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Thread: Tachometer needle jumps sometimes - cause?

  1. #41
    Senior Member 82DMC12's Avatar
    Join Date:  May 2011

    Location:  Olathe, KS

    Posts:    1,104

    My VIN:    11596

    David,

    I could try running my smoke machine while the engine is hot, that should reveal any heat-related vacuum leaks, don't you think?

    Regarding the decel springs, you can see the photo of the springs above. They are not collapsed. Is there any way to test the decel spring theory without replacing the throttle body?

    I did just try a spare idle ECU that painterdave had - no change, it's not the ECU itself. Could it be the idle motor?

    I am going to put a timing light on all six plug wires one at a time and see if I can find if any of the cylinders is crapping out (for the tach issue)
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  2. #42
    Senior Member Rich's Avatar
    Join Date:  May 2011

    Location:  San Francisco Bay Area, Calif.

    Posts:    1,897

    My VIN:    0934

    Club(s):   (NCDMC) (DCUK)

    Quote Originally Posted by Jonathan View Post
    Besides our batteries being the group 78 for physical size and the side post terminals, is there a rating in cranking amps that's right? I see some batteries have more cranking amps, but wasn't sure if more is always better in things like this, like maybe there is a limit where that's too much.
    I'm unaware of a factory spec for a DeLorean battery aside from the Group 78 size. (Group 78 batteries are all side terminal type)
    As mentioned, a smaller side-terminal battery can work.

    Higher cold cranking amps (CCA) or cranking amps (CA) are better, mainly in terms of cranking/starting power as the term implies. Higher ratings typically come with a longer battery warranty.

    No limit. A higher rating won't hurt anything besides your wallet.
    March '81, 5-speed, black interior

  3. #43
    Senior Member
    Join Date:  May 2011

    Location:  Northern NJ

    Posts:    8,259

    My VIN:    10757 1st place Concourse 1998

    Quote Originally Posted by 82DMC12 View Post
    David,

    I could try running my smoke machine while the engine is hot, that should reveal any heat-related vacuum leaks, don't you think?

    Regarding the decel springs, you can see the photo of the springs above. They are not collapsed. Is there any way to test the decel spring theory without replacing the throttle body?

    I did just try a spare idle ECU that painterdave had - no change, it's not the ECU itself. Could it be the idle motor?

    I am going to put a timing light on all six plug wires one at a time and see if I can find if any of the cylinders is crapping out (for the tach issue)
    I was thinking more along the lines of a system that changes as the engine warms up. You could try running the motor in the dark and looking for any sparks or blue glow that would indicate a problem in the secondary ignition system. A system wide problem would not single out one cylinder. A misfire on one cylinder would give you the jumpy tach and the bad idle. I think you are looking for two separate problems, one causing the fast idle and one causing the bad idle/jumpy tach.
    David Teitelbaum

  4. #44
    Senior Member
    Join Date:  Jun 2011

    Posts:    4,747

    My VIN:    3937

    Quote Originally Posted by Rich View Post
    I'm unaware of a factory spec for a DeLorean battery aside from the Group 78 size. (Group 78 batteries are all side terminal type)
    As mentioned, a smaller side-terminal battery can work.

    Higher cold cranking amps (CCA) or cranking amps (CA) are better, mainly in terms of cranking/starting power as the term implies. Higher ratings typically come with a longer battery warranty.

    No limit. A higher rating won't hurt anything besides your wallet.
    Thanks for the explanation. The extra warranty/extra cost sounds a little like how they price power surge bars for home theatre systems. More expensive version = longer/higher warranty.

    Question about battery disconnect switches... could a wonky disconnect switch in some way cause irregular voltages and the like? I'm just thinking out loud as to what other things might affect things intermittently that haven't already been discussed.


    Sept. 81, auto, black interior

  5. #45
    Senior Member 82DMC12's Avatar
    Join Date:  May 2011

    Location:  Olathe, KS

    Posts:    1,104

    My VIN:    11596

    Hey guys, I checked a few more things today.

    1) Cleaned with a wire brush and electronics cleaner all of the bulkhead connectors since white/slate and some of the wiring for the idle parts go through these connectors.
    2) Cleaned Fuse 1
    3) Tested the thermistor at the Idle ECU (black/yellow and black/slate on the inner white plug, disconnected from the ECU) while the engine was still cold. On the 20K scale I get about 8.00.
    4) Started the car and let it idle up until hot. After a certain point the idle get a bit rough and then slowly ramps up to around 1,200 RPM just as it had been before. As it climbs up, the tach starts to freak out and shown in my previous video.
    5) While this is taking place, I hook up my Actron external tach to monitor directly off of the white/slate on the coil. The Actron is not bouncing all over like the dash tach does.
    6) While the car is idling, I hook up my timing light and check each cylinder while the tach is freaking out to see if there is any change to the flashing. There is not. All six plug wires seem to be firing normally with no drop out or monkey business
    7) Unplugged the inner plug on the idle ECU and the RPM's jump up to about 2,000 RPM as expected. The tach is still bouncing around so this tells me the tach bounce is not related to the Idle ECU.
    Shut off the car now that it's hot (fans on) and checked the thermistor again at the Idle ECU on the same pins. Now my 20K scale is reading only 0.86.

    Very interesting because I'm reading some diagnostics information online that suggest the thermistor should read 8K to 12K when cold and 3K to 5K when hot. Clearly I'm not interested in replacing the thermistor unless I am positive my readings are bad and they mean the thermistor is dead/dying.

    Wondering if anyone can confirm and even better if someone whose car is running correctly would repeat the same thermistor test?

    Would a thermistor reading these values result in a randomly high hot idle? Perhaps the tach is still a different issue but maybe the thermistor explains the idle.
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

  6. #46
    Senior Member Bitsyncmaster's Avatar
    Join Date:  May 2011

    Location:  Leonardtown, MD

    Posts:    8,498

    My VIN:    03572

    I have been helping another owner with his idle problem and he got about the same resistance as you on the idle thermistor. But mine at 75 deg. F (cold) is 20.7 Kohm and 1.12 Kohm when my fans turned on and I shut the engine off.

    I would also like to see what some other owners are reading on their thermistor. I used my reading when I designed my ECU software but I don't think that difference makes much change.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  7. #47
    Senior Member 82DMC12's Avatar
    Join Date:  May 2011

    Location:  Olathe, KS

    Posts:    1,104

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    Quote Originally Posted by Bitsyncmaster View Post
    I have been helping another owner with his idle problem and he got about the same resistance as you on the idle thermistor. But mine at 75 deg. F (cold) is 20.7 Kohm and 1.12 Kohm when my fans turned on and I shut the engine off.

    I would also like to see what some other owners are reading on their thermistor. I used my reading when I designed my ECU software but I don't think that difference makes much change.
    Thanks Dave. I appreciate the measurements. I'd be forever grateful if a couple more owners would do the same test and report back!
    Andy Lien

    VIN 11596 Jan 1982 build - owned since Nov. 2000!

    Photography and Backpacking is life.

    Was Fargo, ND
    Now Olathe, KS

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