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HT4100 4.1, 4.5, 4.9 Discussion, last problem with HT4100 (I hope) in Cadillac Engine Technical Discussion; Originally Posted by FrankC Would you explain what you mean by: " 3--brakes work a lot better after rebuilding the ...
  1. #16
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    Re: last problem with HT4100 (I hope)

    Quote Originally Posted by FrankC View Post
    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?
    The brake pedal is firmer and has less travel (and it has improved again since calibrating the ISC). I did not measure vacuum prior to rebuilding the harness so I don't know how much it improved. I rebuilt the harness because there were a lot of brittle/stretched hosing and some mis-sized Tee-connectors that were allowing too much play. Everything is snug now and vacuum stays within 15-25 hg while driving.

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    Re: last problem with HT4100 (I hope)

    theirs no such thing as a "last problem" when you talking about the High Technology 4.1, but best of luck to you hope you can at least get her running happy for a few minutes before the 4.1 throws you another curve ball

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    Re: last problem with HT4100 (I hope)

    Actually the "churning" idle problem is still there just suppressed. I am starting to think that the hillbilly mechanic I tried put the intake gasket in wrong. It will run great cold, and will still run great after the engine enters closed loop, but will progressively worsen as the engine gets warmer. It is definitely temperature related.

    Searching around a bit, I have found a couple of other people who had this happen after the intake manifold gasket was replaced. Oh joy. I was going to just ignore it until I found that.

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

    Your "churning" idle is sadly not an ISC issue (most likely). If your idle goes up and down, it very well may be the distributor gear getting worn. This was the case with my 85. The teeth get worn and cause erratic idle timing.

    As for the car stalling when the A/C is on, make sure that your timing is set at 10 or 11 degrees above base. Anything above or below that and the computer responds incorrectly when the A/C compressor is engaged.

    For instance, with over advanced timing, the computer will retract the ISC plunger just after it extends it a fixed amount when the A/C engages. It retracts because the cmputer sense that the idle has increased too much. Then, the car dies.

    I really doubt it's an intake problem. Timing causes the A/C on stalling and also a lumpy, unstable idle. Check your distributor gear.

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    Re: last problem with HT4100 (I hope)

    If it was the distributor gear then it should behave the same cold or hot

    Idle is great when the engine is stone cold. Idle is great when the engine goes into closed loop. It is only after driving for 5-10 minutes and the engine gets really warm that it starts to develop an uneven rhythmic idle--watching the onboard gauge I see that the RPM fluctuates between 400-600 RPMs, and I hear and feel the engine oscillate.

    When that starts happening, then turning on any accessory (whether A/C or power steering) is likely to kill the engine, but that is because the ~400 RPM down-cycle is not enough to drive the belt so the engine dies. The AC is not killing the engine (I can turn it on while driving just not at a stop), it's the low point of the loping RPM that is killing the engine.

    It also only happens when I am stopped with my foot on the brake in gear. Park/neutral is no problem. Driving down the road is no problem.

    http://www.digest.net/bmw/archive/v8/msg01640.html is another guy that had the exact same problem with a BMW. http://x.nissanhelp.com/forums/front...when-warm.html is another guy with the same problem and a Nissan truck.

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    Re: last problem with HT4100 (I hope)

    It seems that the most common source of these problems with other cars is the PCV valve (or a faulty elbow pipe where they are used). I got to looking around and sure enough my PCV valve was a little bit loose in the grommet. I reseated it and the problem was practically eliminated, although it came back a little later. After reseating it again the car idles at a solid 600 RPM when braked in gear. So I'm guessing that the grommet or valve gets loose when the engine heats up which results in a fluctuating PCV leak, which in turn causes the car to idle unevenly.

    The PCV valve itself was one of the first items I replaced when I started this project, it still looks clean, and it rattles freely. The grommet also seems to have a really good seal around the valve when the engine is running, although I'm guessing it shakes out. Maybe I should put some kind of RTV around the valve to help hold it in and see if that eliminates the problem entirely.

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    Re: last problem with HT4100 (I hope)

    I replaced the valve and grommet and the problem is still very much present. I guess it was just a fluke, although it may be indicative of the actual problem.

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    Re: last problem with HT4100 (I hope)

    Or that PCV hose is leaking at the throttle body end. Can you freely rotate the PCV hose with the valve out of the grommet? If so, its loose at the TB end. My experience with this engine is a vacuum leak raises the idle not lowers it unlike a carb car with a vac. leak. With the engine fully warmed up and idling with the climate control on and set to about 72 degrees, you should see the ICS plunger stay at a fixed extension until a half second before the A/C compressor engages, then it will extend to keep the maintained idle the same. Then just as it disengages, the ISC will retract, again to stabilize the idle. I find it hard to believe that the ISC adjusted to spec would allow a low enough idle to cause stalling. What is the gap you measured with the ISC fully retracted?

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    Re: last problem with HT4100 (I hope)

    I replaced the hose too, sorry. I want to replace the breather hose but I can't find one anywhere.

