Rough Idle w/ Brake Booster Loss
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HT4100 4.1, 4.5, 4.9 Discussion, Rough Idle w/ Brake Booster Loss in Cadillac Engine Technical Discussion; I started this topic in an old thread but am moving it here since it's become apparent that this is ...
  1. #1
    Kiriken is offline Cadillac Owners Member
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    Rough Idle w/ Brake Booster Loss

    I started this topic in an old thread but am moving it here since it's become apparent that this is a separate issue for a separate subforum.

    Synopsis of the issue with my 1985 Eldorado as it was:

    - Hunting/Surging at idle (Drive + brake) after things warmed up (usually a ~10 mile freeway drive).
    - Stalling sometimes at stoplights or when doing parking that required reverse (again after warming up).
    - Smooth idle in Park (I started putting the car in Park at stoplights to avoid stalling).

    Carnut's reply:

    Quote Originally Posted by carnut
    The torque converter cannt cause engine surging and stalling if the harness is disconnected. I think you have idle speed motor issues. The ISC motor is attached to the driver side of the throttle body as you know and extends/ retracts to maintain an idle within 50 RPM regardless of load on the engine, like parking turning in a parking lot or A/C on etc. Do you have the factory service manual? if not do a search for adjusting ISC and TPS switches here.
    Thank you for the advice, I now agree that it's probably not caused by the torque converter. I do have the factory service manual but my copy is missing the ISC adjustment pages! I read the procedure on the forums but have been a little afraid to do it (the instructions were a bit rough and I thought I read something about risking frying the ECM) and at this point I'm also not convinced it's the culprit. I had though perhaps the EGR was to blame but after removing it and checking for deposits/blockage I found the passage was very clean and the plate seemed to function as it should (I just wanted to cross that off of the list).

    Driving around town has been perfectly fine aside from some slight issues at idle, but last night I foolishly took it on a 200 mile trek (leaving the manual behind so I don't have it with me to reference). Well on the way there's one long hill with a steep incline that also requires me getting in the very right lane and because the car maxes out around 40mph with the pedal to the floor. This particular time, I noticed it was running pretty rough when I got to the top but there wasn't much I could do... It smoothed out and after driving around 50 miles further I hit traffic. That's when I noticed the rough idling but this time different. Rather than hunting and surging, it was just very rough, perhaps with a slight rhythm. To make matters worse the dash Brake light came and went as I navigated the traffic. The brake pedal was hard when the light was on and I'm under the impression I was manually applying all the force. At times I would apply the brakes manually and apply a bit of gas just to juice the booster into helping me out.

    Now my thoughts go to vacuum... I did a rough visual inspection of the lines and there's no painfully obvious rot/holes. I have some WD-40 handy to inspect the lines with but I haven't gone ahead and done so yet. I'm thinking either the booster itself or the lines aren't sealed? The ISC seems to be doing its job, I can see the plunger moving in and out to compensate for power requirements. The engine still fires up perfectly fine, seems to idle smooth for a few minutes but now the rough idle also occurs in Park as well (Park was untouched before).

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  3. #2
    cadillac_al is offline Cadillac Owners Connoisseur
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    Re: Rough Idle w/ Brake Booster Loss

    ISC doesn't cause lack of power on hills. It sounds like your catalytic converter is clogged up.

  4. #3
    Submariner409's Avatar
    Submariner409 is offline If it won't run, install a big stereo
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    Re: Rough Idle w/ Brake Booster Loss

    A leaking power brake booster diaphragm is a vacuum leak - an it can be sizable. Vacuum leak = poor idle and poor driveability.

  5. #4
    carnut is offline Cadillac Owners Master
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    Re: Rough Idle w/ Brake Booster Loss

    This car has a large belt driven vacuum pump. The hose from it goes down by the exhaust on the driver side to a metal manifold, then to the booster. You need to measure the vacuum the pump is putting out. A vacuum leak will cause a high idle, not rough.

