4.9 Will Not Idle Below 2500rpm!
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HT4100 4.1, 4.5, 4.9 Discussion, 4.9 Will Not Idle Below 2500rpm! in Cadillac Engine Technical Discussion; I have a 1992 Seville with a 4.9. When I start the car it fires right up and goes to ...
  1. #1
    skor is offline Cadillac Owners Member
    Automobile(s): 1992 Seville
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    4.9 Will Not Idle Below 2500rpm!

    I have a 1992 Seville with a 4.9. When I start the car it fires right up and goes to 2,500rpm and stays there. There are no codes.


    After reading through some threads, I assumed it was the ISC motor, so I replaced it and adjusted it according to factory shop manual instructions. It's still the same.

    The ISC motor seems to be hold the throttle just off the min idle air screw. How can it idle so fast with the throttle plates nearly closed? Do I have a monster vacuum leak? or is something else the problem?

    Thanks in advance.

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  3. #2
    kdcing is offline Cadillac Owners Fanatic
    Automobile(s): 1993 Eldorado, 4.9L, 200k miles
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    Re: 4.9 Will Not Idle Below 2500rpm!

    The idle will run away if there is a vacuum leak. On my '93 Eldorado 4.9L, I left the line for the PCV Valve disconnected and the idle ran to about 2000 rpm, however, if the PCV line is disconnected you would likely hear it suckin' air if you had your ear under the hood. Nonetheless, have a look at the vacuum lines.

    If the vac lines check out, there are a few things that can be checked on the ISC via the HVAC control unit. With key in the ignition and ignition turned to "On" (engine off), press the warm and off...the same way you checked for codes. Get to the "PCM?" message...press "Yes" by pressing the "fan up" to enter PCM functions. Notice if the "Lo" icon is iluminated on the HVAC controller. "Lo" should be illuminated w/ your foot NOT on the gas pedal, "Lo" should not be illuminated w/ your foot on the gas pedal....this tests the internal switch in the ISC motor.

    Next press "Fan Down" button until you get to "PCM Override"...then press "Fan Up" to enter the override. Press "Fan Up" until PS03. When PSO3 shows on the digital readout, press the "Warm" button, then the "Cool" button. Have a helper look under the hood (engine off)...your helper should see the ISC extend, then retract with every press of the "Warm" and "Cool" button.

    If the tests do not pass, then likely 1 of a few things are wrong...It could be:

    1. I've incorrectly listed the procedure (the memory isn't what it used to be), or

    2. The ISC motor is bad (yea...I know you just replaced it), or

    3. The ISC was not properly connected at the connector, or

    4. The cable for the Cruise Control or Gas Pedal is not properly attached to their cams at the throttle body and are holding the throttle open, or

    5. The ISC installation procedure was not peformed correctly, or

    6. I need another Pabst Blue Ribbon.

    One other note, during the post-installation ISC relearn procedure, I found it absolutely necessary to get the idle down to less then 450rpm before the ISC behaved correctly. I searched everywhere on the internet and found this requirement hidden in only 1 post. After getting the idle down to 425 and after redoing the ISC relearn procedure, the engine idled and ran flawlessly.

  4. #3
    skor is offline Cadillac Owners Member
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    Re: 4.9 Will Not Idle Below 2500rpm!

    KDCING, thanks for the reply.

    Here's what happened.

    The car has 62K miles. Recently it developed a jumpy, high idle, between 1200-1250 rpm. A mechanic friend told me to replace the isc motor, since these are common problems.

    After installing the new motor, it ran at 2500 and stayed there. I disconnected the battery, and reconnected, and then did the isc motor adjustments over again. Now the car runs the same as it did before I installed the new motor -- idles between 1200 and 1250 rpm! Obviously, there was nothing wrong with the old isc motor.

    Here's what I checked.

    1) Throttle plates. They are clean.

    2) Binding of throttle and CC cables. Cables are good and do not bind.

    3) Vacuum connections at throttle body. All the connections at the throttle body are good and tight. I found some brittle hoses which I replaced. In addition, I removed the vacuum hoses one by one -- except for MAP -- and plugged the connections at the throttle body -- this made no difference in the way the car idled. I sprayed WD-40 around the base of the throttle body looking for vacuum leaks -- this came up negative as well.