    I've tried a couple of different gap settings. The FSM says .150-.160 so that is what I use for now, maybe a little closer. I also tried .030 as was mentioned in several threads here but I'm guessing that is for the 4.9 not the 4.1

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    Re: last problem with HT4100 (I hope)

    ehall, it's likely a timing issue. remember, when the car is cold, your oil is thick, so there is less play at the distributor gear. here's a way to diagnose this: watch your timing mark with a timing light. if the mark jumps around a lot, you have timing issues. this could be caused by a multitude of things--sticking valves causing uneven burn time, or the distributor gear. it only takes a few minutes to remove the distributor on the FWD 4.1s. I think you'll find that your gear is worn--especially with high mileage.

    the strange timing is fooling the computer. i think you're chasing an incorrect lead looking at the isc.

    it's probably not a vacuum leak, either, given that this would increase your idle speed. i would, however, check/replace your MAP vacuum hose.

  11. #26
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    Re: last problem with HT4100 (I hope)

    I bumped the idle stop screw to 650 RPM and with the new PCV setup it is running really good now. It still chugs a little bit at about every ~third stop light, but most of the time it settles into a ~530 RPM idle braked in gear, and I also have a lot more power and the brakes are a lot stronger. Given that the FSM says base idle should be 450-525 RPMs, that it still chugs a little bit periodically, and that the problem does not usually appear with higher RPMs (see that BMW post for confirmation), then all told I think the higher base idle is just masking the problem.

    I drove around with a vacuum gauge hooked up to the rear vacuum harness and there is no real change in pressure between cold and hot so I don't think vacuum is the problem. I could test the front harness I guess, but it is all sourced from the same manifold (and I can measure vacuum changes from the pump at the rear) so it should not be any different.

    I'll get back with the mechanic and look at the distributor gear and set the timing to +3 according to spec. I don't think this guy has the alldata software so he might have used the old-school method.

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    Re: last problem with HT4100 (I hope)

    The ISC gap should be 0.060 not .150. The min. idle stop screw should be set to 450 to 500. The idle stop screw should never hit the throttle lever with a properly working ISC. Looking at the dist. gear is a good idea. The base timing is not 3 degrees BTC it is 10. Most engines run better at 12 BTC. My factory manual for an 85 suggests looking at spark plug routing to prevent cross fire or induction from the distributor that would affect idle quality. It says wires too close to the wiring going into the distributor will cause rough idle if not in proper retaining clips. It also says the vapor canister purge valve can fail and leak causing erratic idle. This FSM you have, is it a book or ALLDATA? The way you retracted the ISC is not proper for this early engine, it is correct for my 91 but not for my 84.

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    Re: last problem with HT4100 (I hope)

    I wouldn't crank the idle stop screw up to 600, that means the closed throttle switch isn't making contact (ISC is trying to retract to settle the idle in at 530rpms). This will cause a lumpy idle. I really, really think that your problem is a worn distributor gear, or something with the timing. Set it at 10 degrees above base before pulling the distributor and see if your issue goes away. I don't know why your brakes are better now if you vacuum pump is working properly.

    carnut's suggestion to check the wires is a good one. make sure that the distributor terminals aren't wobbling. i bought a niehoff cap once, after which i changed the wires. i noticed 2 terminals were loose. never again. acdelco all the way.

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    Re: last problem with HT4100 (I hope)

    I have the actual printed FSM but the alldata directions are identical. Here is what it says about the gap

    With the ISC motor fully retracted and the throttle fully shut against the throttle stop screw, measure the gap between the ISC motor plunger and the throttle lever. If the gap is .170-.140 inches (4.3 - 3.6 mm), gap is OK; do not adjust. If the gap requires adjustment, reset to .160-.150 inches (4.1 - 3.8 mm).
    The reason base idle is set high is because the car WILL STALL when the lopey RPMs get active after the engine warms up unless I set base idle higher than normal. It is a patch until I can figure out what is causing the weird idle.

    I know about the timing. It is set to +4 right now (ie 14 base), but was set to 0 after the manifold was replaced. However like I said earlier I don't think the mechanic is aware of the intricacies of these engines so he may not have set it correctly (I don't have a timing gun here and am not going to buy another one right now).

    There could be a problem with the plug wiring, or possibly ground wiring. I have noticed a slight buzz in the cabin that comes and goes, which I am starting to investigate. Separately I have also noticed that when the engine gets hot and I step on the gas hard it gets a little noisy like it is missing, but it sounds fine when I am stopped. Both of those could point to plug wires. However the first problem predates this last service, and the second problem is new. The plug wires are less than a year old but I may replace them just to see if the buzzing goes away.

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    Re: last problem with HT4100 (I hope)

    Quote Originally Posted by awadecki View Post
    I wouldn't crank the idle stop screw up to 600, that means the closed throttle switch isn't making contact (ISC is trying to retract to settle the idle in at 530rpms).
    I calibrated the TPS too

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