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Old 01-07-2013, 21:25   #1
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Diesel engine hunting

Between 800 and 1000 revs the Volvo Penta TMD31B diesel engine on my sailboat hunts (revs fall and increase rhythmically with a period of about 1 or 2 seconds).

Below 800 revs the engine idles normally. Above 1000 revs it runs fine.

I have checked fuel lines and filters and all seems OK. I have changed air filter and checked the turbo spins OK.

Could this hunting be due to something wrong in the injection pump?
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Old 01-07-2013, 21:46   #2
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Re: Diesel engine hunting

Probably but why worry? Does it do it under load also? That could be an issue.
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Old 02-07-2013, 00:51   #3
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Re: Diesel engine hunting

Everyone needs to remember that to days diesel is no where near as good as what most of our engines were made to run on !! at least here in the US. It really requires a different tune up and changes in fuel delivery and timeing to make them run right with this stuff they call diesel these day !! Ive had to change the tune on my new to us boats Detroit engine to make it a happy camper ! Just sayin, a little tune up never hurt a diesel engine
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Old 02-07-2013, 02:32   #4
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Re: Diesel engine hunting

Working on car engines, hunting of the rpm has been caused by a faulty throttle stepping motor or incorrectly adjusted air control.
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Old 02-07-2013, 04:45   #5
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Re: Diesel engine hunting

I'm getting a little hunting in my 6 cylinder Ford and I'm wondering if it could be moisture encapsulated by a heavy dose of "Fuel Set."
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Old 08-07-2013, 17:54   #6
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Re: Diesel engine hunting

The hunting does affect the engine under load and sometimes causes it to stall completely.

I am trying to figure out whether this pattern of hunting (just between 800 and 1000 rpm) could be caused by a problem in the fuel supply (like air getting in) or is it most likely to be due to the govenor malfunctioning at these revs. Any ideas?
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Old 08-07-2013, 18:01   #7
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Quote:
Originally Posted by Streetcar View Post
The hunting does affect the engine under load and sometimes causes it to stall completely.

I am trying to figure out whether this pattern of hunting (just between 800 and 1000 rpm) could be caused by a problem in the fuel supply (like air getting in) or is it most likely to be due to the govenor malfunctioning at these revs. Any ideas?
First Check your fuel supply system, after that , Id say faulty governor

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Old 08-07-2013, 18:33   #8
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Re: Diesel engine hunting

I have found that hunting happens before stalling at least you are getting your warning! fuel lines, pumps,filters etc check your oil make sure you dont have too much,as in fuel diluted, injection pump is the last thing on the list after all else has been eliminated
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Old 09-07-2013, 21:25   #9
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Re: Diesel engine hunting

Quote:
Originally Posted by Streetcar View Post
Between 800 and 1000 revs the Volvo Penta TMD31B diesel engine on my sailboat hunts (revs fall and increase rhythmically with a period of about 1 or 2 seconds).

Below 800 revs the engine idles normally. Above 1000 revs it runs fine.

I have checked fuel lines and filters and all seems OK. I have changed air filter and checked the turbo spins OK.

Could this hunting be due to something wrong in the injection pump?
Hunting of a diesel engine is cause by a few things

1) very cold engine oil or diesel fuel
2) worn governor or binding on the injector rack if its a DD
3) mechanical gov not set up correctly
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Old 10-07-2013, 05:03   #10
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Re: Diesel engine hunting

Reading your original post I would not have been overly concerned with this problem but now you have indicated she is stalling under load this is serious... I am familiar with Kuboto engines and on this The fuel pump has a series of springs which control fuel in idel, and as the revvs increase... i would certinly check out the fuel pump first
get a workshop manual for the engine and for reference check out Kubotoman.uk for the technical advice and carrying out a spill test from your pump....regards
john
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Old 10-07-2013, 05:31   #11
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Re: Diesel engine hunting

Maybe problem solved! Thanks for all the contributions.

This maybe only a temporary solution. I opened the final fuel supply connector into the injection pump to have a look. It was clean and so I put it back on. Then I started the engine to make sure it ran without any air block developing. The hunting problem had completely gone away...

For what its worth, my theory is: there may have been some air trapped in the supply hose at this connector.

...although I also note the oil level is too high (sartorst)

Anyway, indications are that problem wasn't in the injection pump.

I'll come back if the problem re-occurs.

Thanks all!
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Old 10-07-2013, 05:34   #12
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Quote:
Originally Posted by Streetcar View Post
Between 800 and 1000 revs the Volvo Penta TMD31B diesel engine on my sailboat hunts (revs fall and increase rhythmically with a period of about 1 or 2 seconds).

Below 800 revs the engine idles normally. Above 1000 revs it runs fine.

I have checked fuel lines and filters and all seems OK. I have changed air filter and checked the turbo spins OK.

Could this hunting be due to something wrong in the injection pump?
Check voltages coming out of the alternator. We had same symptom thus spring. It turned out that the internal regulator was having problems at low RPMs. Voltage would bounce between 14.5 and 15.5. Replaced the alternator and all is back to normal. Still looking to get the old one fixed.
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