  6. #5
    Kiriken is offline Cadillac Owners Member
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    Re: Rough Idle w/ Brake Booster Loss

    Quote Originally Posted by cadillac_al View Post
    ISC doesn't cause lack of power on hills. It sounds like your catalytic converter is clogged up.
    Yeah I didn't think the ISC was related to that. I've had a lack of power on hills for some time now, though maybe that's just how underpowered the HT4100 is. I suspected headgasket failure though this is one of the GM GoodWrench rebuilt HT4100 so from my understanding that's less likely. Clogged cat sounds like a possible culprit for power loss (but probably not brake loss & rough idle), I'll have that checked if my car makes it out of the rut it's currently in.

    Quote Originally Posted by Submariner409 View Post
    A leaking power brake booster diaphragm is a vacuum leak - an it can be sizable. Vacuum leak = poor idle and poor driveability.
    I looked up a procedure to determine brake booster vacuum leaks and followed the tests here: http://www.roadkillcustoms.com/hot-r...oster-test.asp

    1 - (Pass) Pedal drops when car started.
    2 - (Pass) Pedal drops easily when pressed first time, slightly harder and harder after that.
    3 - (Pass) This one is slightly unclear but the pedal feels the same after holding it down with the car powered off for 30 seconds.

    Check Valve - I disconnected the brake booster vacuum line and was unable to blow into the hose (yes I put my lips on the filthy vacuum line :S). I was able to draw air out by sucking so the valve is working properly.

    These tests make me want to rule out the brake booster as the culprit.

    Quote Originally Posted by carnut View Post
    This car has a large belt driven vacuum pump. The hose from it goes down by the exhaust on the driver side to a metal manifold, then to the booster. You need to measure the vacuum the pump is putting out. A vacuum leak will cause a high idle, not rough.
    So it appears the vacuum pump has a metal line which wraps around the engine. There are 2 outlets on the driver's side, one going to the brake booster and the other to a vacuum hose that trees off to various places that require vacuum. I disconnected the tree hose and noticed the high idle you mentioned. Plugging the vacuum tree outlet with a stopper lowered the idle to normal but it still appeared rough and the brake light was on. I doubt there's a vacuum leak between the pump and the hole I plugged (I wasn't able to check too carefully but it didn't seem like there was much between those two points).

    I don't have a vacuum gauge but I'll see if I can't get the suction measured somewhere. In the meantime, I'm going to check the timing on a lark and try to get some RPM readings. I have a feeling whatever is wonky in the engine that's causing the rough idle is dropping the RPMs too low to support enough vacuum for the brake booster (rather than a vacuum leak). I could be wrong though...

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    cadillac_al is offline Cadillac Owners Connoisseur
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    Re: Rough Idle w/ Brake Booster Loss

    What about checking for codes? Is the check engine light on? A clogged catalytic will kill your vacuum but I have no experience with vacuum pumps. Vacuum pumps sound like some kind of Band-aid to me.

  8. #7
    Kiriken is offline Cadillac Owners Member
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    Re: Rough Idle w/ Brake Booster Loss

    No codes unfortunately! No check engine lights either! Only the red brake light comes on intermittently, usually around the time the brake pedal is hard.


    I wasn't aware a plugged cat could cause a rough idle. I always noticed a buzzing from under the car after long drives and had my suspicions about that.


    I tried to check the timing today but I should have guessed that a rough idle would cause it to bounce around too much to check.


    I'm out of town for a week or two but I'll update this thread when anything new develops. Thanks for the tips/help everyone, I appreciate it.

  9. #8
    carnut is offline Cadillac Owners Master
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    Re: Rough Idle w/ Brake Booster Loss

    The red brake light is probably due to weak vacuum to the booster. You have a sensor in the booster that senses low vacuum and will turn on that light. Again, you need to know what that vacuum pump is putting out. Should be around 18 and reasonably steady.

  10. #9
    Kiriken is offline Cadillac Owners Member
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    Re: Rough Idle w/ Brake Booster Loss

    I'm back and had some time to fiddle around.

