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Thread: 2002 Tige 21V 350 mag mpi Backfires and bogs until warm and then runs perfect

  1. #1
    Join Date
    Aug 2008
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    Holladay, Utah
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    27
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    02 21V Riders edition

    2002 Tige 21V 350 mag mpi Backfires and bogs until warm and then runs perfect

    recently I fixed the "crank but no start" with some great help from you all. It was the ignition module in the distributor and so after the updated module and new coil, plugs, wires, cap and rotor I thought I had it solved, but now I am dealing with the boat runs perfect at Idle and no load but if its cold and not warmed up, it will hesitate and bog down and backfire at full throttle until the engine is "warmed up". After letting it warm up it will run great. Anyone have any ideas as to what direction I should look? TPS, IAC, fuel delivery? I've read some things about Throttle Position Sensor, Idle Air Control. I also just recently changed the Fuel/water separator. I checked the Fuel pump with a gauge and it is around 32 psi.
    One other symptom that may be related is that when the boat is running perfectly and warm, I noticed last time that after I had shut it off between wakeboarders, I had to give it throttle for it to start as it didn't seem to want to start in just neutral. But with throttle it'll start and then does fine as I put it back into neutral.
    Thanks!

  2. #2
    Sounds like a vacuum leak or a bad MAP sensor to me. Then again I am no Merc mechanic lol!
    It's dangerous to go alone! Take this.

  3. #3
    Join Date
    Jul 2017
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    Nashville
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    2000 21v Riders
    Could be the MAP sensor. Go back and check your work. Make sure the module is tight. Did you put a timing light on it the first time? If so. Is the advance cap plugged in good? The map could cause a backfire cold. but first check timing related items. and I don't believe the two are related I would look at the IAC for the crack throttle. Start issue. Once you find the timing advance plug is out. or put a new MAP. Put an IAC you have every thing else new except TPS. Hang that sucker and be done.
    Basically what I am saying once you do the Module and the IAC the rest of the sensors are cheep pull the throttle body give it a good cleaning put a new IAC and TPS on it. Put your new MAP on. Go another 500 hours.
    Last edited by gumby; 10-17-2018 at 09:01 PM.

  4. #4
    Join Date
    Aug 2010
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    Long Beach, CA
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    2015 Z3, 1999 2100i
    The fuel warm up curve on those doesn't settle down until after 140* so the injector pulse width is double during that time. A backfire is incorrect timing and as Gumby said, check the module is secure. We ALWAYS check timing following a module replacement. However, reading your post, you say it bogs and backfires at full throttle cold!!! Please elaborate on this. Do you mean under hard acceleration like pulling up a boarder or skier or do you actually mean full throttle?

    When you check fuel pressure, does the pressure bleed down within 30 seconds when the engine is off? If so you have a leaking injector, possibly multiple leaking injectors. Replace all of them if thats the case, if one goes the others are not far behind. If the fuel pressure does not bleed down, do a compression test. A backfire can also be a hanging intake valve but the symptoms really seem to be injector or timing related.
    Fixing everyone elses boat just so I can use mine...

  5. #5
    Join Date
    Aug 2008
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    Holladay, Utah
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    02 21V Riders edition
    Just backfire at full acceleration pulling up a wakeboarder cold engine. No backfire when just reving motor in neutral. Mine is an 02 mag mpi serial #OL687430 and looking at a parts diagram I dont see a map Sensor?? This is a naive question, but is there really much of a timing adjustment I can make with this motor? I thought it was controlled by the module and the ecm?
    The good news is, it runs awesome after a few minutes of running, and the compression checks out good.
    Thanks for all the input.

  6. #6
    Join Date
    Aug 2010
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    Long Beach, CA
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    43
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    2015 Z3, 1999 2100i
    On the timing, yes it is controlled by the ECM BUT only after base timing is set. The total advance on those engines should be no more than 34* at 3000rpm. The base timing gets set at 10* through the mercury diagnostics computer or by jumping two of the terminals on the DTC connector. Then the distributor module advances the timing with engine rpm based on load parameters measured through the sensors and ECM. However, at 3000 rpm in neutral and realistically maybe 2800 or slightly earlier, the total timing seen should be 34*. If its above that, the potential for too much advance when cold is there, hence the backfire.

    The map sensor is located at the front of the engine, top of the intake manifold. its number 12 in this diagram.
    https://www.perfprotech.com/mercury-...0?umodel=12336
    Fixing everyone elses boat just so I can use mine...

  7. #7
    Join Date
    Aug 2008
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    02 21V Riders edition
    Well, it’s been way too cold to try much, but I did want to update this thread after a lot of trial and error and throwing parts at the situation. LONG story short, I just started over but along the lines of the advice on this thread as to securing the module and checking my work. I had two days of 45 plus days and decided to do one better and just replace the module with a mercruiser part and not the Sierra one that I put in the first time. Sure enough the Mercruiser part fit in much more snug as it had a little foam on the outside edge and fit it very tight. I double checked all the wires etc and hooked up water and it runs like brand new. The mercruiser part was only around $80 and so I’m convinced that was it. I had ordered the other module off amazon and it was around $50. It appears that my running issues are solved. It ran perfect hooked up to the fake lake for 10-15 minutes. I’ll do a lake test as soon as this thing they call winter is over!
    Just for reference, these are the part numbers that I am referring to for the ignition module.
    New Part Number: 87-892150K02—- notice the “k” indicates mercruiser. A “q” would be a quicksilver part
    Sierra part#
    18-5116-1
    Thanks to all for your input.

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