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Old 08-11-2015, 07:50 PM   #1
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Onan 20kw generator- How quick should it start ?

Have two of them actually, 1993 vintage with about 1600 hours each. After sitting unused for say 3 days or so, takes about 3 tries of perhaps 5 seconds starter time each to get them going. And that's after holding the preheat position for 10 seconds before trying.

Which is fine, no big deal...but just wondering if that is "normal" for these ? I wonder partially because my two Detroit 12v71TA's start the instant I press the start button with no preheat. But then they have less than 300 hours since major overhaul.

Is that the difference...the hours....or is what I describe perfectly typical for the little Cummins engines in Onan 20kw generators ? Or is it a 2 stroke vs 4 stroke diesel thing ? or ?

How would a brand new Onan 20kw start ? Would it start the nanosecond you pressed the start toggle ?
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Old 08-11-2015, 08:01 PM   #2
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A good diesel should start within a couple of seconds, even cold.


Have someone crank the engine when cold while you watch the exhaust. Is heavy white or grey smoke coming out. If heavy smoke is coming out of the exhaust and it isn't starting then that is probably a low compression issue.


If no heavy smoke comes out, then that is probably a fuel issue. You may have a leak in the fuel hose or tubing that is leaking down and it takes 10 seconds or more of cranking to prime the engine.


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Old 08-11-2015, 08:56 PM   #3
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I have a 20 kw also and it usually starts about the second or third try also (three or four seconds of starter per try. I agree that it is probably a fuel issue. I always figured it was just a lot further from the fuel pick up/tank.
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Old 08-12-2015, 06:47 AM   #4
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And that's after holding the preheat position for 10 seconds before trying.

Depending on the engine 10 seconds of pre heat is very little.

I had an Izissu that had pre combustion chambers and needed 30 seconds , even in the carib.

Check the owners manual, to find their advice for preheat time..
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Old 08-12-2015, 07:27 AM   #5
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guys. I have two 20kw northern lights gensets. When I got the boat, the port genset started within a second of turning key after ten seconds preheat. No smoke.

the stb genset took a looong time to start. Then lots of puff of white smoke. It is a chamber heat issue producing white smoke.

I checked my glow plugs and sure enough only one was working. Replaced all four and now good as new and zero smoke. As fast as port side.

so check your pre heat plugs.
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Old 08-12-2015, 07:48 AM   #6
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The Hatt has a 20kw Onan MDL4 with Cummins engine of that vintage that has always started promptly. However the manual says 5 seconds is well within the range of time for it to crank.. they say no more than 30 seconds. Fuel is the usual culprit for these engine to have trouble starting when warm. While 5 seconds is OK, it might be time to change out the on-engine fuel filter and check the Racor (does it have a vacuum gauge?)

I always used about 10 seconds on the pre-heat, but that is a minimum based on temps 86 degrees or higher. Between 50 and 86 they say 15 seconds.
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Old 08-12-2015, 10:20 AM   #7
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On our onan the manual says 25 second preheat AND 10 seconds after starting. Typically unless it's really cold I do about 15 seconds and none after starting. Doing either results in a start in about 3 seconds.. first try

A restart within 8 hours of running requires no preheat if even moderately warm.

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Old 08-12-2015, 10:26 AM   #8
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The Hatt has a 20kw Onan MDL4 with Cummins engine of that vintage that has always started promptly. However the manual says 5 seconds is well within the range of time for it to crank.. they say no more than 30 seconds. Fuel is the usual culprit for these engine to have trouble starting when warm. While 5 seconds is OK, it might be time to change out the on-engine fuel filter and check the Racor (does it have a vacuum gauge?)
There is a filter restriction gauge for the prime movers but not the generators. Both Racor and the odd looking rectangular secondary fuel filters were changed about 30 hours ago. There is no smoke that I am aware of with either generator...perhaps a hint of smell when first started but no visible smoke.

