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  1. #1
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    Default Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    Engine serial number 0L373392
    Merc 7.4 mpi 2000 2855 Ciera SUN BRIDGE approx 300 hrs on engine.

    Started missing, out of the blue, had been a while since tune up so replaced rotor and cap (showed corrosion) still has miss.
    Pulled plugs and port side plugs looked to be covered in black soot but dry starboard side plugs were correct cocoa brown color. Wire brushed, hit with brake clean, gapped to .045, reinstalled. Plan to replace with new.
    Replaced plug wires with new, even swapped in new fuel/water separator filter, ran better but still missed, just less.
    Had new ignition module and dist coil pick up and ignition coil from previous project build, swapped in and set timing to 8degress, starts right up, comes up to temp 170 on gauge cluster, idles and revs just fine but still has intermittent miss regardless of at idle or up in the rpm, still sooting up all port side 1, 3, 5 & 7 plugs only.

    Figured best to check for codes at this point, came up with code 14, "Engine Coolant Temperature (ECT)" so going to run the traps on Sensor and Sensor Circuit - Low Temperature Indicator testing procedure unless any one has different advice for me.

    Using Merc service manual Number 23 pages 783-785
    https://d3gqux9sl0z33u.cloudfront.ne..._8.2_litre.pdf

  2. #2
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    Pulling codes and following the factory manual are always good starting points.

    a little weird that the port bank is running richer than the stbd bank...the only thing common between all odd (even) cylinders is the B+ feed to the injectors.

    a low ECT value will widen the pulse width sent to the injectors...

    Only extra thought is to do the "wiggle test" for every connector you touch...

  3. #3
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    Quote Originally Posted by makomark View Post
    Pulling codes and following the factory manual are always good starting points.

    a little weird that the port bank is running richer than the stbd bank...the only thing common between all odd (even) cylinders is the B+ feed to the injectors.

    a low ECT value will widen the pulse width sent to the injectors...

    Only extra thought is to do the "wiggle test" for every connector you touch...

    Much appreciated 411, will dive inter over the easter wknd, pull injector plugs, clean, inspect, test sensor and signal, scored a set new plugs, maybe just time as they are original MERCRUISER plug at 300+ hrs on vessel.
    Question though, i did not clear codes after all of this so would this cause an issue? or irrelevant if all parts working?
    thx again.

  4. #4
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    The code is an indicator of a problem...say it was the ECT sensor and you replaced the sensor...things will be just fine even though the code remains...after enough cycles, the code will clear by itself...that said, its still best to follow the factory manual re clearing the DTCs.

    I'd be inclined to hold off on the plugs until you get the rich condition resolved...

  5. #5
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    How about throwing a new set of plugs in and retesting. You cannot clean spark plugs. Chances are this is your issue. Doing all the other things on your list first is not how you go about it.
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

  6. #6
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    Quote Originally Posted by Docksidemarineservices View Post
    How about throwing a new set of plugs in and retesting. You cannot clean spark plugs. Chances are this is your issue. Doing all the other things on your list first is not how you go about it.
    Oddly enough may have been a contributor but definitely the only culprit as the miss was significant prior to swap of new items and now is considerably less prevalent. However, did score a set of OEM plugs and ect on the way home just in case, will address this evening, will swap plugs, retest, hopefully will conclude findings as all other items appear functioning as designed.
    If not have returnable ect on stby to continue.
    Oddly enough, depending on point of view, unfortunately or fortunately, my current reality is I am a dyed in the wool gearhead tinkerer that will "geek out" on ANY opportunity to wrench on, new to me toys, just for the sake of learning experience coupled with my need to know its as fresh as can be being I acquired boat used and do not have history other than hr. meter.
    All that said I have time on my hands these days so made perfect recipe to skip deducible reasoning and get to wrenching on recent toy as im still waiting on 4l80 tans kit to show for LS swap build.
    Otherwise when "wrenching on a budget" or OCD kicks in to over ride the need vs the logic then yes by all means start with pulling codes and the basics.
    Much appreciate the point of view, TBC

  7. #7
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    update, replaced with new plugs, mild improvement but still surges up and down and misses, starts, comes up to temp, once comes off closed loop, pulled codes, none only 12, so swapped in new ECT for the hell of it at this point, no change. Once warm ran it up to 2500 rpm, no problems spooling up and holding then all of sudden surge up then down and what seems like miss fire, throttle down and same at idle just less prevalent.
    Is it possible knock sensors, fuel pump or injectors may be the issue and would any of those item NOT trigger a code since only one registering is 12?
    stumped at this point.

  8. #8
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    The ecu doesn't control the fuel pump pressure, just the ON and OFF aspect.

    Fuel pressure gauge is a handy tool...

    may also be time to replace the pickup coil in the EST distributor...pretty sure that's an earlier ECU so there isn't much "sophistication" in it...

  9. #9
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    Quote Originally Posted by makomark View Post
    The ecu doesn't control the fuel pump pressure, just the ON and OFF aspect.

    Fuel pressure gauge is a handy tool...

    may also be time to replace the pickup coil in the EST distributor...pretty sure that's an earlier ECU so there isn't much "sophistication" in it...
    understood and yes agre on coil pick up so swapped in a new complete distributor for the sake of shaft play, coil pick up, module, etc no change. thx for the input. Have a mechanical fuel gauge i will attempt to adapt to fuel rail and monitor during surge / miss / and idle hunt.