    I tried to yank the catalytic converter out and my plan was to put some temporary piping in to test if that was the root cause. I ended up removing everything in the exhaust system behind the cat (I had a muffler welded on so that's just one big piece now) in order to have room to pull the cat off the Y pipe. The bolts were a pain to remove but the trouble was that once the bolts were out, the cat wouldn't budge. The air pipe wasn't holding it back either, I was going to disconnect it on the engine bay side but I noticed the mounting bracket wasn't attached to anything so there was plenty of give. I believe the exhaust U brace might have been tightened enough to crimp the cat pipe over the exhaust. I was able to take a peak into the "out" side of the cat and it looked perfectly intact. Obviously the "in" side of the cat would be the clogged part but I thought that was worth mentioning. Today's Sunday so most mechanics are closed, hopefully tomorrow I can squeeze in and get the exhaust backpressure measured.

    I purchased a vacuum gauge and tried to get some vacuum readings. I wasn't quite sure how this vacuum pump works so I think I'm going to try and get another reading later because I may be missing something. I found a diagram of the system which I've included below.



    I yanked #8 off of the pump and expecting vacuum to come out of the pump I noticed it was coming instead out of #7 which I believe is an inline filter. I measured the vacuum coming out of the filter and it was fluctuating between 7 - 10 inHg. in sync with the engine's idle fluctuation. I reconnected that line and checked the vacuum at the line near the booster. Same reading. I made a short video just to demonstrate the fluctuations and how revving affects vacuum (it increases, presumably because the belt is moving the pump harder). Brake light didn't go away when I revved twice it in the video but it does after a bit of driving. In the beginning and after 28 seconds in is how it idles without my interference.



    Link to the video is: https://www.youtube.com/watch?v=GfLOYrklSCA in case the embedding doesn't work for anyone.

    I believe I should be checking the vacuum coming out of #10 instead of #8. I'll do that in a bit and report back.

  11. #10
    carnut is offline Cadillac Owners Master
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    Re: Rough Idle w/ Brake Booster Loss

    You need a vacuum pump. Should be at least 18. Verify that in your service manual. The diagnostic test is in there!

  12. #11
    Kiriken is offline Cadillac Owners Member
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    Re: Rough Idle w/ Brake Booster Loss

    Just checked the vacuum on #10 (the other port on the vacuum pump). It started around 15 but after warming up returned to the 7 - 10 inHg which was what the other port was giving me. If both the inlet and outlet ports are giving me the same reading, that makes me think you might be right, the vacuum pump doesn't seem to be doing any work...

    When I said I was "back", I meant back to my car and the forum but still far from home (where I left my manual -last time I ever do that). I'll look through the diagnostics when I get home to my manual and order a reman pump if I still think it's the problem.

    I'm wondering if it's even possible that my torque converter problems could be caused by inadequate vacuum. I did some searching and read somewhere that the 325-4L uses a vacuum switch for the torque converter lockup. I'm not sure how intertwined these systems are but if my issue has been vacuum all along then I may hit the goldmine if/when I replace the pump. I'll report back when I have anything to update on.

  13. #12
    Kiriken is offline Cadillac Owners Member
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    Re: Rough Idle w/ Brake Booster Loss

    Short update:

    I finally drove home (~ 200 miles). When I arrived the car died after pulling up to the curb. Tried to start it 20 minutes later and after 3 tries of the starter wouldn't fire up. Pressed down on the gas a bit, tried again and it caught but was pretty rough. After it sat for a while I checked the factory service manual and ran through the vacuum diagnostic. I was doing it properly last time but this time the readings were between 1 - 3 inHg. Waaay too low.

    Just ordered a reman vacuum pump and I'm hoping that's the issue. Strange how the only mention of the vacuum pump in the service manual is in regards to brake booster. No symptoms listed for failing pump other than low vacuum readings.

    Also I ran through the sensor readings and verified things were within spec. Everything looked reasonable. Idle RPMs were a tad higher than I expected ( ~700, fluctuating +/- maybe 50). Perhaps it's trying to rev up some vacuum.

    I'll update once the pump gets here and I've got it on.

  14. #13
    carnut is offline Cadillac Owners Master
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    Re: Rough Idle w/ Brake Booster Loss

    The pumps primary function IS to provide adequate vacuum for the brake booster under all conditions. If the vacuum test is that low, you must have a really hard brake pedal, An inop. pump will not cause a rough running engine.