Re fuel, it is interesting that the port generator would not start when I first took possession of the boat (it did start for the survey a week earlier). Figured out quickly it was not getting fuel. And this, after brand new filters. Even filling the Racor bowl by hand and pumping the manual primer pump lever hundreds of times did not result in one drop of fuel to the injector (with main injector input unscrewed slightly)

Then I figured out the darn things had electric primer pumps I was unaware of. Once the appropriate valves were opened and the electric pump button pushed, finally...fuel and she started right up !

Supposedly the reason the port generator got to the no fuel situation in the first place is the generator pickup tube in the fuel tank is higher up than the pickup tube for the engines.

My fuel gauge, which was on 7/8 full when I bought the boat....was completely wrong as I really had more like 1/8 tank !! (surprise, surprise...$2,600 I was not expecting down the drain in fuel costs !!)

(in case this is confusing, the above "getting no fuel via manual primer" episode was after the fuel tank was filled and had plenty of fuel)

This brings two more interesting questions-

1. Is it typical for generator pickup tubes to be higher in the tank than the main engine tubes ? (makes sense they would be but just confirming or not)

2. Why would that port generator not get fuel via the manual primer lever and was absolutely dependent on the electric primer ?
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Old 08-12-2015, 10:32 AM   #9
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On our onan the manual says 25 second preheat AND 10 seconds after starting. Typically unless it's really cold I do about 15 seconds and none after starting. Doing either results in a start in about 3 seconds.. first try

A restart within 8 hours of running requires no preheat if even moderately warm.

HOLLYWOOD
It would be interesting to me if you guys with these sorts of reports would mention the model year and hours on your generator.

As mentioned in original post, mine are 1993 vintage and have aprox 1600 hours each.

Which brings up yet another question... is 1600 hours considered low, moderate or high hours for one of these ? If fuel filters tended to and oil and filters changed on schedule, how many hours can they go before needing rebuild ?
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Old 08-12-2015, 11:24 AM   #10
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1. Is it typical for generator pickup tubes to be higher in the tank than the main engine tubes ? (makes sense they would be but just confirming or not)

I cannot say as I don't recall running our tanks down low enough to have a genny shut down in the few boats we have owned nor ones I have crewed on. However, this is a feature on all the diesel motorhomes I have owned. It allows a "reserve" of fuel to get one back to a truck stop to refuel. On the first bus I bought I lost the genset near Los Cruces on the delivery from LA to La. Of course my dad and I, having dealt with fuel issues on boats, immediately determined a no fuel condition and I went into commando mode running to various parts houses to find a new Onan fuel pump and secondary filter while my dad kicked up his feet and began to RTFM! By my third parts house he suggested we fuel up (as we had less than 1/4 tank of 125 gal left). I did and the genny fired up after several extended cranks.
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Old 08-12-2015, 12:26 PM   #11
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Which brings up yet another question... is 1600 hours considered low, moderate or high hours for one of these ? If fuel filters tended to and oil and filters changed on schedule, how many hours can they go before needing rebuild ?
Low. My guess, 20,000 +, if you can put those hours on in 20 years, and if, in addition to proper maintenance as you mentioned, they are run with about a 75% load -- not a lot more, but not much less either. After that, age will be the primary determinant.

I see your gensets have 20+ years. They may have another 10 years in them, but they will definitely die of old age before the hours get them, IMO/IME.
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Old 08-12-2015, 12:34 PM   #12
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There is a filter restriction gauge for the prime movers but not the generators.
Just an FYI, the term "prime mover" is used to refer to the engine that runs the armature end of a genset.

And taking 5 seconds or so to start doesn't sound like an issue to me. But as others have noted, check the glow plugs.
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Old 08-12-2015, 01:40 PM   #13
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Just an FYI, the term "prime mover" is used to refer to the engine that runs the armature end of a genset.

