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Old 12-11-2015, 12:04   #1
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Odd diesel starting behaviour.

Our boat's been out of the water for a few months so far whilst we kit it out for extended liveaboard cruising. Not long before haul out, I noticed that on first start when cold it seems to start, rev very high momentarily and then stall. On an immediate subsequent start it's fine and will continue to run fine thereafter. Started the motor after a few months the other day and same thing, although it turned over for a little while before firing, no doubt due to the time since last run.

The engine is a mechanically injected Mitsubishi 33 hp. It's only a few years old and under 200 hours. My google-fu has let me down on this one as I haven't been able to find a possible cause. One thing that may be relevant is it might be idling a bit low. Workshop manual says 1000 rpm, marinised owners manual says 925 and I think it's idling around 825 to 875 (the tacho is hopeless for accuracy). It was also bouncing around a bit on the mounts for a little while after starting. Hopeing it might just be a fuel quality problem, but would like to hear knowledgeable opinion!

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Old 12-11-2015, 16:17   #2
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Re: Odd diesel starting behaviour.

Test: Before next start bleed fuel system at filters and fuel pump. Do not crack injector lines, this test should eliminate air in system. If this does not fix problem the F/pump Governor fly weights may be sticking causing them to over react, now this test is crude but works, take a length of wood using it as drift to protect the surface find a solid part of the pump housing and give wood a few good blows with a weighted hammer.
If motor has been laid up with very dirty oil in system or has had water in sump or bad fuel all of these will and can cause your symtoms.
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Old 12-11-2015, 16:34   #3
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Re: Odd diesel starting behaviour.

Cheers Shakey.

The Mitsu S3L2 has an autobleeder. Not sure how it works, but it does. I'm going to change the fuel filters before next start. Come to think of it, I better check the oil as it literally hasn't been checked in months! Not neglect, I check it before each trip it's just that I didn't check when I started the motor the other day on the hard and it's been 4 months since last run.

The S3L2 also has an electric fuel pump so I assume you're referring to the injector pump?
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Old 13-11-2015, 00:00   #4
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Re: Odd diesel starting behaviour.

Injection pump is the one. self bleeding systems have there limits an air bubble the size of 1/2 dozen pin heads will cause your problem bleed system manually or try the following Pushing the stop button while turning key at 10sec ON-OFF intervals repeat 5-times. Have you checked all fuel fittings for tightness? When next run and warmed though advance throttle until it is running smooth note Rpm If it over 1000 max you may have a soft engine mount
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Old 13-11-2015, 00:50   #5
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Re: Odd diesel starting behaviour.

Cheers again mate.

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Old 13-11-2015, 09:39   #6
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Re: Odd diesel starting behaviour.

Really should not make a different how long a diesel has been sitting around. If it fires then dies, you got a fuel issue, possibly a bit of air in the system. The knocking around most likely misfiring in one of the cylinders(again probably air). Get the engine running and let it run under load for an hour so it heats up to op temp. That should solve your problem. If not, get a real mechanic to look at it.

There are pros and cons about long term storage of diesels. One side says drain the fuel lines to prevent algae and air from forming. Another says leave lines full since algae needs air and full lines keep air from forming.
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Old 13-11-2015, 09:57   #7
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Re: Odd diesel starting behaviour.

You can get one of those photo tachs pretty cheap, like $10 and eliminate any doubt about how fast the engine is turning over. Sounds like you are getting a fuel bleed back, and it is introducing a small amount of air into your system. I would check for leaks, starting from the tank to the engine and then your secondary fuel filter gaskets, and eliminate all the possibles one by one. The usual suspects are filter housing gaskets, any rubber fuel lines, sometimes if someone has over tightened a hose clamp it can cut into the hose.
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Old 13-11-2015, 09:59   #8
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Re: Odd diesel starting behaviour.

It sounds to me like you have a fuel starving issue, only after it sits a while.
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Old 13-11-2015, 10:02   #9
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Re: Odd diesel starting behaviour.

