. #1
    awadecki is offline Cadillac Owners Enthusiast
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    HT4100 Driveability Question/Problem

    Hi everyone--

    Well my HT4100 (in my 85 Coupe Deville) has been running pretty decently, but I'm still battling a mid-range soft spot and I still don't think I have all 125 hp. For those of you who have read my posts before, my car is 100x better than it once was, but it's still not quite up to snuff.

    I think I've finally pinpointed the issue, and I need some help on what could be causing it.

    Today I drove around with an analog vacuum gauge taped to my window and noticed something strange: while my car pulls good vacuum at idle (don't remember the exact value, but parameter P.0.2 reads about 35-38 MPa at idle vs. 99 MPa atmospheric), it really doesn't have much vacuum at part throttle.

    If I step on the throttle to the point where the pedal is about 1/3 of the way down, my car's vacuum is not far off from atmospheric pressure. I hooked the vacuum gauge up to my 92 STS equipped with a 4.9, and it still has a lot of vacuum at only 1/3 throttle.

    Interestingly, when my car is cold and I first take off, it has considerably more vacuum at part throttle than when it is warm--and feels peppier too. I have no idea why.

    I've tried the WD-40/carb. cleaner spray trick around the throttle body gasket to see if there are any leaks, but there aren't any. I've also swapped MAP sensors out, changed the MAP vacuum hose, and cleaned the port out, but this hasn't helped.

    What could be causing my problem? Since I'm only a 1 man show, I haven't done a compression test, but my starter cranks evenly over all cylinders (no skips), and the car starts instantly cold or warm.

    Could it be a bent valve? Leaking intake gasket (although my coolant bottle never moves and my oil looks near new after 1,000 miles since the last change)? Leaking throttle body gasket? Intake manifold bolt torque too low? Weak valve spring? I really am clueless...Why would it be worse when warm, rather than cold?

    By the way, the car only has 34,000 miles on it now.

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  3. #2
    carnut is offline Cadillac Owners Master
    Automobile(s): 1988 Allante' (sold), 1984 Eldorado, 84 Sedan DeVille
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    Re: HT4100 Driveability Question/Problem

    I would verify fuel pressure is at least at minimum spec.(engine temperature fully warmed up) When cold the injectors spray longer. When warm and in closed loop the spray duration is less. So if fuel pressure is weak, it will be even weaker when the engine gets to temperature. Pulse width, ie how long the injector is cycled on, depends on eng. coolant temp, and feed back from the oxy. sensor once warm enough to go into closed loop. If fuel pressure is below spec, even say 2 lb.,the car will be lean, hense less power. Also this engine will easily stand 2 to 3 degrees more timing advance. If I recall timing spec is 10 degrees, reset it to 12.

  4. #3
    awadecki is offline Cadillac Owners Enthusiast
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    Thread Starter

    Re: HT4100 Driveability Question/Problem

    Hi carnut--

    Thanks for the tip. However, I checked that and it reads a constant 12 psi (so well within spec). The car also has all new ignition parts (new distributor, cap, rotor, wires, plugs, and coil). I really think this vacuum thing is my issue. My injector spray pattern is also nice and conical.

    I also have put on a new OE-spec ArvinMeritor catalytic converter, so backpressure shouldn't be an issue.

    I'm currently running 12 degrees advance.

  5. #4
    cadillac_tech is offline Cadillac Owners Fanatic
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    Re: HT4100 Driveability Question/Problem

    Quote Originally Posted by awadecki View Post

    Today I drove around with an analog vacuum gauge taped to my window and noticed something strange: while my car pulls good vacuum at idle (don't remember the exact value, but parameter P.0.2 reads about 35-38 MPa at idle vs. 99 MPa atmospheric), it really doesn't have much vacuum at part throttle.
    What is most important to look at initially is how fast the vacuum gage reacts. If it reads 18 in Hg at idle, it should snap to 0 and back to 18 on a sharp flip of the throttle (not in gear). Driving it will produce much lower vacuum readings at 1/3 throttle, closer to atmosphere.
    Clogged converter will cause your described symptom and so will the MAP port you checked, although their symptoms are slightly different. One other idea is an IAT sensor reading out of range.
    Your cold start is controlled primarily by CTS and TPS.. Check the coolant temp and intake air temp when cold. They should be the same.

  6. #5
    awadecki is offline Cadillac Owners Enthusiast
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    Thread Starter

    Re: HT4100 Driveability Question/Problem

    Checked the IAT and CTS when cold--both read the same value. Then I pulled the TBI off and found the MAP port had some varnish on it. Sprayed the port out, wiped it off well, and reassembled everything. I also changed the rubber portions of the MAP hose.

    The car now responds much better to throttle inputs...it used to bog a bit, then take off...seems to be largely cured

    This car is always an adventure.

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