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Thread: What are the main causes a car would start, but not idle?

  1. #1
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    What are the main causes a car would start, but not idle?

    I just bought a car but I am finding a lot of details (car stereo, speedometer, etc.), But there is one that is affecting me the most. The car will start, it will go all the way to 1200-1500 rpm and will go down on the rev to 600 and it might oscillate up and down until the engine shutdown. I have to open and close the key (force the pump) 2-3 times and it will start, once again revs going up and down - it fluctuates (up and down).

    I think my issue should be associate with a solenoid, at least that is the problems with other cars; but I am not sure if it is the same in Thunderbird. In order to keep the pump from continuously pumping gasoline in to the engine when the key is on but the engine is not actually running, there is a relay that shuts off the power to the fuel pump unless it gets a signal that the engine is turning.

  2. #2
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    Grounds, check the grounds.

    An EVTM will help locate them.

    Check the ground for the injectors back of the driver's side head.

    Check the ground for the IRCM on the ganged connector, front of the header panel, next to the radiator (check the other one on the other side too).

    Check the ground cable from the pass. side motor mount over to the body.

    Check that the center bolt in the EEC's connector is tight.

    Confirm the accy. brackets haven't been painted.

    etc. etc.

    And while we're on the subject of idle, check for vacuum leaks and read codes. Example of one suspect:

    Which is Not a Symptom of a Potentially Bad ECT Sensor

    • Continuous rich air-fuel mixture
    • Continuous lean air-fuel mixture
    • Rough idle
    • Engine misfire
    • Engine stalling
    • Increased emissions
    • Increased fuel consumption

    Answer is: misfire

  3. #3
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    check to see if there is a hole in the air intake.
    also, if there is a vacuum leak beyond the MAF it will make your rpm shudder a bit at idle as well with the O2 sensor trying to compensate, or your MAF could be bad/misreading as well.
    That's my guess based on what you've said.

  4. #4
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    I agree, a vacuum leak is most likely cause. Spray some carb cleaner or starter fluid around the IC tube connections (most common leak points) and listen for engine rpms to dramatically change when starter fluid is sucked in.

    David
    1991 SC AOD 4.2..2.3 Whipple..........10.910 @ 125.61
    2016 SRT Challenger Hellcat.............10.597 @ 135.78

    My Garage

  5. #5
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    I am going to start reviewing all the vacuum hoses. I bought the book “Haynes repair manual” but it doesn’t have a good description of the vacuum hoses. I also Bought the EVTM , I should receive it during the week. is that a better guide for troubleshooting the hoses?



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  6. #6
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    Quote Originally Posted by yayo_ayala View Post
    I am going to start reviewing all the vacuum hoses. I bought the book “Haynes repair manual” but it doesn’t have a good description of the vacuum hoses. I also Bought the EVTM , I should receive it during the week. is that a better guide for troubleshooting the hoses?



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    Sounds similar to what I experienced on my 1989 Manual SC when the cam sensor failed.

    Easy enough to trouble shoot by unplugging it and trying to start the car without it hooked up. Usually takes a few attempts but might be worth a try.

    -Tim
    Current Collection
    1994 Thunderbird SC - Auto converted to 5spd

    1995 Thunderbird SC - Auto

    1994 Thunderbird SC - 5spd
    1989 Thunderbird SC - 5spd - Daily Driver
    1990 Thunder/Cougar XR7 - 5spd
    1994 Thunderbird SC - 3v Project

  7. #7
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    Quote Originally Posted by yayo_ayala View Post
    I am going to start reviewing all the vacuum hoses. I bought the book “Haynes repair manual” but it doesn’t have a good description of the vacuum hoses. I also Bought the EVTM , I should receive it during the week. is that a better guide for troubleshooting the hoses?



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    Those aren't vacuum hoses. Intercooler tube connections is where it usually leaks.
    1991 SC AOD 4.2..2.3 Whipple..........10.910 @ 125.61
    2016 SRT Challenger Hellcat.............10.597 @ 135.78

    My Garage

  8. #8
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    I would clean up the rusty bolts holding the DIS. I think one of those is a ground.

  9. #9
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    Checking for an electrical failure

    After cleaning the DIF and the bolts, I started following the EVTM. the first thing that I did was to clean all the ground terminals, brush them and replace the bolds. But the problem was still there. I follow the EVTM manual and check for the wires between the DIF and the Camshaft sensor NP.
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    I am attaching a video with the problem. The car cranks the engine but it takes ~30 sec to start and when it starts the rev's go up and down.
    https://youtu.be/No6bnx4Y9N8

    Yesterday after checking and cleaning electric wires, I discard that the issue was electrical. I started looking for Vacuum hoses, and did not see any cracks or signs of aging. Just for testing, I close the below vacuum valve - the car worked; the car started with a dime. The problem is not notorious after driven the car for a while; it accentuates in the morning when you are starting the car for the first time.

