last problem with HT4100 (I hope)
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HT4100 4.1, 4.5, 4.9 Discussion, last problem with HT4100 (I hope) in Cadillac Engine Technical Discussion; A couple of weeks ago I had the intake manifold gasket replaced, and the entire throttle body replaced, and the ...
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    ehall's Avatar
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    last problem with HT4100 (I hope)

    A couple of weeks ago I had the intake manifold gasket replaced, and the entire throttle body replaced, and the EGR tubes cleaned (again). Since then the car has had a low idle whenever it is under load. If I am at a red light in Drive and my foot on the brake, it will make a cycling vibration like rrrr-RR-rrrr-RR at a low pulse rate. If the AC or power steering comes on during this condition the engine will stall. I have also noticed that if I am cruising down the highway and have my foot just barely into the throttle the engine will start bucking a little bit and I have to either press into it more or take my foot off the gas in order to calm it down. The stalling under load also happens in Reverse, but there are no problems in Park or Neutral.

    I have since had the ISC, TPS, injectors, and cat/muffler replaced with new components thinking they would clear up the last of the residual problems, and while they have reduced the symptoms the problem remains.

    Honestly I think my idle is just too low. I talked to the mechanic briefly this morning and he said the ISC screw might be too far back, but that it might also be EGR related.

    I've also started having problems with my brakes getting soft and they went to the floor once. It could be coincidence, or it could be that there is a vacuum condition that is creating these other problems and manifesting itself in the brake booster primarily. Note that cruise control and A/C controls are all working fine, and the car does not have classic problems with poor engine vacuum (it runs great). However the vacuum pump may be compensating for that a little.

    I can't put much more into this right now so I'm wondering what do you guys think?

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    awadecki is offline Cadillac Owners Enthusiast
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    Re: last problem with HT4100 (I hope)

    Eek. That's a toughie. The idle speed is controlled by the computer at idle, so, as long as the ISC plunger is touching the throttle stop and your closed throttle switch is working (check to make sure the "Off" light illuminates in diagnostics with your foot off of the gas), there's nothing you can do. The computer will try to keep the idle at 530 rpms in drive.

    The only thing I can think of is low voltage causing your problem. Check the appropriate parameter in diagnostics, and make sure your voltage reading is above 13.8 volts.

    The other potential cause is a stuck EGR valve diaphram. Push up on the underside of the valve and make sure it moves freely.

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    ehall's Avatar
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    Re: last problem with HT4100 (I hope)

    I did some reading and I am going to bet that the ISC needs to be calibrated. This mechanic (who is not my usual) is fast and cheap but I'm not sure he understands everything about these engines. What caught my ear in particular is that he said he moved the ISC arm to the same position as the old unit when he replaced it. The books all say that it has to be calibrated with the TPS. I'm betting that he did not do that.

    Soft brakes could be explained by low vacuum that is generated when engine goes to low RPM.

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    carnut is offline Cadillac Owners Master
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    Re: last problem with HT4100 (I hope)

    Low engine vacuum will give you a hard pedal,(not enough vacuum to fill the booster for assist) not soft. Also low vacuum should also give you a red warning "brake" light. The ISC is adjusted by manually retracting it with one hand (engine running) and quickly disconnecting the wire harness to it with the other once fully retracted. If the ISC is retracted and not quickly unplugged the ISC can be shorted! The car should idle VERY slowly and there should be a slight gap between the ISC shaft and the throttle arm. .030. I'll bet the gap's larger than that. Soft brakes can be as simple as rear drum brakes not adjusted or as serious as a bypassing master cylinder.

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    ehall's Avatar
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    Re: last problem with HT4100 (I hope)

    FWIW I was researching some stuff and found this symptom in the Chilton's book under EGR System Diagnosis--"Engine runs rough on light throttle acceleration and has poor part load performance" which are my problems exactly.

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    carnut is offline Cadillac Owners Master
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    Re: last problem with HT4100 (I hope)

    If you replace it try to get the same EGR valve. The numbers stamped on the valve determines its opening rate. Try not to get a "universal" one.

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    Re: last problem with HT4100 (I hope)

    It's a relatively new AC Delco unit. I think the mechanic knows he left something unbolted since he brought it up so quickly.

