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Low Engine RPM's

Discussion in 'Engines' started by Keysbearfl, Oct 21, 2014.

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  1. Keysbearfl

    Keysbearfl New Member

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    I have a problem that has me really stymied. I have a Hatteras 43 with 6V92's (535 hp) The starboard engine will not turn above 17oo RPM's There is no smoke coming out of the exhaust, no excessive vibration or unusual fuel burn or oil usage. The tach is slightly fluctuating between 1500-17oo RPM's but even with the throttles fully advanced the engine will not go up to WOT. Any suggestions of what to look at? The starboard engine is the slave engine on the synchronizer, but when the sync is not engaged the problem is the same.

    Any help is appreciated.

    thanks,

    Keysbearfl
  2. TeKeela

    TeKeela Member

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    I assume the observation that you are not reaching WOT is based on something other than the tach?
  3. Marmot

    Marmot Senior Member

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    All of the symptoms point at the engine not getting fuel. That could be caused by a laundry list of issues including a control lever slipped, a fuel filter almost blocked, governor issues ...

    One important thing that was missed and will help with troubleshooting - does it reach full rpm in neutral?
  4. Capt Ralph

    Capt Ralph Senior Member

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    Any fuel filter vacuum gauges? Observations?
    Have you searched any (of many) similar threads already posted?
  5. Capt J

    Capt J Senior Member

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    I'd look at fuel filters first. If its been a while, change them both....(primary+secondary). Then make sure it's actually getting full throttle at the rack. Also make sure the air filters are clean. If that's all good, I would think the turbo would be the next thing I'd look at.
  6. Marmot

    Marmot Senior Member

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    Both of those items will lead to black smoke unless there is a charge air pressure compensation feature on the governor. If so that is easily tested. Considering the OP clearly stated "there is no smoke coming out of the exhaust" it is fairly safe to say that charge air conditions may be placed far down the list of potential problems.

    That leaves fuel starvation as the most likely scenario. Either the engine is not getting the memo about how much fuel to deliver or it is not getting the fuel that is called for.

    If the engine reaches and can maintain high idle in neutral it is most likely a fuel filter issue.
  7. CSkipR

    CSkipR Member

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    What rpm will the starboard engine turn at idle? Will it go higher than 1700rpm?
  8. Capt Ralph

    Capt Ralph Senior Member

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    Should see a no load high idle @ 2300 rpm.
  9. NYCAP123

    NYCAP123 Senior Member

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    If you're reaching full RPM in neutral I'd see if the prop were fowled or the shaft bound.
  10. K1W1

    K1W1 Senior Member

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    There is quite a difference between neutral full rpm and full load rpm in amount of fuel required to achieve each level, If the prop were fouled or shaft bound there would be no shortage of fuel and therefore one would expect a wee bit of puff, the earlier statement that the revs fluctuate ( hunt ) by 200 rpm is also an indication of fuel starvation.
  11. NYCAP123

    NYCAP123 Senior Member

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    Agreed, but more info is needed, such as what happens when he brings her up in neutral. Also, I didn't get a clear statement about whether that 200 rpm fluctuation is when he's running manual or on sync. We had the same fluctuation, and I changed Racors twice before it dawned on me that the boss almost always ran synced. When I checked I found it was a problem with the sync (not yet resolved).
    While we're waiting for the OP to come back, and since it may be relative if it turns out that he's got two separate things going on, any thoughts on what would cause that with the sync?
  12. K1W1

    K1W1 Senior Member

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    Loose wire , dirty connections or chips on the speed sensor if its of the screwed into the flywheel housing type
  13. Capt Ralph

    Capt Ralph Senior Member

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    Glendening stuff on old Detroits. #1; engage cable slipping. Read Glendening support instructions on how to tighten it up.
    #2; Mechanical tach cables whipping. Pull the tach cables, make sure there is not any bends, pour in 5 drops of oil and re-insert the cable from the same end you just oiled.
  14. NYCAP123

    NYCAP123 Senior Member

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    Thanks guys. I've never done diddly with an auto-sync, in fact rarely use them (prefer my ears). So today I learned something new. Another good day.
  15. Keysbearfl

    Keysbearfl New Member

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    Thanks to everyone for responding. Greatly appreciated.

    I will go to the boat in the next several days to see if it goes over 1700 when at the dock, in neutral.

    The Racors are clean and the secondary's were replaced very recently-it did not resolve the issues.

    The Airseps are clean

    Tach fluctuation is only when above 1000 RPM's and occurs when the engine is not on sync as well as when the sync is used.

    It was suggested to blow out the fuel return line which we will try this weekend I hope. We had an issue with Algae in the tank, which resulted in having all the fuel pumped out, and polished. We have actually changed the Racors multiple times-they are clean and there are also no air bubbles seen which would indicate a fuel supply issue.

    I appreciate the help, and will update what we find.

    Thanks
  16. TeKeela

    TeKeela Member

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    Assuming a Glendenning, it could be the flex shaft in the tach cable drive adapter. Past experience with an 892 it would wear down and tach stops working properly and then the synchronizer doesn't know what is going on. I think they use the same/similar configuration.
  17. Capt Ralph

    Capt Ralph Senior Member

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    Your not over fueling or under load.
    Suggested by a s$#%^&^.

    The return line will not cause a fuel starvation issue.
    Detroits use a restrictor at the left head already, say 0.080 (or less).

    Per your description of clear exhaust; you have a rack (governator) issue or more likely a fuel starvation issue.
    You may have already trashed the injectors. They have their own lil filter.

    Anybody pull the pickup tube yet.

    No response yet on vacuum readings or pressure readings.

    Read any of the other threads yet?

    Or believing what someone suggested in blowing out the return lines...
    Last edited: Oct 22, 2014
  18. Capt J

    Capt J Senior Member

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    A vaccuum gauge on the racor (which is good to have anyways), or anywhere in the fuel supply line will tell you if you have a fuel starvation issue. A worn turbo would cause a loss in power. Are you sure that it's getting full travel on the throttle cable? I don't think the return line has anything to do with your issue.

    Usually the skipping tach (or even low reading) is the tach pickup needs to be adjusted at the flywheel. But also the cable going to the glenndenning is another popular source of that issue.
  19. Old Phart

    Old Phart Senior Member

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    I dunno
    Give that person bagpipes to blow. o_O

  20. Marmot

    Marmot Senior Member

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    Only if it is downstream of the lift pump. It won't tell you squat about what is going on in the delivery side. Vacuum gauges are for monitoring filter condition or tank valve status and that is all they are useful for.

    And a great deal of smoke if the fuel metering and delivery is functioning normally.

    No one said the rpm indication was "skipping" it was described as "slightly fluctuating between 1500-17oo RPM" which lacking further clarification normally means a smooth increase and decrease between those points. This is what can be expected if there is a fuel starvation issue.