Bit concerned about his idle rpms. Our old Detroit clicks over at 300 rpm under load and idles(with lots of marbles rolling around) at 100 rpms. He could also take each cylinder off line, one at a time to figure out which one is causing the rock and roll.
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Old 13-11-2015, 10:22   #10
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Re: Odd diesel starting behaviour.

Too low idle rpms, most likely. Don't know how you stop that particular engine (kill handle or solenoid/switch), but low idle stops most diesels. I think this because it starts and runs the second time all the time. Either manually keep the idle up with the throttle or reset it on the engine. That would be my first step ITWMB. Good luck.
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Old 13-11-2015, 10:37   #11
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Re: Odd diesel starting behaviour.

Quote:
Originally Posted by Stu Jackson View Post
Too low idle rpms, most likely. Don't know how you stop that particular engine (kill handle or solenoid/switch), but low idle stops most diesels. I think this because it starts and runs the second time all the time. Either manually keep the idle up with the throttle or reset it on the engine. That would be my first step ITWMB. Good luck.
Tried and true way to kill is cut fuel or cut the air. Older diesels were built to go at low rpms. Newer light weight things are basically gas engine blocks built to run like a gas engine--high rpms. If his is running from 800 plus, then even a yanmar will run well at that idle. At lower rpms than design you will get problems with fuel injection timing. Still think, as others have posted, that a minor air bubble was in the system. Usually running an engine under load till it reaches and stays at operation temperatures for an hour will solve most problems. diesels are really hard to screw up unless there is water in the system.
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Old 13-11-2015, 11:10   #12
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Re: Odd diesel starting behaviour.

I am not acquainted with them. Just maybe a stab in the dark. Does the governor override to start and settles down to low. A little sticky from sitting.
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Old 13-11-2015, 11:17   #13
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Re: Odd diesel starting behaviour.

Just my guess/opinion.
You may have one leaky injector. 2 possible causes.

External leak-Check for fuel stains around each injector feed pipe connection-both at injector & inj. pump ends before starting.Tighten the fittings.

Internal leak-the injector itself is dripping fuel thru it's nozzle. Run the engine for a few hours & hope it clears. If still there & no other suggestions work,remove & test injectors.

My theory is that a tiny slow fuel leak while sitting is starving one inj. until "new" fuel drives air out & reaches inj.

Cheers/ Len
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Old 13-11-2015, 11:27   #14
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Re: Odd diesel starting behaviour.

I'm going to guess like a check valve if it has one is kinda sticking. If the crank takes a while on first start up there's air getting into the fuel line somewhere.

As well as a leaking injector can also cause it. Add power service to your fuel. For me it's been the best diesel additive I've ever used. Run it with the power service up to operating temp next time. This low sulfur fuel is doing poor work with the injectors and injector pumps.
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Old 13-11-2015, 12:23   #15
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Re: Odd diesel starting behaviour.

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Originally Posted by shakey doug View Post
. If this does not fix problem the F/pump Governor fly weights may be sticking causing them to over react, now this test is crude but works, take a length of wood using it as drift to protect the surface find a solid part of the pump housing and give wood a few good blows with a weighted hammer.
Not good. Better to give the engine an hour or so run and vary its speed. You can do that against the marina lines in gear up to maybe 2000 rpm. Obviously with good clean engine oil in the sump. Being in gear gives some load which is good.
Doing that at the marina will sort out any other problems such as an air leak in the fuel filter which might strand you if you are at sea. Any stickiness in the governor should go away with a good run like this.


Avoid over cranking when starting. You can flood your exhaust with cooling water. If you do need to crank more than say 5 seconds, turn off your water intake and once the motor fires shut it down. Then open the water. Then restating should be easy. Maybe your start battery has been standing too long and needs replacing.
Consider a solar panel to keep your batteries charged while not in use.
( my motor starts instantly and idles around 950 rpm)
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