98 STS RPM/Voltage Drop and Intermittent Stall
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Cadillac Seville / Cadillac Eldorado Forum Discussion, 98 STS RPM/Voltage Drop and Intermittent Stall in Past Cadillac Vehicle Discussion; Symptoms: When I'm coasting or braking, usually 35 MPH or less, the RPMs will randomly drop from about 1000 to ...
  1. #1
    98cadillac is offline Cadillac Owners Fanatic
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    98 STS RPM/Voltage Drop and Intermittent Stall

    Symptoms:

    When I'm coasting or braking, usually 35 MPH or less, the RPMs will randomly drop from about 1000 to about 400, and the voltage will also drop from about 14.2 to 12.7, then recover quickly. About once per trip, it does not recover and the car stalls. The car usually starts right back up, but a couple times it would start right up then die immediately. It seems to be somewhat worse when making a sharp or long right hand turn (like a downhill exit ramp). When I have cruise control on at highway speeds, there is not a hint that anything is wrong.

    Details:

    -It can happen about once every 3-4 seconds at times
    -DOES NOT EVER happen when my foot is on the accelerator
    -There is no bucking or jerking while accelerating
    -No apparent warning that it is going to stall

    What I have done so far:

    -Replaced the battery (misdiagnosed as being bad)
    -Had the alternator tested (came back as being good)
    -Replaced the spark plugs with AC Delco brand plugs
    -Cleaned the throttle body
    -Replaced the IAC valve with a used one from another car and cleaned it
    -Checked the fuel pressure at the rail and came up with 45-50 steady PSI (car off, key in ON position, released pressure and repeated 3 times)
    -Checked the transmission fluid level (used proper procedure with transmission warm/engine idling. etc. Just below top line of "Full hot".)
    -Checked the power steering fluid level (added some)

    I have pulled the following codes:

    AMP:

    -Current: NONE
    -History: B1327, U1255, U1064, U1128

    DDM:

    -Current: NONE
    -History: U1301

    IPC:

    -Current: NONE
    -History: B1652, B1983

    IPM:

    -Current: NONE
    -History: B1344

    PCM:

    -Current: P0135, P0146, P0147
    -History: P0134, P0603, P1258, P1599, P1611

    RSS:

    -Current: C1780
    -History: C1326, C1658, C1715, C1716, C1727, C1735, C1761, C1762, C1783, C1784, C1788

    IRC, DIM, RFA, RIM, SDM, VTD, MSM, TTM: NONE

    As of this point, I'm not even sure which is the symptom and which is the problem. In other words, I don't know if the engine speed is dropping due to the voltage dropping, or if the voltage is dropping due to the engine speed dropping. There does seem to be about 1/2 second or less of a delay for the voltage to drop after the RPMs drop.

    Although I would describe myself as a "capable backyard mechanic", I am by no means an expert and I'm sure my knowledge and ability is far exceeded by many of the forum regulars around here.

    I apologize for the long post and thank you for your time!

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

    This sounds suspiciously like a failing CKP, but no P 0335, P 0336, P 0385, P 0386 codes......... and O2 sensor codes should not cause intermittent stalls.

    The stall event occurs, the alternator slows radically, voltage drops to whatever the battery can punch out with the existing electrical load at that instant.

    Please - always post code definitions - we don't have them all memorized (yet) ............


    http://myweb.accessus.net/~090/dtcobd2.html

  4. #3
    98cadillac is offline Cadillac Owners Fanatic
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    Re: 98 Seville STS RPM/Voltage Drop and Intermittent Stall

    Thank you for the reply.

    Codes (with definitions this time):

    AMP:

    -Current: NONE
    -History: B1327 (Vehicle system voltage below 9.0 volts), U1255 (Class 2 Communication Malfunction (Serial Data Line Malfunction)), U1064 (Loss of Communications with DIM), U1128 (Loss of Communications with IRC)

    DDM:

    -Current: NONE
    -History: U1301 (Class 2 Short to Battery)

    IPC:

    -Current: NONE
    -History: B1652 (Loss of Keep Alive Memory (KAM)), B1983 (Device Power Circuit Low)

    IPM:

    -Current: NONE
    -History: B1344 (HeaterDefrost/AC Door Movement Fault)

    PCM:

    -Current: P0135 (Heated Oxygen Sensor (HO2S) Heater Performance Bank 1 Sensor 1), P0146 (Heated Oxygen Sensor (HO2S) Circuit Insufficient Activity Bank 1 Sensor 3), P0147 (Heated Oxygen Sensor (HO2S) Heater Performance Bank 1 Sensor 3)
    -History: P0134 (Heated Oxygen Sensor (HO2S) Circuit Insufficient Activity Bank 1 Sensor 1), P0603 (Control Module Long Term Memory Reset), P1258 (Engine Coolant Overtemperature - above 268°F - Protection Mode Active), P1599 (Engine Stall or Near Stall Detected), P1611 (Loss of CVRTD Serial Data)

    RSS:

    -Current: C1780 (Loss of Steering Position Signal)
    -History: C1326 (Battery Out of Range), C1658 (RSS Control Module Calibration Fault), C1715 (Right Front Damper Actuator Short Circuit to Battery), C1716 (Right Front Damper Actuator Short Circuit to Ground), C1727 (Right Rear Damper Actuator Open Circuit), C1735 (Compressor Relay Short to Battery), C1761 (Right Front Position Sensor Input Fault), C1762 (Left Rear Position Sensor Input Fault), C1783 (ICCS2 DL Left Output Short Circuit to GND), C1784 (ICCS2 DL Right Output Short Circuit to Bat), C1788 (Damper Control Relay Short to Bat)

    I replaced my CKP sensors about 3 years ago or so when I was having an issue with the car being hard to start and the tach occasionally being dead when it did start. I'm not experiencing those symptoms now; could the sensors have gone bad in that amount of time?

