slow to start, stalls w/ deceleration, air in lines

[robs523]

Hello, I have twin 3208 N/A engines and one of them now stalls when i decelerate but only after it's been running a while. This is the same engine that had a piston head break last year, followed by a new water pump. I also replaced the throttle shaft seal last year and have not had problems with that since. It was doing great for 50-60 hours after replacing the piston, including starting up without issues, but over the summer it stalled when backed all the way off the throttle while getting ready to anchor, after running smooth for the past 5 hours. Since then, this is what happens:

when it's hot, I have to bleed the lines, restart, and it runs in gear and neutral. I can accelerate and it will run, but as soon as I back off, it stalls.

If it's cold, it starts (after bleeding). In neutral, it runs fine while docked, whether I give it throttle or not. It's not until after it's been going a while that it stalls with deceleration.

I have checked all the lines and can't find anything needing tightened or obvious where air is getting in. I have not seen any evidence of a fuel leak when it happens. Before I had to replace the throttle shaft seal, I could get away with only bleeding the easily-accessible lines and the bleeder on top before starting, now I have to bleed both sides (I don't know if this matters or not...). Sometimes it cranks for a while before getting going, which I've seen asked several times in similar posts I've researched, but I'm not sure of the significance. From what I can piece together, I'm getting air from somewhere, but only with deceleration and only after it's been running a while. Possibilities might be lift pump, fuel pump, solenoid, or a loose line I missed. But if it was a loose line, wouldn't it always get air and run rough or stall?

I always seem to have some white smoke (not steam) with this engine. This winter I'm going to probably do as much as an overhaul as I can, hopefully it doesn't involve removing any pistons again! Until then, I 'd like to try to fix this if it's something I can diagnose or rule out easily, and if I have to address something really major, do it when I tear the engine apart in a few months. Any help is greatly appreciated, thank you,

Rob

 

[C lectric]

You can use a piece of clear vinyl tubing in the fuel system to prove air is building/leaking in. be able to make a loop so it collects. Of course initially there will be some but it should disappear shortly. If not then there is an air leak. Move the tubing to another position and do it again. When the air bubbles stop the last device is the cause.

Do not just go by no outward signs of fuel leaking. It may not show. When vacuum is on the system from the engine running air can be pulled in. But when the engine is stopped there is no pressure or vacuum so fuel may not leak out.

Engine heat from a run may evaporate the fuel if there is a leak out. That happened to me and it took me crawling alongside the engine to spot it. Otherwise the fuel was gone by the time I could shut down and drop in. I could smell it, just not see it untill I did a running inspection. Wear good hearing protection AND plugs and be CAREFULL.

Check the fuel filters. You don't say what you have but some Racors have had trouble if not FULLY resealed from time to time. Seals harden and fail to seal anymore when they can no longer conform to minute shape differences.
There is a check ball which has a seat that can be reversed when the mounting direction label is missed.

Idle leak testing may not be a good test. Very little fuel is pulled through the system. It isn't untill the engine really starts to work that any more serious vacuum is developed which may then pull in air.

If I remember correctly these engines , along with others, can clear small amounts of air especially at higher than idle engine speeds. When revs are dropped enough air may have accumulated somewhere in the system that it overwhelms the ability to clear.

That leak can also allow fuel to drain back out of the system in overnight [or longer] stops which means the pump must reprime itself leading to slow and bumbling/rough starts.

THere is a return flow also. Sometimes use of the vinyl tubing in both the feed line AND the return line can show if there is a problem.

Just my thoughts.

 

[Conall63]

I would be looking at the lift pump as the culprit.

Lift pumps are pretty inexpensive, and easily changed out...so much so that doesn't make much sense not to have a spare on board just in case.]


Conall

 

[robs523]

thank you for the ideas! I'm hoping to work on it next week when I get some time off before having to travel again.

 

[Ski in NC]

Second the clear line on fuel supply. Will show you if air is coming in. But also put a piece of clear line on the return. 3208 if an injector hangs open, compression blows back into injection pump and air loads the whole thing. In that case, you will see no air on supply, but will see air on the return.

Lift pump on 3208 is integral to injection pump, not a separate item.