98 STS RPM/Voltage Drop and Intermittent Stall - Page 2
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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 mike5514 I dont know if a 98 has a Idle Air Control Valve like my 97 does. ...
  1. #16
    98cadillac is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    Quote Originally Posted by mike5514 View Post
    I dont know if a 98 has a Idle Air Control Valve like my 97 does. I would definitely zero in on that baby. The pintle gets gummy and will stick you can clean it

    You can remove the connector and test the resistance between the IAC terminals a to c, a to d, b to c and b to d. Resistance should be infinite if not the IAC is faulty.

    The IAC is controlled by the power train control module do a visual check of the wires. Check for a restricted air intake system and/or a vacuum leak. mike
    I cleaned the unit I swapped in from another car, but I didn't know about checking for resistance. I'll do that when I have a chance today. I guess I'll have to remove the split tubing and inspect the wires as well.

    Quote Originally Posted by Mafiaman111 View Post
    I have the exact same problem, no codes and only does it between 20-35 MPH only, RPM goes down to 400 then back up with my foot off the gas but has never stalled and i have bucking and jerking under light load, I think my Crank Postion Sensors might be going south ans i think yours are too
    That's the next thing I'll have to replace if there is nothing wrong with the IAC valve. I've been putting it off because I can't afford to go the route of "diagnosis by replacement", and also because the crank sensors are a pain to replace due to their location (based on the last time I replaced them a few years ago).

  2. #17
    98cadillac is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    The IAC valve appears to be fine.

    I did notice when looking under the car to see if there was any damage to the CKP sensor wiring that the front engine mount (which really gets in the way when you're trying to change the CKP sensors) appears to be broken. This is also borne out in what I feel when driving. In the factory service manual it says to check the engine mounts as part of the diagnostic steps for stalling issues - could the mount have something to do with it?

    I guess I'm going to have to try to replace the CKP sensors in the meantime and hope that fixes the issue.

  3. #18
    mike5514 is offline Cadillac Owners Enthusiast
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    Check the ground on the Ignition Control Module its the black wire going to the ground distribution center. Its on one end of the module.

    The crank and cam position sensors go to this module and then to the pcm

    Not much chance wires will go bad inside the tubing its mostly at the connectors or a bare wire ck for loose,corroded broke wires.

    the cam sensor is located at the front of the engine firewall side of the serpentine belt. Mike

  4. #19
    98cadillac is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    Quote Originally Posted by mike5514 View Post
    Check the ground on the Ignition Control Module its the black wire going to the ground distribution center. Its on one end of the module.

    The crank and cam position sensors go to this module and then to the pcm

    Not much chance wires will go bad inside the tubing its mostly at the connectors or a bare wire ck for loose,corroded broke wires.

    the cam sensor is located at the front of the engine firewall side of the serpentine belt. Mike
    Thanks for the reply, Mike. I had a spare cam sensor so I changed that (super easy) but it did not fix the issue. I'll check the ground wire as suggested.

    The CKP sensors should arrive on Tuesday (AC Delco). I'm beginning to suspect more and more that Submariner was right about the CKP sensor, since I have to crank it longer for it to start when the car warms up than I do when I first start it each day. This corresponds with the RPM/stalling issue getting worse when the car warms up.

    I'll know for sure on Tuesday or Wednesday. The only way I can figure out to get at the sensors without removing the front engine mount is to remove the oil filter (as I did a few years ago when I last changed the sensors). Fortunately I'm due for an oil change as well, so I'll do that at the same time.

  5. #20
    Mafiaman111 is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    im curious if that fixes your problem, ill be waiting for your result, and if it does fix your problem then i will know for sure that i need ones too

  6. #21
    98cadillac is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    I'll post here as to whether that fixes it or not. I sure hope it does.

  7. #22
    98cadillac is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    So I changed my crankshaft position sensors (brand new AC Delco) and...

    ...it didn't do anything at all to fix the problem.