    The car acts like it has a vacuum leak, but if that is the case, I can't find it.

    What else can cause this condition? Can EGR, bad MAP or cam sensor be bad? If so, why are there no codes?

    How do you get the idle down to 450 for the idle relearn?

    Any ideas will be greatly appreciated.


    BTW, I haven't seen Pabst Blue Ribbon in years, where do you buy it?

  5. #4
    HUF's Avatar
    HUF
    HUF is offline Cadillac Owners Connoisseur
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    Re: 4.9 Will Not Idle Below 2500rpm!

    scor,

    Looks like you know what you are doing... It's really strange tha there are no codes. Have you checked the TPS? If nothing helps, I would disconnect ISC motor and drive without it setting minimum air at some 550-600 RPM.

    Just in case... Because the problem is most likely electrical, check all your cables, grounds. Particularly one on the engine block by the starter. Sometimes they look good, but disconnecting, cleaning reconnecting never hurts.

  6. #5
    skor is offline Cadillac Owners Member
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    Re: 4.9 Will Not Idle Below 2500rpm!

    Quote Originally Posted by HUF View Post
    scor,

    Looks like you know what you are doing... It's really strange tha there are no codes. Have you checked the TPS? If nothing helps, I would disconnect ISC motor and drive without it setting minimum air at some 550-600 RPM.

    Just in case... Because the problem is most likely electrical, check all your cables, grounds. Particularly one on the engine block by the starter. Sometimes they look good, but disconnecting, cleaning reconnecting never hurts.
    I thought that the ground may be bad, so I ran a jumper cable directly from the engine block to the battery ground -- no difference.

    When I go into diagnostics and look at TPS values, it looks good. The TPS holds steady with foot off the gas pedal and rises and falls smoothly when I work the gas pedal -- no weird jumps or spikes.

    Next I'll try to pull the EGR and block of the openings and see how it idles.

  7. #6
    skor is offline Cadillac Owners Member
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    Re: 4.9 Will Not Idle Below 2500rpm!

    Update, I pulled the EGR and installed a steal plate and gasket over the EGR ports to block them off.

    Results: The same, the car still idles at 1200rpm. Next I'm going to remover the throttle body and give a good scrubbing. I'll reinstall everything with new gaskets and torque everything to spec.

    Cadillacs new motto should be: "Cadillac, The Cars That Defy All Attempts at Repair!".

  8. #7
    skor is offline Cadillac Owners Member
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    Re: 4.9 Will Not Idle Below 2500rpm!

    Here you go, pics of the freshly scrubbed throttle body. I'll try and get everything reinstalled later tonight, but I have a feeling I know how this is going to turn out -- the idle will still be high.

    If there is a secret to this, I wish someone would tell me.




  9. #8
    HUF's Avatar
    HUF
    HUF is offline Cadillac Owners Connoisseur
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    Re: 4.9 Will Not Idle Below 2500rpm!

    Sounds like ECM thinks it is 10F outside... What if coolant temp sensor failed? Just a guess...

  10. #9
    kdcing is offline Cadillac Owners Fanatic
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    Re: 4.9 Will Not Idle Below 2500rpm!

    That throttle body looks great. It's easy to clean the top of the throttle plates and not see the carbon buildup on the underside of the throttle plates...over time, the carbon buildup will cause the throttle plates to open up slightly and cause a high idle. Liquid/Spray carb cleaner is a must in this area. Cleaning the 2 EGR tubes, located just below the throttle body, is a good thing to do since they are exposed w/ the throttle body removed...unfortunately, cleaning the egr tubes will probably not fix a high idle issue.

    I believe that we need to determine 2 conditions:

    1. Does the ISC's internal switch work? If it is stuck closed, then the ISC will never retract, resulting in a high idle. I know the symptoms (i.e. 1200 rpm idle) are the same between the 2 ISC's, so it could be the wire that carries the internal switch's signal could be damaged. Yesterday's post described the way to use the HVAC control unit to determine the switch's signal by monitoring the "Lo" indicator.

