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Old 08-19-2019, 08:10 AM   #1
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Cummins 4bt3.9 150hp

A general question on a fuel issue. Cummins engine starts fine and runs smoothly up to rated rpm's. After shutdown while engine is warm, it will not start again without priming{squeeze bulb inline for this issue}fuel. Once it restarts it runs fine until the next shutdown and attempted warm restart. This issue is on a boat I am considering purchasing. I will have this checked out by a Cummins mechanic before a purchase deal but was curious if anyone here has any opinions on what the problem could be?
Thanks
John
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Old 08-19-2019, 08:31 AM   #2
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if squeeze bulb always rectifies problem, there is an air leak. this will be between tank and engine driven lift pump. First place to look is racor filters as they will let air in around big gasket or t nut. Could be a valve packing, loose fitting or lift pump suction. Do not need a cummins mech necessarily but a good marine mechanic can find it.
Methodical inspection and tightening from tank to engine. also look for any indication of fuel anywhere on lines, clean them all and keep looking. will only weep fuel if at all, when shut down.
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Old 08-19-2019, 08:48 AM   #3
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if squeeze bulb always rectifies problem, there is an air leak. this will be between tank and engine driven lift pump. First place to look is racor filters as they will let air in around big gasket or t nut. Could be a valve packing, loose fitting or lift pump suction. Do not need a cummins mech necessarily but a good marine mechanic can find it.
Methodical inspection and tightening from tank to engine. also look for any indication of fuel anywhere on lines, clean them all and keep looking. will only weep fuel if at all, when shut down.
Thanks. That makes sense to me and was my initial thought also. But wouldn't the leak be present on initial cold start up as well as when warm? Engine reportedly starts without hesitation when cold, only doesn't start without priming when warm.
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Old 08-19-2019, 11:47 AM   #4
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Some injection pumps have a valve that enriched the fuel when cold. My truck Cummins had one don’t know if your engine does.
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Old 08-20-2019, 01:24 AM   #5
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Someone offline suggested the possibility of a fuel filled vent line? The boat reportedly is very difficult to fill with fuel, supposedly taking a long time. I'm not sure if this would create enough vacuum to stop the lift pump from supplying fuel to the engine? It has new tanks so maybe a valley in the vent line that is filling with fuel?
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Old 08-22-2019, 03:49 PM   #6
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Someone offline suggested the possibility of a fuel filled vent line? The boat reportedly is very difficult to fill with fuel, supposedly taking a long time. I'm not sure if this would create enough vacuum to stop the lift pump from supplying fuel to the engine? It has new tanks so maybe a valley in the vent line that is filling with fuel?
This bears investigating, but still points to an air leak.
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Old 08-22-2019, 04:39 PM   #7
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I don't think a hard to fill tank would have any bearing on this.

If it starts when squeeze bulb pumps it up, could be air or a weak lift pump. Both should do the same thing.

Suspect air in there somehow. Also some inj pumps do advance the rack further when cold, less so when warm. Might be something to that here.
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Old 08-22-2019, 04:57 PM   #8
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Check every joint for air leaks and fit a non return valve.
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