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Thread: running better before warming up

  1. #1
    Senior Member Parzival's Avatar
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    running better before warming up

    When I start my car fresh on a day, the idle sits right where it should and acceleration is smooth. However when it reaches warm up temp the idle starts to hunt a bit, not a huge range stays between 750 and 900, and I get a little bit of jerking in acceleration intermittently.
    I hear a lot of people say the opposite, telling me that the car runs a bit rough, but then is perfect when warmed up. I still have the tamper proof plug in my fuel air mixture adjustment, so its still set from the factory. I have new stainless fuel lines, and new spark plugs. Could this mean the air fuel mix needs adjustment? or is there something else at play?

  2. #2
    Senior Member
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    I would bet it's your idle microswitch not staying fully engaged once the engine is warm. Likely from a slightly weak or sticky return spring that's not quite strong enough to pull it back against the vacuum produced by a hot engine. You can try one of two things (or both): spray some lube in on the spring that sits at the pivot point of the idle microswitch lever arm and/or dial out the little peg that touches the idle microswitch itself so it pushes it in slightly earlier (and then when the engine is at full temp, and full vacuum, it's still pressing in enough to tell the idle speed motor to go.


    Sept. 81, auto, black interior

  3. #3
    Senior Member
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    I wouldn't mess with the adjustment (just yet) if it has never been touched. Being in Florida, you're pretty close to sea level and shouldn't need to adjust under normal circumstances. The fact that your car runs well when cold (richer thanks to the CPR) and runs jerky when warm tells me you might be running lean, vacuum leak being the most likely culprit. Could you get a dwell reading once at operating temperature and confirm? You should get a high reading if this is the case (above 50 - means its adjusting for lean)

  4. #4
    Senior Member Parzival's Avatar
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    Quote Originally Posted by Jonathan View Post
    I would bet it's your idle microswitch not staying fully engaged once the engine is warm. Likely from a slightly weak or sticky return spring that's not quite strong enough to pull it back against the vacuum produced by a hot engine. You can try one of two things (or both): spray some lube in on the spring that sits at the pivot point of the idle microswitch lever arm and/or dial out the little peg that touches the idle microswitch itself so it pushes it in slightly earlier (and then when the engine is at full temp, and full vacuum, it's still pressing in enough to tell the idle speed motor to go.
    I think that could be if it were not for the fact that I get a jerky acceleration especially if its under load, but its intermittent not always. While driving the microswitch is not supposed to be pressed anyway.

    Quote Originally Posted by dmcnc View Post
    I wouldn't mess with the adjustment (just yet) if it has never been touched. Being in Florida, you're pretty close to sea level and shouldn't need to adjust under normal circumstances. The fact that your car runs well when cold (richer thanks to the CPR) and runs jerky when warm tells me you might be running lean, vacuum leak being the most likely culprit. Could you get a dwell reading once at operating temperature and confirm? You should get a high reading if this is the case (above 50 - means its adjusting for lean)
    I don't have a dwell meter but I could get one if I had to. I had some vacuum leaks that were coming from the hose between the cold start valve and the idle sped motor back when I did the valley job, I got those tightened up, or at least enough so that I cant notice any hissing there. I put clamps at all connections.
    I replaced all the vacuum routing when I did the valley job with Dana A.K.A DMC-81. All I hear people say about finding leaks is to spray carb cleaner around the connections and listen for a change. Is this reliable? I'd really like something more quantitative than that. Does anyone have a good recommendation a dwell meter? I've tried to explain this problem to DMC FL but unfortunately the car is always behaving perfectly whenever I'm there, go figure. They always tell me that my car is one of the best running cars they see, but then a few days later i'm driving and the jerky acceleration comes back. Through out the day, I'll make stops that let the car cool down, and I come back and she's perfect on startup and for the first few red lights, then she's warmed up and starts hunting a bit, and the jerky acceleration is back! If only I could get her to do it when the experts are looking. On a side not I think the problem presents it self a bit more noticeably now that the AC system is functional, because of the added load on the engine... just like to get this last thing figured out and she'll be pretty much all worked out mechanically... I hope

  5. #5
    EFI'd dn010's Avatar
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    I bought a dwell meter off amazon. If you don't want to spend the money on something you'll hardly use, feel free to borrow mine if you want - although I can't recall if you're in or near Tampa or further away where it wouldn't make sense to get it.
    -----Dan B.

  6. #6
    Senior Member Parzival's Avatar
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    Quote Originally Posted by dn010 View Post
    I bought a dwell meter off amazon. If you don't want to spend the money on something you'll hardly use, feel free to borrow mine if you want - although I can't recall if you're in or near Tampa or further away where it wouldn't make sense to get it.
    thats awesome! I'm on the east cost so its a bit of a trek. Someone PMed with this recommendation a dwell meter, seems pretty inexpensive.
    https://www.harborfreight.com/lcd-au...kit-95670.html

  7. #7
    '82 T3 FABombjoy's Avatar
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    I wouldn't buy a $40 dwell meter for this. Really you just need a duty cycle meter which can be had for less. Something like the UT136A should work.

    The dwell meter is "utilized" because 1981 auto shops had them. Unless you really need to adjust ignition points I can't see the investment in a dwell meter being worth it.

    D:04:09 even depicts and describes the system using duty cycle (0-100)

    A dwell meter reads 0-90 and a DC reads 0-100, so you just have to change the manual's reference numbers accordingly:

    Code:
    35	38.9
    45	50
    	
    40	44.4
    50	55.6
    	
    87	96.7
    	
    20	22.2
    	
    50	55.6
    60	66.7
    Luke S :: 10270 :: 82 Grey 5-Speed :: Single Watercooled T3 .60/.48 :: Borla Exhaust :: MSD Ignition :: MS3X Fully SFI Odd-fire EFI :: DevilsOwn Methanol Injection

    Console5.com - Game console parts, kits, games and more. [shop] [wiki] [RSS] [f] [t]

  8. #8
    Member
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    If you've seen a video of someone measuring dwell on a Delorean, it always shows an Actron CP7605. I got mine from Amazon for $23, but they're no longer available there. You really want an analog meter for measuring things that are fluctuating. Unfortunately, after a quick search, I can't find anyone that still sells these.
    Robert
    1981 DeLorean #1890
    1976 Datsun 280Z
    1968 Pontiac Le Mans convertible

  9. #9
    EFI'd dn010's Avatar
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    running better before warming up

    +1, same meter I have. You can get them on eBay now for triple that price. Maybe I should sell mine.
    -----Dan B.

  10. #10
    Senior Member Parzival's Avatar
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    ok so, I needed a new multimeter anyway, so I got this one.
    These are the readings cold and hot.
    I sprayed carb cleaner around the vacuum connections and I never saw any effect. I didnt get any idle hunting today, that usually ony happens after driving a fair bit, but I did take it around the block and got some jerky acceleration. Can anyone make anything of these readings?
    Last edited by Parzival; 04-24-2019 at 08:11 PM.

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