  15. #14
    Kiriken is offline Cadillac Owners Member
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    Re: Rough Idle w/ Brake Booster Loss

    The reman vacuum pump was around $50 from Amazon but the free shipping took forever. I swapped the vacuum pump out today. Note to others who do this: Do not stick anything in the holes on the pulley to keep it from turning while removing the pulley from the pump, I bent mine up pretty easily doing that. Anyway, I was able to bend it back so the damage didn't cause any mechanical issues. The new pump is giving me around 25 inHg at idle which is awesome. I believe the service manual said it should put out 22 inHg but I don't have it directly in front of me to reference now. Idle is a bit high which could explain the small additional vacuum. In any case my brakes are back to safe operation. Thanks carnut

    The bad news is that the vacuum pump seems to have been the cause of the brake issues but not much in relation to the rough idle (as I expected after reading the last post). I noticed a code 34 (high MAP sensor reading I believe) set on my test drive after replacing the vacuum pump but that was probably caused by having the vacuum lines wide open momentarily when I was measuring the vacuum produced by the new pump. After I noticed that, I disconnected the battery to reset the ECM entirely and have it relearn anything that could have been thrown off by the low vacuum. If the code comes back I'll look into that more closely.

    It's still hard starting after any drive (even a short, 5 minute one). I have to press a bit on the gas pedal as it cranks the starter or it will put-put-put and stall. I'm thinking there might be some misfiring based on the sound of the engine when it is running and the somewhat heavy gas fume smell in the exhaust. I picked up an inline "lightbulb" spark plug tester and I plan on trying to get a better idea of what's going on with the ignition system tomorrow.

  16. #15
    Kiriken is offline Cadillac Owners Member
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    Re: Rough Idle w/ Brake Booster Loss

    The light on my inline test sparkplug was very weak on the few sparkplugs I checked and not visible on others (didn't check all of them). I went ahead and did an ignition tuneup figuring the spark plugs and wires had probably 35,000 miles on them and the distributor cap and rotor were nearing 20,000 so they would probably be good candidates for replacement soon anyway.

    New rotor, distributor cap, wires and plugs. Rotor and distributor cap looked almost new but I replaced them anyway. Plugs on driver's side had oil on the threads so I replaced the valve cover gasket while I was at it. I was too afraid of over-tightening so when I started it up I got a cloud of smoke after a minute of oozing oil . Hopefully it's tightened down enough now to prevent that. I believe I can redo it if I must with the spare gasket since I only did the one side (hopefully it's identical if I "flip" the remaining gasket). Here's what the plugs looked like

    Passenger Bank


    Driver Bank


    I don't think they look too bad. I caught some arcing a few months ago which is likely what caused that blackened part on at least one of the ceramic insulators. Had to slide the wire boot around to make it stop. Tip for anyone replacing sparkplug wires. Avoid the Omnisparks from O'Reilly. I picked them because they were the only ones in stock and had a < $20 price tag but regretted it based on the very poor quality. When removing the wires (by firmly grasping the boot!) I had almost half of them leave the metal connector attached to the plug and rip off of the wire. I went with "BWD Select" parts this time for everything except the standard AC Delco spark plugs. I can tell you that the BWD Select wires are much much more sturdy than the Omnisparks and a lot easier to work with! I was thorough this time with dielectric grease at electric terminals and added a liberal amount of anti-seize lubricant to the spark plug threads.

    Fired it up and it ran slightly better, but not by much. After I handled the aforementioned untightened valve cover leak, I tried to fire it up again but got a few seconds of very low idle until it sputtered and died out. Subsequent tries resulted in the same until I gave it some gas (little by little, too much throttle and it stalls out). Checked codes. Code 30. ISC Circuit Error. I unplugged the ISC and it seemed to start without a hitch, however I would describe the idle as "runaway", it seemed to just keep going up so I turned it off. Carnut, you might have been right on the money with your initial advice. I'm sorry I didn't take a closer look at that! My manual is missing most of the ISC-relevant pages but I've found a few posts on the forums that describe the procedure(s). I'll try to check the ISC tomorrow but I have a feeling that if it's throwing codes and acting funny all of the sudden after not being touched for years, it's likely failing.

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