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Starts cold, runs great, won't start warm

Larry Gaddis

New member
I have a puzzling problem with one of my AD41B engines. It starts well and runs great. If I shut it off and try to restart it right away, it starts. But one hour later, it won't start until it cools down completely. The next morning, it starts fine. I've changed all the filters to no avail. There is no air in the fuel return line, but I have been able to restart it hot by disconnecting that. The problem persists whether I use the forward or the rear fuel tanks, so I don't think it's a fuel vent or a fuel pickup issue. Any ideas for further ideas for diagnosis of this issue would be very welcome. Thanks.

Larry in Honolulu
 
Larry,

I'm not sure, but I would bet you have a vapor lock somewhere. Your fuel line is probably running to close to a heat sorce and forming a vapor lock during cool down.

BobT
 
Check if any back-pressure is built up in the return lines as well as temperature on return diesel.
If not enough diesel flowing through you will experience a vapor lock.
It may be enough to keep engines running, but not for cooling down the fuel systems and prevent vapor when engine is stopped.
 
I just replaced all fuel lines with new crimped style and went up one size with new banjo fittings, but could not get a penta return banjo. The shop and dealer say ther is very little pressure in the return, so for the sake of a 1/16 went ahead and used the wrong hose size (which seemed tight enough). Well it blew off the barb.
Whats my point.... I'm with Morten... you have a block in the return. It must build pressure somehow and the flow is not getting back to the tank. Try running with it connected to a jerry can. Good luck.
 
Note that the problem may not only be in the boat system.
Check out the return pipe going between the injectors as well.
 
Thank you all for your suggestions for resolving my issue. I think Haffiman and others who focus on my fuel return are on the right track. I'm very appreciative of your explanation of a partially restricted return leading to warm fuel which then results in a vapor lock that prevents starting. It makes a lot of sense. I've blown compressed air through the return line back to the tanks. It didn't "seem" restricted and it didn't resolve the problem. Next I'm going to remove the return pipe from the injectors and make sure it's clear. After that, I guess I could replace the entire return line.
I hate to think it's the injector pump itself. Rebuilding and reinstalling & timing the injector pump is a big job and costly especially if it doesn't fix the problem.
 
larry,
I know this is a strange question but it is leading towards answering your post.Did this problem start after having work done on the raw water pump?

Shakey.
 
It might even be the capacity of your fuel feed pump.
Since having 2 engines, easy to test.
Disconnect both return lines and monitor fuel return from both engines.
 
Thanks Shakey. I don't think I've done anything on the raw water pump or strainer for that engine. I did fix a coolant leak on that engine to the left of the thermostat housing but this starting problem didn't begin until a month or more after I repaired that leak.
 
larry'
I have seen a problem when refitting vp raw water circ pumps if joint is to thin or shaft is not fitted in correct fore & aft position, ie 0.020'+the fuel pump operation is affected.
 
Here's an update and a new clue. I've isolated the problem to the engine and probably to the injection pump. I ran it with the fuel intake and return going directly to a separate can of diesel with clear lines. Little or no air is visible, but the problem remains. Here's the clue: When the engine is warm and won't start, if I place ice cubes on the injector pump, it will start right away. I switched the solonoid stops between the two engines, but the problem didn't follow the solonoid, so that's not it. I wish I knew what could be happening with the injection pump that a little ice is able to temporarily correct.
 
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