    2. Does the ISC is move? If it is frozen @ an extended position (again more likely the ISC wiring, since the ISC has already been swapped.) then the Idle would remain high. Follow the procedure from yesterday's post or get more hands-on and perform the following (this procedure will actually test both the switch internal to the ISC and the ISC itself):

    A. With engine running, grab the throttle cam with your hand and increase engine speed to 2200rpm or higher. With your other hand depress the ISC's plunger (i.e. you'll be closing the switch inside the ISC). With the 2200rpm engine and with the ISC plunger depressed, the ISC should slowly retract the ISC plunger. If the ISC DOES NOT retract, then we need we know the high idle speed is related to the ISC and not vacuum lines or other stuff. If the ISC's plunger DOES retract, then let it retract as far as it will go and disconnect the ISC wiring at its connector (Drink some PBR and you'll grow a 3rd hand to do this!). When you let go of the throttle cam, the throttle plate should now be stopped by the idle speed screw, not the ISC plunger. To get the throttle down to 450, back the idle speed screw out.

    Btw, I hope you don't mind, but I printed the pictures that you posted of the clean throttle body. The pictures are now framed and are on top of our piano next to the pictures of my wife and kids! That throttle body is beautiful!

  11. #10
    skor is offline Cadillac Owners Member
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    Re: 4.9 Will Not Idle Below 2500rpm!

    kdcing,

    I didn't quite get her all back together again tonight, so I have to finish up tomorrow morning and see what I got.

    I checked both ISC motors for switch function via the HVAC display as you indicted, both checked out good.

    I still think this is something other than ISC motor. The ECM is seeing something that is making it hold that throttle open.

    BTW, you are welcome to the pictures of my throttle body, provided that your wife doesn't mind having throttle body porn around the house.

    HUF, I thought of that, the coolant temp readout on the dash is the same as always, so I assume that's not the problem. What if it were a faulty Manifold Air Temp sensor?

  12. #11
    cadillac_tech is offline Cadillac Owners Fanatic
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    Re: 4.9 Will Not Idle Below 2500rpm!

    Oh to have this in my hands.. This stuff is usually so simple once it is all in perspective. How could you have the ISC adjusted if the idle never got below 550 - 450 range. It is hard for everyone to read what you've done and give a complete answer without hands on.

  13. #12
    noahsdad's Avatar
    noahsdad is offline Cadillac Owners Connoisseur
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    Re: 4.9 Will Not Idle Below 2500rpm!

    cadillac_tech is so right about simple stuff in perspective. I drove myself nuts with this problem on my '87 until discovering I'd completely left off a major vacuum hose.

    This sounds like a vacuum leak or misadjusted TPS. If the ISC is functioning normally, these are the next in line. EGR/MAP issues usually cause a stumble or loss of idle rather than a surge, and will normally set codes. Triple check all vacuum connections to the TB, and be sure the base gasket is not misaligned. Then move on to TPS.

    I'm a bit confused though. The title of this thread (and your first post) says 2500 RPM, then a couple of posts back you say that disconnecting the battery and resetting the ISC brought it to 1200. That's quite a drop. While that doesn't eliminate the possiblity of a vacuum leak, it at least tells you that the ISC circuit is somewhat fuctional.

    When you checked the TPS parameter through the ODB, what were the readings at idle? Have you set the TPS with a voltmeter or with the simpler - but no less effective - awadecki method?

  14. #13
    skor is offline Cadillac Owners Member
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    Re: 4.9 Will Not Idle Below 2500rpm!

    noahsdad, you're correct, the first post is confusing, ignore it, I was was half asleep when I typed it.

    OK, now for the update.

    I got everything back together. I used new gaskets under the throttle body and EGR. I replaced all the rubber hoses at the throttle body.

    I disconnected the battery and did a ISC relearn. The car idles between 1250 and 1300 -- same as before. I can not see the ISC motor move while the idle goes up and down. While the car ia at idle, diagnostics show the TPS to be at 2.5. This reading is rock steady although the idle moves up and down by 50 RPM.

