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Engine cranking slow


Robbo

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Anyone got any ideas what this could be, it just suddenly happened the other day..

My diesel generator has suddenly started to crank really slow when trying to start, before finally stop cranking (with a hot starter), cranking is around 15-30secs before this.   The battery is good, connections good, starter new.   I've removed the starter and checked the bushes which look okay, and I'll revisit all connections.  Is there anything else that can cause this?   When cranking and struggling to turn it's pulling over 400amp.

It's a Yanmar 3TNE68C-KM if that helps.

Edited by Robbo
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11 minutes ago, Paul C said:

Try turning over the genny by hand, ie using a suitable tool on the bolt on the end of its crankshaft. A lack of oil would/could cause a part-seizure, but there's many other possibilities too.

Once started it runs okay, and restarts easier when hot as it starts instantly.   I can't tell if it's easier to turn when hot as it starts to quickly.

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1 minute ago, Robbo said:

Once started it runs okay, and restarts easier when hot as it starts instantly.   I can't tell if it's easier to turn when hot as it starts to quickly.

Sounds like you've not tried it by hand, only by using the starter motor?

I had an engine which would be difficult to start, and crank slowly, when hot. When it was cold/not-too-hot it was alright. Its basically down to the tolerances in the engine being not quite right, meaning that it starts to seize up at one or other end of the extremes of temperature it operates in. In other words, the initial moments of an engine beginning to seize up. I solved the issue by selling it.

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Thanks for all the advise guys, i could easily turn the engine by hand so it looked to electrical which I should have spotted first hand :-/.  I moved the negative connection (required a new end!) to the starter motor bolt as the previous one was on the engine mount.  I think this did the trick as well as wire wool all the connections.

 

i was surprised how easy it was to turn by hand ( I just held the pully and was able to turn it, no levers required ),  are small diesels always this easy to turn?

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6 hours ago, Robbo said:

Thanks for all the advise guys, i could easily turn the engine by hand so it looked to electrical which I should have spotted first hand :-/.  I moved the negative connection (required a new end!) to the starter motor bolt as the previous one was on the engine mount.  I think this did the trick as well as wire wool all the connections.

 

i was surprised how easy it was to turn by hand ( I just held the pully and was able to turn it, no levers required ),  are small diesels always this easy to turn?

Depends on whether it has decompression levers. If it has, yes, it should be easy to turn  over manually. 

If it is a modern diesel without, then no.

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38 minutes ago, cuthound said:

Depends on whether it has decompression levers. If it has, yes, it should be easy to turn  over manually. 

If it is a modern diesel without, then no.

Unless it has poor compression ;)

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7 minutes ago, WotEver said:

Unless it has poor compression ;)

Very true. However the OP was asking if it was it it was normal for small diesels to turn over by hand easily, and I was saying no it is not normal unless they have a mechanism to decompress the cylinders.

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47 minutes ago, cuthound said:

Depends on whether it has decompression levers. If it has, yes, it should be easy to turn  over manually. 

If it is a modern diesel without, then no.

It's a modern diesel, I will have to see if I can do a full turn as I only tried a little.    The replacement and cleanup of the negative cable has given it a real burst of speed now when trying to start.  I think this is what burnt out the old starter motor with getting hot trying to start it due to poor connections.   I replaced it January and it seemed to be working fine since.

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Drove a lorry once with similar issues, one day the solenoid would not disengage so the starter motor was permanently running. It got quite hot before I could get at it with a big stick and destroy a connection. Turned out that a bad solenoid connection inside the thing had welded itself together. Why aren't vehicles fitted with a master switch?

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I see that a lot. I have customers who bring in a starter destroyed by this for an expensive rebuild and get it back with a letter explaining that the system needs checking. Every three months or so I rebuild it again and send it back with another letter. There are three or four going round like that at the moment, still, it's an earner.

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On 21/05/2017 at 19:34, cuthound said:

Very true. However the OP was asking if it was it it was normal for small diesels to turn over by hand easily, and I was saying no it is not normal unless they have a mechanism to decompress the cylinders.

It is normal to easily turn a 4 stroke engine if it's on the induction or exhaust stroke, but obviously not on the compression stroke. 

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On 22/05/2017 at 10:44, Bee said:

Drove a lorry once with similar issues, one day the solenoid would not disengage so the starter motor was permanently running. It got quite hot before I could get at it with a big stick and destroy a connection. Turned out that a bad solenoid connection inside the thing had welded itself together. Why aren't vehicles fitted with a master switch?

I have always been under the impression that lorries and buses did have master switches. 

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32 minutes ago, Machpoint005 said:

If it's a four-pot four-stroke diesel there will always be one of each!

Yes, that is indeed the case but you will still get a lot of movement to be able to check whether the engine is tight.

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