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  1. #1

    Default Gray marine 671 rpm issue

    I have a gray marine diesel with a manual transmission. At idle I can bring the throttle up and get over 1400 rpms but when it's in gear it won't get over 1100. It's as if when I put it in gear it reduces the rpms and idles lower. Does anyone know what this could be? I just replaced the throttle cable because it was slipping back and tightened the controls. Any help would be greatly appreciated. Thank you!

  2. #2
    Join Date
    Nov 2009
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    temecula, ca., usa
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    Default Re: Gray marine 671 rpm issue

    It sounds as if your engine governor has gone out of adjustment. You should be able to throttle up the engine with NO LOAD (neutral) and obtain around 2100 rpm. There will be a natural drop in rpm at ANY throttle setting when a load (in gear) is added. Detroit calls this drop off "droop" and it can be around 100 to 200 rpm from a 2100 rpm setting. The droop difference will get greater as the Max speed gets lower. So your 300 rpm drop sounds about right for a mis-adjusted unit.

    Of course, the governor can be adjusted to obtain pretty much any maximum rpm desired... up to a point. And, it is possible the 1400 no load setting was done on purpose...you don't say what application the engine is being utilized for...but 2100 no load is a "standard" setting for any variable speed use IE: trucks...boats....off road equipment.

    The governors on these use springs to put pressure on the throttle control rods and spinning weights to counteract the springs as the engine increases it's speed. It's a simple matter of harnessing centrifugal force. What usually happens in a case such as yours is that the HIGH SPEED spring LOCK NUT gets loose and allows the HIGH SPEED SPRING adjustment to BACK OFF.

    These engines are not complicated but it seems few people that own them understand how they work or how to adjust them. You may want to find a qualified Detroit mechanic to take a look at it for you but owning the engine manual is a PLUS for any skipper faced with doing their own maintenance. How to adjust idle setting and high speed setting is in the manual.

  3. #3

    Default Re: Gray marine 671 rpm issue

    Thanks for your input. It can go higher than 1400 at idle but I haven't tried to push it too hard at the dock. I usually run it at 1250 when steaming and towing. It's in a small dragger. I have a few manuals to consult while I adjust the governor. Thanks again.

  4. #4
    Join Date
    Nov 2009
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    temecula, ca., usa
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    Default Re: Gray marine 671 rpm issue

    Well then....I'm afraid I didn't really understand your question. So...let me speculate;

    What you were trying to convey is that something has recently changed? I'm guessing that you start it...."idle" it up to 1400 and then...when you're ready to "roll" you pop it into gear and go. But...instead of just dropping to 1250...like it used to...it drops to 1100.

    Is that close to what you meant? Because if it is....the problem...and the solution... might be quite different than what I first thought.

    As I stated previously,these engines have TWO springs in the governor...the LOW SPEED spring...for "idling"....and the HIGH SPEED spring for all higher speeds. As a result...they also have TWO sets of controlling weights rotating on a shaft to counter balance the effects of the springs on the fuel control rod that goes to and operates the injectors.

    From start up to...coincidentally...around 1100 rpm....the LOW SPEED spring is essentially pushing on the fuel rod to "idle" the engine. It's counter balance partner in this...the LOW SPEED weights...are spinning and pushing BACK against the spring to maintain the LOWER SPEED setting the operator has selected. But, when the operator pushes the throttle to select a speed somewhere ABOVE the limits of the low speed spring...a transition takes place.

    At some point...usually around 1100 rpm....the governor control has to transition from being influenced by the low speed spring and weights and the HIGH SPEED SPRING AND WEIGHTS come into play. As the throttle is advanced, there is GAP that is crossed. That's what Detroit Diesel calls it...the GOVERNOR GAP setting. It is the place that NEITHER the low speed spring NOR the high speed spring is actively "pushing" on the fuel rod. If that gap is not properly adjusted...it COULD cause the sort of symptom you are describing.

    I say COULD because it isn't the ONLY thing that could cause your complaint. It could be that the engine is simply down on power for whatever reason...IE: dirty injector(s)... clogged fuel filters...low fuel pressure...poor quality fuel...and the list goes on.

    Also, my "example" is for a single shaft and weight set governor in the later model engines. If your engine utilizes the older...twin shaft governor, the concept is somewhat the same but the "gap" setting is dealt with in a much different manner. You will need to consult the manuals you have on board and compare the information to your engine to be sure you are approaching your adjustments in a logical manner.

    You should first go through a "loss of power" checklist before making any adjustments to the governor. Replace fuel filters...check fuel pressure and "spillback" for presence of air....clean or replace air induction filtration....ensure there are no exhaust restrictions.

    If any of these terms or tasks are unfamiliar to you it would be in your best interest to "bone up" on them as they are the essential components to keeping a Detroit running in the "sweet spot".

    Good luck.

  5. #5

    Default Re: Gray marine 671 rpm issue

    Thanks for Your help. i fixed the problem it was a fuel restriction issue. Thankfully it wasn't the governor because I'm not ready to dive into that type of project yeti do wouldn't be able to get a mechanic for a few days. Although I did learn quite a bit along the way. The engine fairly new to me so I'm glad to have found this message board.

  6. #6
    Join Date
    Nov 2009
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    Default Re: Gray marine 671 rpm issue

    Very good to hear you got it sorted out.

    Just keep in mind...these engines are not all that complicated...they don't even have intake valves! But many people tend to "overthink" them. Including some fairly astute mechanics I have known.

    Feed the old girl clean, quality fuel with no water in it and keep the oil changed and she will ALWAYS get you home.

    Speaking of home....I'll give you some "homework". The fuel system actually RELIES on RESTRICTION to fuel flow to raise the fuel pressure to around 55 to 60 psi.

    This is accomplished through the use of a RESTRICTED fitting that the fuel RETURN line connects to before it is routed back to the tank. See if you can locate and identify it.

    The fitting looks pretty much like any other of the brass/bronze "flared" fuel line fittings but with one SIGNIFICANT difference. Inside is a precisely drilled passage to causes the fuel to "back up" behind the fitting and increase pressure going to the injectors. If you were able to examine the fitting closely, you should find a stamping on the exterior describing the size of the drilled "orifice". This is typically anywhere from 0.050" to 0.080" depending on application. I'm guessing yours is 0.060" or 0.070".

    The reason I bring it up is twofold; the fitting will...over time..."errode" and the orifice will enlarge. This will cause your fuel pressure to be low. Also, it can get debris in the orifice and, while your pressure will go way up, the flow will go down and starve the engine for fuel. This could damage the injectors as they need fuel to cool themselves and lubricate the plungers.

    The RESTRICTED FITTING is also your "test point" for determining if the fuel system is working properly and if air is entering the lines. Learning where this fitting is located and reading up on how to perform the fuel "spillback" test is Detroit Diesel 101 should you ever encounter a problem. Air entering the fuel lines is the #1 reason for no start/hard starting and low power complaints.

    Hope you get many years of trouble free service from the Gray Lady.
    Last edited by jgmo; 10-27-2015 at 02:31 AM.

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