    I was trying to figure out if there was any detail which I hadn't posted yet. When I am driving and I cancel the cruise control with the lever, I hear somewhat of a "thunk" sound. Also, when I see the RPMs on the way down, once they get to 500 or below, if I hit the accelerator to try to "catch" it, it will hesitate for a moment with no change in the RPMs, then "wake up" with a jerk like I hit the gas hard from a stop.

    Maybe there's nothing wrong and it's supposed to stall on me 4 times a day.

  8. #23
    Mafiaman111 is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    well crap now im hesitant to change the sensors on my caddy now if that wont fix the problem. this problem might be harder to find then I thought.

  9. #24
    Mafiaman111 is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    Okay, I have a good update and it may help you greatly. I decided to pull the intake and clean everything including the throttle body. I found many problems inside all the ports were completely clogged shut with very thick build up. Fixed a vacuum leak, One of the metal fuel lines was very loose and put it all back together and runs fine now

  10. #25
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    Quote Originally Posted by Mafiaman111 View Post
    Okay, I have a good update and it may help you greatly. I decided to pull the intake and clean everything including the throttle body. I found many problems inside all the ports were completely clogged shut with very thick build up. Fixed a vacuum leak, One of the metal fuel lines was very loose and put it all back together and runs fine now
    Ya know, the more you described the stalling issue, the more I was thinking Vac leak. I'm glad you found the clogged passages and the vac leak.... sounds like you might have this one knocked out.

    1 more thing tho, have you since cleared the codes, driven, and retested to see which still exist?

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

    Quote Originally Posted by Mafiaman111 View Post
    Okay, I have a good update and it may help you greatly. I decided to pull the intake and clean everything including the throttle body. I found many problems inside all the ports were completely clogged shut with very thick build up. Fixed a vacuum leak, One of the metal fuel lines was very loose and put it all back together and runs fine now
    That's good news! Where was the vacuum leak?

  12. #27
    Mafiaman111 is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    All my symptoms were jerking under light throttle, hesitation to get up to speed, engine shaking under load, rpm dropping when coasting between 25-35 MPH, when between 41-50 i could feel the engine braking and letting go if that makes any sense, sometimes the idle would bounce around then stabilize, and I always could hearing a hissing noise from the engine.

    The funny part is I never had any codes, I tested the fixes last night and remember the problems only started after it got up to operating temp. I let it coast between 25-35 MPH and it stayed steady, then I gave it light throttle and no jerking, no idle bouncing, no engine jerking. If anything I swear the car drove smooth like butter, it got up to speed like it was nothing.

    This is what I did, I removed the whole throttle body and intake. Upon inspection I found very heavy deposits of gunk in the all the vacuum lines inside the throttle body and the lip of the intake, I believe the clogged holes were brake vacuum, FPR vacuum port, MAP sensor port, also took off all the other vacuum lines like PCV Feed tude and the tube with the PCV valve on it and throttle body spacer was leaking, cleaned it and put it all back together and fired her up, she ran like a dream
    Tankboy40 and Tankboy40 like this.

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

    My car has no hesitation or jerking at all on acceleration, and the engine does not shake at all under load. In fact, when I have my foot on the accelerator, that is the only time that I have no issues at all.

    I also don't hear any hissing noise from the engine as far as I can tell. Someone suggested that there could be an exhaust leak, and since I think my front motor mount is broken, it could be putting stress on the exhaust connections, so that's probably where my next inspection will take me.

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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    Quote Originally Posted by 98cadillac View Post
    My car has no hesitation or jerking at all on acceleration, and the engine does not shake at all under load. In fact, when I have my foot on the accelerator, that is the only time that I have no issues at all.

    I also don't hear any hissing noise from the engine as far as I can tell. Someone suggested that there could be an exhaust leak, and since I think my front motor mount is broken, it could be putting stress on the exhaust connections, so that's probably where my next inspection will take me.
    IMHO I dont think the exhaust will cause that, but feel free to chime in if thats an incorrect assumption guys...

    Usually when you have an "off idle" situation, its due to a vac loss in general.

    Once you hit the gas, more vac is supplied causing the engine to smooth out.
    but that is a general statement, IDK if there are other possibilities due to being N*[COLOR="Silver"]

  15. #30
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    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

    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 ................)

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