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    Re: last problem with HT4100 (I hope)

    Well I checked the EGR pretty thoroughly and it seems okay. If I jab the diaphragm the engine chokes down a bit and then comes back when I release it. The solenoid works. I replaced the old worn vacuum hoses and it runs a little better but still has the problems. I also reset the ECM memory and it runs a little better more but still problem. In a fit of desperation I blocked off the vacuum line from the solenoid to take the EGR valve out of the picture, and while the problems are greatly diminished they are still there.

    Based on all this I'm guessing that the TPS/IAC needs to be tweaked, and that the EGR valve is probably sticking a little thereby aggravating the problem.

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    ehall's Avatar
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    Re: last problem with HT4100 (I hope)

    I did some distance driving tonight and this is what I've deduced

    1--rough idle/stalling at red light happens with EGR disconnected (but is worse when EGR is connected) so it must be ISC/TPS. I also noticed that when I take my foot off the gas at cruising speed the TPS angle goes to 2* (where it is supposed to be) but it drifts down to 0* when I stop at a red light.

    2--engine chugging with mild acceleration/deceleration only happens when the EGR is hooked up. This may be a sticking EGR valve, or it may be that the computer is warbling the EGR due to bad ISC/TPS signals.

    3--brakes work a lot better after rebuilding the rear vacuum harness

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    carnut is offline Cadillac Owners Master
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    Re: last problem with HT4100 (I hope)

    Remember the EGR should never operate at idle, so if the idle is affected by EGR then theres something allowing vacuum to it. There should never be a vacuum signal from the solenoid to the EGR at idle. You've paid to have both the TPS and ISC replaced so let him adjust them to spec.

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    ehall's Avatar
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    Re: last problem with HT4100 (I hope)

    It's certainly possible that the solenoid is leaking vacuum

    It could also be that the TPS signals make the ECM think that the car is not at idle. I don't know if the ECM also looks at speed or if it only looks at TPS to determine "idle" for this purpose.

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    Re: last problem with HT4100 (I hope)

    Just a little curious... Does this car have sentimental value? Not trying to throw off on your car, but if you like an '85, you would love an '88. Same basic body, and the 4.5 stands head and shoulders above the 4100.

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    Re: last problem with HT4100 (I hope)

    I'm not starting over with another one, if that's your suggestion

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    Re: last problem with HT4100 (I hope)

    ehall...

    Would you explain what you mean by: "3--brakes work a lot better after rebuilding the rear vacuum harness"?

    Does it mean the pedal does not go down as far, the brakes are no longer soft, or that the brakes are more easily applied? And do you actually measure a higher level of vacuum now than before replacing the hoses?

    Thanks.

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    ehall's Avatar
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    Re: last problem with HT4100 (I hope)

    I went to the shop this morning and we got it straightened out. The ISC stop screw and the TPS were both way out of whack. After we set them better the chugging at stop light and the bouncing at tip-in acceleration have both disappeared. It will still stall if I am at a red light and I turn the AC on, so we're not perfect, but it does not die if the AC is already on, and power steering does not kill it anymore, so I am close enough. Throttle response is much better too. I think we ended up with a base idle of about 600 RPM so I will do some fine tuning later but as of right now I'm very happy.

    I used a procedure that was a combination of what's in the FSM and what people here have written.

    1) Bring engine to operating temperature then turn engine off

    2) Disconnect ISC connector

    3) Retract ISC plunger by applying +12v to ISC terminal C, and press ISC plunger while applying GND to ISC terminal D

    4) If plunger still contacts throttle arm, manually retract until fully clear. Pull and snap throttle arm to ensure no contact

    5) Turn engine on. Make sure AC is not running.

    6) Manually adjust stop screw until engine idle is 525 RPMs

    7) Turn engine off

    8) Backprobe and adjust TPS for proper output voltage using FSM instructions

    9) If TPS was changed, disconnect battery ground wire to clear stored settings

    10) Turn ignition on (but do not start engine) and verify that TPS angle is zero degrees

    11) Adjust ISC plunger for throttle gap between .150 and .160 inches [values from FSM]

    12) Extend ISC plunger by applying +12v to terminal D and GND to terminal C to ensure operation, and verify TPS angles at full extension

    14) Start engine and verify ISC and TPS operation in Park and while driving.

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