    This morning, before starting the car, once again I open and close the valve multiple times, cracked the engine and ... it started immediately. Definitely the problem is a vacuum issue and I am just sharing this info, incase someone else has a similar problem. I will not say that I am out of the woods, I have to figure out if it is that particular valve or a hose, etc.

    I checked the valve with a vacuum pump that I used for bleeding the brakes, and the valve starts retracting ~10 Psi and fully retracts at 15 Psi. I review the EVTM but is not very good source for vacuum problems - it is fantastic for electrical problems. I am not sure where to find information about this valve and information on how to replace it. I couldn't identify a bolt to remove it, it looks very hidden. In addition when I open and close multiple times the valve, I see smoke - I couldn't figure out where is coming from.

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  10. #10
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    I review the EVTM but is not very good source for vacuum problems
    Right, in which case I move to the Engine/Emissions Diagnosis (year model) Shop Manual.

    I am not sure where to find information about this valve and information on how to replace it.
    That would be shop shop manual.

    It's pretty easy to test general operation. Engine off...closed - engine running at idle, full closed, sure you know about that. But if you have a vacuum tester, just do a pull and see if it holds or leaks down. If it leaks, how fast? Noticeable? Replace it. Just be sure to test any replacement.
    Last edited by KMT; 05-03-2020 at 09:48 PM.

  11. #11
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    Quickest way to find vacuum leaks is start the motor and keep it running by playing with throttle linkage under the hood, while spraying flammable carb or brake cleaner on the inter cooler pipe connections and all areas in the picture above or with vacuum hoses. You will hear engine rev up when cleaner is sucked in where it's leaking. Most common place for leaks is at IC tube connections.

    David

    PS: Do not spray hot exhaust manifolds or headers.
    Last edited by David Neibert; 05-06-2020 at 01:19 PM.
    1991 SC AOD 4.2..2.3 Whipple..........10.910 @ 125.61
    2016 SRT Challenger Hellcat.............10.597 @ 135.78

    My Garage

  12. #12
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    Quote Originally Posted by David Neibert View Post
    Quickest way to find vacuum leaks is start the motor and keep it running by playing with throttle linkage under the hood, while spraying flammable carb or brake cleaner on the inter cooler pipe connections and all areas in the picture above or with vacuum hoses. You will hear engine rev up when cleaner is sucked in where it's leaking. Most common place for leaks is at IC tube connections.
    This actually doesn't work real well due to the fact the EEC will compensate instantly by pulling timing. This will kind of work if the vacuum leak is so extreme that retarding the timing won't be enough to compensate.

    When the car won't start and idle immediately, the 02 sensors are not the problem because they don't kick in until 30-90 seconds later.

    Usually a surging idle is caused by too much fuel. On an old car there are many reasons why it may get too much fuel:

    - stuck injectors
    - bad fuel pressure regulator
    - fouled plugs
    - bad cats
    - bad 02 sensors
    - bad maf
    - stuck EGR

    If it were me I would start with verifying fuel pressure. This is easy and will rule out injectors and fuel pressure regulator.

    Then I would check the MAF - first physically remove it and check/clean the sensor wires. If you have a digital VOM you can check output voltage at idle, it should be around .9v and vary with throttle input.

    Then I would pull one plug and look at it. They might be original.

    Next I would take all the tubes off, change the plugs and wires while it's apart, and reseal all the tubes. You can be pretty sure that they are leaking after 30 years. If you can do a smoke test, or have one done, this will also help you greatly in finding a vacuum leak.

    Now as you know, a vacuum leak won't make the engine run rich, it will make it run lean. But, with your verbal description one can't be 100% confident in your assessment of the symptoms and since vacuum leaks are almost 100% probable on an SC, I still would not rule it out.

    One of the side effects of a vacuum leak is that it will cause the computer to learn incorrect fuel trims. Try disconnecting the battery and see if it acts differently. Disconnecting the battery will clear all learned parameters. This will give you some hints as to what is going on.

    Bad 02 sensors, or weak ones, will cause the system to learn rich. If it's been running like this for awhile, it can damage the cats. The cats become clogged and excessive back pressure will cause the engine to run rich and struggle if you try to force it to run. Not likely, but something you can't rule out completely.

    Stuck EGR - you mentioned seeing smoke around the bypass valve. This seems odd to me, unless perhaps the EGR valve is open when it shouldn't be allowing exhaust into the intake plenum. That is of course if your car even has EGR.
    Quote Originally Posted by Miller View Post
    Ya thats why i tape mine down. People think its bc i dont have a moonroof seal (which is true) but its really to keep my roof from ripping off .
    Email me here.

  13. #13
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    Thanks Dave

    Thanks Dave, I took my car to a shop, to take care of multiple things such as: brakes, AC, tensioners and idle issue. The gentleman has been working with Fords for many years and he fixed the car by replacing the Fuel Pressure regulator. Your advise was on the money!

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