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Cadillac Seville / Cadillac Eldorado Forum Discussion, 98 STS RPM/Voltage Drop and Intermittent Stall in Past Cadillac Vehicle Discussion; Originally Posted by Submariner409 Idle = highest manifold vacuum Opening gas pedal = decreasing manifold vacuum WOT = least manifold ...
  1. #31
    98cadillac is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    Quote Originally Posted by Submariner409 View Post
    Idle = highest manifold vacuum
    Opening gas pedal = decreasing manifold vacuum
    WOT = least manifold vacuum

    Think about it: At idle that good ol' atmospheric pressure (14.7 psi) is trying to push air into the running engine but the closed throttle won't let it move much air. As the throttle opens further, more air is pushed into the engine, lowering vacuum. At WOT there is essentially no restriction to air moving into the engine and "vacuum" drops to < 1" in a good NA intake.

    These Northstars carry about 22" of vacuum at idle, from 15" to 19" at highway steady running, and less than 1" at WOT redline. The higher the road load vacuum, the better the gas mileage.

    (Post #2 for a theory on stall/voltage drop ................)
    Thank you for the info on engine vacuum - certainly more extensive than my knowledge on the subject.

    Unfortunately, when I changed the CKP sensors, it didn't fix the issue.

    I may have put this in an earlier post, but when it stalls, there's no sputtering/shaking beforehand, and it doesn't feel like a "hiccup"; it feels more like the car was sitting in a comfortable chair and drifted off to sleep. If I wasn't so aware of the issue and glancing at the display all the time, it could easily stall on me and I wouldn't know anything happened until I went to steer or press on the accelerator.

    To add to the list of things I changed:

    -MAF sensor module - swapped from my previous working one I saved when the housing broke (not the whole unit, just the module on the side)
    -Brake booster vacuum hose

    When I tested it after changing these last night coasting at about 25 MPH on a straight, slightly downhill road, it was still doing "1000 RPM - dip to 500 RPM - recover to 1000 RPM". As it did in the past, it was doing it almost like it was on a timer. I wasn't using a stopwatch, but it seemed like it would run at 1000 RPM for 4 seconds, smoothly dip to 500 RPM and recover to 1000 RPM within the space of 2 seconds, run at 1000 RPM for another 4 seconds then repeat the cycle. It's not randomly coughing and sputtering down the road, if that makes any difference.

  2. #32
    98cadillac is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    Here's a video of my car idling; it idles pretty smoothly. It was not completely warmed up when I made the video.


  3. #33
    Mafiaman111 is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    This is what my car did before It got worse and before I fixed the problem


    http://www.youtube.com/watch?v=iml-s9-vpfw

  4. #34
    98cadillac is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    Yep... That's almost exactly what mine is doing. I guess I'll have to try taking the manifold off and cleaning it. Did you use a spray cleaner?

  5. #35
    Mafiaman111 is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    yeah cleaned it with carb and choke cleaner

  6. #36
    Mafiaman111 is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    Update my car started running like crap while I was driving it around today and what do you know the check engine light came on. I pulled the codes and P0171 and P0174 came up. Now im chasing down a vacuum leak that is causing the car to run lean on both banks

  7. #37
    98cadillac is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    Is it doing the same thing as it was before, or are these new symptoms?

  8. #38
    Mafiaman111 is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    No, these are new symptoms

  9. #39
    98cadillac is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    UPDATE - I FIGURED IT OUT!

    Last Friday, as I was considering taking out my intake manifold to clean it, I found an old post on a forum somewhere about someone having the same issue. They mentioned that they had cleaned the EGR valve, so I figured with two bolts and a connector being all that needs to come off to remove it, I might as well try.

    I held the valve in basically the same position that it is installed (meaning I didn't turn it upside down) and sprayed in the opening with throttle body cleaner (not carb cleaner). I then used a "pipe cleaner" to clean the inside, and I also worked the plunger up and down after I sprayed it.

    I had a few "false alarms" in the past thinking it was fixed, so I wanted to wait until I was sure before posting. I now have to lose the habit of constantly glancing at the tach to see if it's about to stall.

    Thank you to everyone who posted here to help me. It would be interesting to know why cleaning the EGR valve fixed it.

  10. #40
    Submariner409's Avatar
    Submariner409 is offline If it won't run, stick on fender vents
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    EGR - Exhaust Gas Recirculation - is an engine control and emissions system that bleeds a measured amount of exhaust gas back into the intake air/fuel mixture, slowing down combustion and cooling the cylinder combustion charge in the process - it reduces emissions of oxides of nitrogen - nOx. The system uses a movable, tapered valve (pintle) attached to either a vacuum or electric motor (solenoid). EGR valves get sticky or hung up due to carbon fouling, so they need to be cleaned once in a while, particularly in cars that lead a "quiet" life. The sticky valve actually dilutes the intake charge enough to cause engine slowdown or stalls - but for our cars that problem should set a P code - the valve is a monitored emissions part. If the valve is stuck shut it causes part-throttle fuel detonation - ping or rattle - from rapid combustion coupled with ignition advance - most often when accelerating moderately after a stop.
    98cadillac and 98cadillac like this.

  11. #41
    mike5514 is offline Cadillac Owners Enthusiast
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    Glad its fixed!

    The EGR valve is often overlooked during trouble shooting and it is easy to clean should be right up there with a IAC clean and check.Mike

  12. #42
    98cadillac is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    Thanks for the explanation, Submariner. Mike, I sure wish I had tried that before I spent any money on it. At least I only spent $135.00 in part replacement which was not needed. I guess I should be happy considering that there are people who have spent a couple thousand to try to fix an elusive problem.

  13. #43
    ManoViper is offline Cadillac Owners Member
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    Thank god I found this thread. I'm having the exact same problem. Has the problem come back at all? Or any further issues with your vehicle?

  14. #44
    Mafiaman111 is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    on my car the problem never came back

  15. #45
    98cadillac is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    Quote Originally Posted by ManoViper View Post
    Thank god I found this thread. I'm having the exact same problem. Has the problem come back at all? Or any further issues with your vehicle?
    Nope. Over two months with no stalling issues.

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