    Could there be a bad spot on the throttle position sensor that is telling the IAC valve that the throttle is open when it really is closed? Would there be any other symptoms if that were the case?

  5. #4
    Ranger's Avatar
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    Re: 98 Seville STS RPM/Voltage Drop and Intermittent Stall

    Low voltage can create havoc in the electrical system. Since it is a historical code, I'd clear all the codes and pull them again after you have the problem again and see what returns. I'm guessing the B1327 (Vehicle system voltage below 9.0 volts) is from the RPM drop or stall.

    P1258 is rather disturbing.

  6. #5
    rodnok01's Avatar
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    You can check the tps easy enough. Don't think it's your problem, as usually it's more random. Seems to be a wiring issue, hard to see all those codes about loss of comm and not think wiring.

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

    Quote Originally Posted by Ranger View Post
    Low voltage can create havoc in the electrical system. Since it is a historical code, I'd clear all the codes and pull them again after you have the problem again and see what returns. I'm guessing the B1327 (Vehicle system voltage below 9.0 volts) is from the RPM drop or stall.

    P1258 is rather disturbing.
    P1258 is from when it overheated a few months ago when my water pump belt came off the pulley. The belt and pulley have since been replaced.

    I am dreading a wiring issue, as it will be very difficult for me to locate if that is the case.

  8. #7
    rodnok01's Avatar
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    Easiest route is to replace the entire frt wiring... if in doubt rip it out. No way to tell what was damaged and where. Sure it will take some time but peace of mind would be worth it

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

    Quote Originally Posted by rodnok01 View Post
    Easiest route is to replace the entire frt wiring... if in doubt rip it out. No way to tell what was damaged and where. Sure it will take some time but peace of mind would be worth it
    Depending on what you mean by "the entire frt wiring", I don't think I'm about to take the time or expense to replace all of the wiring and wiring harnesses in the front of my car. In addition to the many hours of time, the parts alone would probably cost a couple thousand dollars (if they are even available anymore).

    Perhaps I am misunderstanding your post.

  10. #9
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    Re: 98 Seville STS RPM/Voltage Drop and Intermittent Stall

    Have you noticed if it happens more when the car is cold or at operating temp?

    I agree with Ranger, clear and road test then re-check codes.

    Just my $0.02

  11. #10
    rodnok01's Avatar
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    Re: 98 Seville STS RPM/Voltage Drop and Intermittent Stall

    No they are prob not available, but salvage yards sell stuff like that pretty cheap and you can pick and choose the best one's and install at your leisure. I bet you could do it over a weekend.

  12. #11
    98cadillac is offline Cadillac Owners Fanatic
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    Re: 98 Seville STS RPM/Voltage Drop and Intermittent Stall

    Quote Originally Posted by Tankboy40 View Post
    Have you noticed if it happens more when the car is cold or at operating temp?

    I agree with Ranger, clear and road test then re-check codes.

    Just my $0.02
    It definitely happens more when the car is at operating temp.

    As far as the current codes, the ones that it shows after the original codes were cleared are:

    AMP:

    -Current: NONE
    -History: U1064 (Loss of Communications with DIM)

    IPC:

    -Current: NONE
    -History: B1652 (Loss of Keep Alive Memory (KAM)), B1983 (Device Power Circuit Low)

    IPM:

    -Current: NONE
    -History: B1344 (HeaterDefrost/AC Door Movement Fault), B1652 (Loss of Keep Alive Memory (KAM))

    PCM:

    -Current: P0135 (Heated Oxygen Sensor (HO2S) Heater Performance Bank 1 Sensor 1), P0147 (Heated Oxygen Sensor (HO2S) Heater Performance Bank 1 Sensor 3)
    -History: P0134 (Heated Oxygen Sensor (HO2S) Circuit Insufficient Activity Bank 1 Sensor 1), P0603 (Control Module Long Term Memory Reset), P1599 (Engine Stall or Near Stall Detected)

    RSS:

    -Current: C1780 (Loss of Steering Position Signal)
    -History: C1715 (Right Front Damper Actuator Short Circuit to Battery)

  13. #12
    98cadillac is offline Cadillac Owners Fanatic
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    Re: 98 STS RPM/Voltage Drop and Intermittent Stall

    Today as I was driving in a town where the speed limit is 25 MPH in most places, it stalled on me three times.

    Since my last post, I have removed the brake booster vacuum hose and verified that there was a lot of vacuum by holding my thumb over the end, added fuel injector cleaner, and swapped the throttle position sensor with a spare, but the RPM dip keeps happening.

    I was driving on a slight downhill road today at around 25-30 MPH and I could pretty much time it:

    1000 RPM - three seconds - dip to 500 RPM and recover to 1000 RPM - three seconds - dip to 500 RPM and recover to 1000 RPM, etc.

    What I can't figure out is why it never happens if I have my foot even slightly pressed on the accelerator, and why it accelerates so smoothly without a hitch. I don't even know where to look next.

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

    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

  15. #14
    Ranger's Avatar
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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.
    It does.

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

    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

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