  10. #10
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    Have you considered a faulty IAC valve? If it isn't doing its job it can cause all kinds of displeasure.
    Practicing the ancient art of ren-ching

  11. #11
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    Quote Originally Posted by o2batsea View Post
    Have you considered a faulty IAC valve? If it isn't doing its job it can cause all kinds of displeasure.
    wouldnt faulty IAC trigger fault code? currently only 12 that indicated cpu communication is functional is present.

  12. #12
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    After complete, new distributor, new plugs, new wires, new ECT sensor, throttle body clean out, vacuum test fuel pressure regulator (holds vacuum ) and good vacuum signal while running (18 lbs. ) still has intermittent stumble cold or up to temp.
    Only thing i have left to do is pressure test fuel pressure both initial key on and running, record any changes as stumble occurs.
    Is there a way to test IAC? If malfunctioning wouldn't it record a code?

    If fuel pump and IAC checks out thinking Injectors?
    Last edited by Magonsterz; 04-17-2022 at 09:14 PM.

  13. #13
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    The IAC goes open when running at cruise speed. So it should not cause a surge at cruise speed.
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

  14. #14
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    Quote Originally Posted by Docksidemarineservices View Post
    The IAC goes open when running at cruise speed. So it should not cause a surge at cruise speed.
    That makes sense so onto fuel pressure rail monitoring against vacuum signal to confirm pump integrity it is.

  15. #15
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    Update, cyl compression checks out, low 121, high 140, all others 130 to 135.
    Checked existing static fuel pressure in rail cold and had bled down to zero over night.
    Pulled neg vac to 18 hg on both the fuel rail pressure regulator and fuel pump pressure regulator, both held 18 and 22 neg vac with slow bleed settling at neg 11 after a 15 minutes.
    1st key on fuel pressure read 36
    2nd key on pressure 38
    3rd key on pressure read 38 no change.
    Fired engine up with rail pressure gauge installed, pressure read 36 (weak), factory states 43 psi running.
    Unplugged vacuum hose from rail pressure regulator, while running, plugged leak, pressure jumped to 40 from 36(weak)
    Attached vacuum gun to fuel rail pressure regulator, drew neg vac to 15, fuel pressure dropped to 34, running rough.
    Increased neg vac to 18, fuel pressure dropped to 30 psi, engine stalled.
    Looks like i found the culprit, have a TITAN fuel pump with cooler kit on order, to be continued.

  16. #16
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    finally all back together with fresh fuel, firing order and timing are spot on, fires right up.
    fuel pump replacement fixed low pressure problem now rock solid 38 at idle, jumps up to 43 when throttled up.
    now onto next problem, all four port side injectors confirmed not firing as was noted before but unable to confirm until fuel pump pres. issue was resolved.
    I can fire it up and unplug each wire one by one and there is zero change in engine on all port plugs and typical stumble on all starboard sides.
    spark present and confirmed to all plugs with inline probe and a few good zaps!
    no codes are present and not inclined to believe all four injectors (1, 3, 5, 7) on same bank of engine failed at the same time, have yet to test signal or lack there of at injector plug.
    injector fuse is good as there is only one and all starboard side is firing correctly.
    leaning toward ECU Injector driver but will continuity test any and all wires related for 12v and Ground, if checks out will have to send ECM off to get gone through at OBD Diagnostics near me.

  17. #17
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    the injectors are controlled as two banks of four...within a given bank, two will be on one head and two will be on the other head...i don't see how an electrical problem screws up all four injectors on one cylinder head...

    with that MEFI ECU, i don't think you will get any codes for the injectors/drivers...

  18. #18
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    Quote Originally Posted by makomark View Post
    the injectors are controlled as two banks of four...within a given bank, two will be on one head and two will be on the other head...i don't see how an electrical problem screws up all four injectors on one cylinder head...<br>
    <br>
    with that MEFI ECU, i don't think you will get any codes for the injectors/drivers...
    <br><br>"MAKO" I saw that on wiring schematic last night, one set port and starboard - front and rear injectors controlled by one set of output leads from ECM and one set starboard and port - front and rear injectors controlled by the other set of leads, so doesn't make sense that one lead broken lead would knock out all four port side bank. Still plan to run the traps on all leads for sake of due diligence but in the interim will be shipping off ECM to "OBD Diagnostics" in Redondo Beach that specialize in Marine applications to run though its paces and repair if necessary.
    Click image for larger version. 

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  19. #19
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    update, pulled intake, hooked up fuel pressure gauge, hooked up injector pulse test unit and bingo, 3 failed to pulse and one was intermittent with weak fuel pressure discharged volume.
    Remaining 4 discharge pressure drop were within 1.5 psi of each other.
    Fixing to send into injector clinic since they did pass coil continuity test average 12.4 to 12.7 ohm, spec states 12.1 nominal resistance.
    with any luck will be back on the water by august after some pre planned traveling is done.

  20. #20
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    got injectors back from RC injector service out of Torrance, Ca. today and they were able to save all 8.
    highest flow cc was 251.0 lowest was 250.0, avg 250.4 system balance is .4
    4 were found to be dripping
    2 had poor flow
    2 had bad patterns and sub par flow.

    plan to put them in over the weekend and fire that pig up.Click image for larger version. 

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  21. #21
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    fingers crossed...good luck!

  22. #22
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    Default Re: Mercruiser 7.4 L Mpi L29 MISS FIRING in Bayliner 2855 Cierra

    Any Updates? How did she run?

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