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Thread: Stuttering under acceleration when warm

  1. #1
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    Location:  Allen, TX

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    Stuttering under acceleration when warm

    A little background first. I've done a lot of things to my car since I purchased it and within a few weeks of each other, I upgraded to a Spec 1 exhaust, installed new fuel lines, spark plugs, plug wires, rotor and cap. Sometime after that, I noticed a hesitation or stutter when accelerating after the car was warmed up (driving for about 10 mins or so). The stutter is most noticeable when accelerating in the 1800-3000 RPM range but once I'm above that range, the car seems to even out and accelerates fine. Car is rock solid at Idle when cold or warm and stays right at 750 or so.

    I'd read somewhere that after installing a performance exhaust like the Spec 1, some sort of adjustments might need to be made, but for the life of me, I can't find where I read that anymore. If I should make adjustments, can anyone point me in the direction of what I should be adjusting? I'm assuming it's just the mixture, but I'm scared to touch that as I've read that should always be the last thing you touch.

    I recently acquired a Dwell meter and the following video shows my results as the car heats up.



    TL;DWatch -- Cold, I get a steady 45 Dwell reading. 3 mins later the reading oscillates between 35 and 45. Approximately another 7 mins after that, dwell oscillates between 40-48 and another 10 minutes after that, dwell oscillates between 44-48. On both the last two readings, it seems like the dwell wants to stay at the higher end of the oscillation longer (hard to describe but video shows this). Disconnecting the O2 sensor or pressing the WOT switch gives me a steady reading around 48-50.

    These readings seem to indicate issues when the engine is warmer, but seems to be correct when it's cold or just warming up.

    Any recommendations given all of the above information?

    Thanks,
    Bueller

  2. #2
    Delorean Guru
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    Some things to check:
    Injector spray patterns
    Vacuum leaks
    Vacuum, mechanical advance, and initial base timing.
    David Teitelbaum

  3. #3
    Senior Member Bitsyncmaster's Avatar
    Join Date:  May 2011

    Location:  Leonardtown, MD

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    Your dwell reading is a little high. You should be running around 35 deg with a hot engine. I would not think that dwell setting would cause the hesitation with the warmed up engine. You don't have to wait to adjust the mixture last. Any time you change things on the engine, just hook up your dwell meter and adjust it. Your new exhaust probably changed the dwell setting.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  4. #4
    Senior Member NckT's Avatar
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    Location:  Yorkshire UK

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    Set the primary fuel pressure from 5.2-5.3 bar to 5.3-5.4 bar (76-77 psi ish) then reset the fueling with the dwell meter.

    I'm not sure if the US octane rating is the same as UK Octane rating, but we have 95ron as basic unleaded, so for us here, ignition timing can be advanced to 14-14.5 btdc no vac advance.

    The above verified in a rolling road.

    Good luck
    RIP Rob van de Veer Top bloke

    I say Sir, I must be mad, one loves fixing K-Jet !

    Make sure there's plenty in the tank for the weekend chaps....

  5. #5
    Not a DeLorean Guru
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    Quote Originally Posted by NckT View Post
    Set the primary fuel pressure from 5.2-5.3 bar to 5.3-5.4 bar (76-77 psi ish) then reset the fueling with the dwell meter.

    I'm not sure if the US octane rating is the same as UK Octane rating, but we have 95ron as basic unleaded, so for us here, ignition timing can be advanced to 14-14.5 btdc no vac advance.

    The above verified in a rolling road.

    Good luck
    Hello friend,

    The U.S. uses different formulas to calculate octane ratings:

    https://www.etuners.gr/fuel/
    -Mike
    1981 DeLorean, heads/cams/exhaust, EFI
    1999 Corvette, heads/cam/exhaust, 440 BHP
    2005 Elise, stock
    2016 Chevy Cruze

  6. #6
    Member
    Join Date:  Feb 2015

    Location:  Allen, TX

    Posts:    36

    My VIN:    #4595

    Thanks for the responses. I've checked for vacuum leaks but haven't checked spray patterns or timing yet.

    I was leaning towards adjusting the dwell as my first step given the new exhaust, etc. Should I be checking the spray patterns and timing before that too or is it okay to try the Dwell first?

    Also, I don't currently have anything that can test fuel pressure but if I need to go down that route, I'll definitely purchase what I need. How likely is it that I need to adjust the primary pressure too given the background on work that was performed? She's a low mileage car (under 20K) and she was running perfect as far as I could tell before the upgrades and updates.

    Thanks!

  7. #7
    Senior Member NckT's Avatar
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    Location:  Yorkshire UK

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    Quote Originally Posted by opethmike View Post
    Hello friend,

    The U.S. uses different formulas to calculate octane ratings:

    https://www.etuners.gr/fuel/
    Thanks for that
    RIP Rob van de Veer Top bloke

    I say Sir, I must be mad, one loves fixing K-Jet !

    Make sure there's plenty in the tank for the weekend chaps....

  8. #8
    Senior Member NckT's Avatar
    Join Date:  Jun 2011

    Location:  Yorkshire UK

    Posts:    118

    My VIN:    No. 4068

    Quote Originally Posted by bueller View Post
    Thanks for the responses. I've checked for vacuum leaks but haven't checked spray patterns or timing yet.

    I was leaning towards adjusting the dwell as my first step given the new exhaust, etc. Should I be checking the spray patterns and timing before that too or is it okay to try the Dwell first?

    Also, I don't currently have anything that can test fuel pressure but if I need to go down that route, I'll definitely purchase what I need. How likely is it that I need to adjust the primary pressure too given the background on work that was performed? She's a low mileage car (under 20K) and she was running perfect as far as I could tell before the upgrades and updates.

    Thanks!
    I'd test the fuel pressure as even if the car had low mileage, the primary fuel pasture spring weakens over time and needs additional shims to even bring it back from stock. Typical values noted over 30 plus cars have been only 4.7-4.8 bar where setting pressure should be 5.2-5.3 for stock engine.

    If you've got a fuel pressure gauge on you can also test the control pressure regulator (CPR) also known as the warm up regulator (WUR) / accumulator capacitance and rest leak down rates too.
    RIP Rob van de Veer Top bloke

    I say Sir, I must be mad, one loves fixing K-Jet !

    Make sure there's plenty in the tank for the weekend chaps....

  9. #9
    Member
    Join Date:  Feb 2015

    Location:  Allen, TX

    Posts:    36

    My VIN:    #4595

    Thanks NickT. Makes sense.

    I'm going to order some shims from DeloreanGO to have ready to go after I get the fuel pressure tester. It will be a bit before they get here from over the pond. I'll test and adjust these things and get back with the group with the results.

    Thank you!!

  10. #10
    Senior Member NckT's Avatar
    Join Date:  Jun 2011

    Location:  Yorkshire UK

    Posts:    118

    My VIN:    No. 4068

    Quote Originally Posted by bueller View Post
    Thanks NickT. Makes sense.

    I'm going to order some shims from DeloreanGO to have ready to go after I get the fuel pressure tester. It will be a bit before they get here from over the pond. I'll test and adjust these things and get back with the group with the results.

    Thank you!!
    Good luck

    I'm 'Admin-bloke' on www.deloreanclub.co.uk/forum in case you search there too.

    NickT
    RIP Rob van de Veer Top bloke

    I say Sir, I must be mad, one loves fixing K-Jet !

    Make sure there's plenty in the tank for the weekend chaps....

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