And taking 5 seconds or so to start doesn't sound like an issue to me. But as others have noted, check the glow plugs.
Re prime mover, literally you are right, but historically the term refers to any engine that converts fuel to useful work. And since the most "work" is done by the 900 hp Detroits I think in terms of them being the "prime" movers !

Re 5 seconds...note I mentioned two to three "reps" of 5 seconds before they will keep going.

For the gory details...the typical scenario is, 10 seconds of preheat, 5 seconds of starter and they kick in for a second or two and then shut off. Then another 10 seconds of preheat, 4 seconds of starter, they start for another 4 seconds and shut off.

Then yet another 10 seconds of preheat, 2 to 3 seconds of starter, they come on and stay on. Once on they rev up and down a little for perhaps 5 seconds before settling in to a constant RPM and run perfectly from there on.
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Old 08-12-2015, 01:50 PM   #14
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Are you starting them into a load? Otherwise sure sounds like fuel. But I have never had a glow plug related issue so won't rule that out, but if they light up for a few seconds I don't know.. By the way on the lift pump priming lever, it has to be engaged. Sometimes you have to crank the engine over a half a turn to get it to do so.
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Old 08-12-2015, 05:03 PM   #15
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Are you starting them into a load? Otherwise sure sounds like fuel. But I have never had a glow plug related issue so won't rule that out, but if they light up for a few seconds I don't know.. By the way on the lift pump priming lever, it has to be engaged. Sometimes you have to crank the engine over a half a turn to get it to do so.
Not starting into load (generator circuit breaker off) Re pump prime lever, tried combining that with engine turn over as well...still, even with full fuel Racor filter, could not get the first drop of fuel into the secondary fuel filter via the manual pump. Only the electric pump did the trick.

Re the glow plugs...I am assuming they are working but maybe not....I should try my usual sequence of events and see if the gens start the same with no preheating.
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Old 08-13-2015, 06:51 AM   #16
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A simple test (even easier than using a meter to check the V or resistance of the glow plugs can be done.

With a dead cold engine 48+ hours of no operation , give a TINY !!!! squirt of starting fluid to the intake and crank.

If the engine starts and runs it is most probably a lack of pre heat that is making starting take a while.

NEVER EVER EVER EVER EVER use Ether to start a warm engine , or one that has run out of fuel to spin more rapidly for priming.

Only a tiny shot ONCE .
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Old 08-14-2015, 10:41 AM   #17
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A simple test (even easier than using a meter to check the V or resistance of the glow plugs can be done.

With a dead cold engine 48+ hours of no operation , give a TINY !!!! squirt of starting fluid to the intake and crank.

If the engine starts and runs it is most probably a lack of pre heat that is making starting take a while.

NEVER EVER EVER EVER EVER use Ether to start a warm engine , or one that has run out of fuel to spin more rapidly for priming.

Only a tiny shot ONCE .
WD-40 is a good substitute for ether. While it isn't quite as immediately effective, it can be used much more liberally.
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Old 08-14-2015, 12:30 PM   #18
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Don't use ether at all on those glow plug diesels. Just don't.

Those MDL series use an Onan designed engine. They got labled as a "A" series Cummins when they bought Onan, but it was never a Cummins and they are not well supported at this time. A good machine, though.

They have a mechanical lift pump. Little trick here: Snug up the diaphram screws. Rubber shrinks and a snug can cure air leaking in. I have not found a source for that pump, so unless someone can find one, replace with electric should it fail.

Five seconds of cranking is no big deal if machine is otherwise reliable.
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Old 08-14-2015, 02:29 PM   #19
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Five seconds of cranking is no big deal if machine is otherwise reliable.
It's more than 5 seconds total, please re read my posts to clarify what I mean.
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Old 08-14-2015, 03:17 PM   #20
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It's more than 5 seconds total, please re read my posts to clarify what I mean.
I appologize for my poor comprehension. Good luck with the troubleshooting!!
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