    Here is all the diagnostic info while the car idles at operating temp:

    ED01 Throttle Position -- 2.5 degrees

    ED02 MAP -- 26kpa

    ED03 Computed BARO -- 103kpa

    ED04 Coolant Temp -- 101C

    ED05 MAT -- 105C

    ED08 Spark Advance -- 34 Degrees

    ED10 Battery Voltage -- 13.4V

    ED11 Engine RPM -- 1250-1300 RPM

    ED12 Vehicle Speed -- 0

    ED19 Power Steering Switch -- 0

    ED30 Left Injector Pulse Width -- 3.3-3.6 mSec

    ED31 Right Injector Pulse Width -- 3.3-3.6 mSec

    ED32 Left o2 Sensor Voltage -- .11-.90 Volts

    ED33 Right o2 Sensor Voltage -- .20-.78 Volts

    ED34 Left o2 Cross Counts -- 2/Sec

    ED35 Right o2 Cross Counts -- 2/Sec

    ED36 Left Fuel Integrator -- 126

    Ed37 Right Fuel Integrator -- 126

    ED38 Left Block Learn Fuel -- 133

    ED39 Right Block Learn Fuel -- 132

    ED70 Cruise Control Feedback -- 22%

    ED71 PRNDL1 Gray Code -- 11

    ED72 PRNDL2 Gray Code -- 00

    ED98 Ignition Cycle Counter -- 5

    ED99 PCM Prom ID -- 1503

  15. #14
    awadecki is offline Cadillac Owners Enthusiast
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    Re: 4.9 Will Not Idle Below 2500rpm!

    OK, hang on, let's get back to basics here...if you're sure there's no vacuum leak, then I think you have a problem with how you've set the TPS/ISC. I didn't see anyone else post on this, so I'll post my method, which is really simple.

    The FIRST step is to get the ISC plunger to retract all of the way. Start the car, warm it up all the way to operating temperature, and hold the throttle open with your finger so that it's just off of the ISC plunger, and push the plunger in. You'll see it start to retract. When it stops retracting, pull the connector so it can't move.

    Then, slowly drop the throttle back until it rests against the throttle stop screw. Your car should now be idling at 450 rpms--very slowly. Check the RPM parameter for this reading (don't recall which # it is). My guess is that your min. air idle is WAY off--probably around 900 rpms. If this the case, you need to turn the torx screw on the throttle body so that the car idles at 450 rpms. If you can't achieve that idle speed, then you still have a vacuum leak somewhere.

    Once you've achieved this idle speed, shut the car off. Turn the key on and enter diagnostics and observe parameter P.0.1. It should read (according to factory specs) around 0. My guess is that this is also off. Loosen the throttle position sensor bolts and move the sensor until this reads 0.7 (this is the way I set mine to avoid throttle sailing--the factory spec is 0). Tighten your bolts, reconnect the ISC, then restart the car.

    Now, perform the idle learn, which I believe consists of letting the car idle 60 seconds in Park, then 60 seconds in Drive, then 60 seconds in Drive with the A/C on (not sure, haven't done it in a while).

    See if this helps.

    If it doesn't--I just thought of this--pull the vacuum hose at your fuel pressure regulator. Or if you have a fuel pressure gauge, check the rail pressure. It could be reading too high because the FPR diaphram might be sticking. If you notice the vacuum hose is full of fuel, then change it, the diaphram is bad. Otherwise, spray a lot of WD40 in the regulator, then put the vacuum hose back on. Restart the car and blip the throttle about 10 times. Sometimes this will free a stuck diaphram if it hasn't actually broken, and will help prevent varnishing on the diaphram.

    By the way, nice car...I too am a 92 Seville owner. Do you have an STS? Here's mine...
    Attached Images

  16. #15
    Tailfin's Avatar
    Tailfin is offline Cadillac Owners Enthusiast
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    Re: 4.9 Will Not Idle Below 2500rpm!

    I'm with awadecki on this, follow that advice. The throttle plates can look "closed" but still be letting more air in than you realize. I think your MAP sensor is fine, because 103kpa comes out to about 14.9 psi, which is, indeed, close to typical atmospheric pressure (you must be pretty low elevation).

    One other thing is make sure the throttle cable is not binding on the throttle arm.

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