Intermittent rough running, coughs when starting

[BeezFun]

Two symptoms, not sure if they're related:

• 
  When cold starting, engine runs rough and sputters for a few seconds, kind of like a gas engine that has too much fuel. Requires glow plug even in hot weather. Sometimes takes several starting attempts. After a few seconds it runs smooth. Warm starts are fine.

• 
  Occasionally while running at operating speed, it will run rough like it's only running on 2 of the 3 cylinders, then it clears up in 5-10 seconds. No noticeable smoke and exhaust pipe has some soot in it, but no fuel smell, and no oil.

I cracked open each of the injector nuts while it was running, and the change in sound/speed on each cylinder was the same. I don't have a compression tester, but I would have thought if the compression was bad in one cylinder it would show up when I cracked the nut on that cylinder. Also adjusted the valves, which didn't really need adjusting.

Any thoughts on how to diagnose this? The engine is 3cyl Shibaura with 2500 hours in a New Holland, but this forum is much more active so I thought I'd have a better chance posting here. I did change the fuel filter on it when I bought it a few months ago, and it was totally packed with fine sediment and grass, surprised it ran at all. I flushed the fuel lines backwards, cleaned the tank, and replaced filter, but that doesn't seem to have any affect.
tnx

 

[Pepsiboy]

How many hours on tractor?? I had similar problem with my BX 2350. turned out primary fuel filter was plugged. Changed that and problem was gone for about 4 days. Turned out REAL problem was that electric fuel pump was going out. New was $250 from Kubota Found aftermarket one at Auto Zone for $50. It had higher flow rate and higher pressure than original. No more starting OR rough running problems.:thumbsup: Took me about 45 minutes to change pump, including draining fuel tank first.

Best luck with your problem. :2cents:
Dave

 

[Rocky Hoffler]

If it starts fine when hot and only gives problem when it has been sitting for a time you might have a fuel drain back issue could be a check valve in the return to tank line

 

[BeezFun]

How many hours on tractor??

2500

Turned out REAL problem was that electric fuel pump was going out.

My pump is mechanical, but I suppose it could still have a problem.
tnx

 

[BeezFun]

If it starts fine when hot and only gives problem when it has been sitting for a time you might have a fuel drain back issue could be a check valve in the return to tank line

Doesn't have a check valve in return line. But while checking the manual to find out, I discovered there's a troubleshooting table for the fuel system. One of the things that fits my symptoms is an air leak on the suction side of the system. Not sure how I check for an air leak, but I'll look into that.

 

[Rocky Hoffler]

There hard to find will suck air and not leak if its rubber line try looking for damp area or dirty area if you can bend hose look for cracks or just replace hose cheap and then you know that line is ok.
If possible clamp off line at tank and then put a small amount of pressure on line and look for leak Good Hunting

 

[Pepsiboy]

2500

My pump is mechanical, but I suppose it could still have a problem.
tnx

BeezFun,

Does your tractor have JUST the mechanical fuel pump to the injectors? Or is there another fuel pump between the fuel tank and the injector pump? Mine has an electric fuel pump between the fuel tank and the injector pump. That is what I was referring to. If that pump has failed or is starting to fail, it could cause your problem. With the high hours on your tractor, I would guess that the fuel line MIGHT by the problem, also. :2cents: As others have said, I would also check the fuel lines for leaks.

Good luck. Please let me know how you do with this problem of yours. Thanks.

Dave

 

[BeezFun]

I installed a piece of clear plastic tubing between the filter and fuel pump. That allowed me to see any air bubbles if it's sucking air. No bubbles, so there's no air being introduced prior to the fuel pump. I guess there could still be something wrong with either fuel pump or injector pump, not sure how to diagnose that without guages to measure pressure in the line. I think I'm going to check compression next, I must know someone with a guage and adapters I can borrow.

 

[Pepsiboy]

I installed a piece of clear plastic tubing between the filter and fuel pump. That allowed me to see any air bubbles if it's sucking air. No bubbles, so there's no air being introduced prior to the fuel pump. I guess there could still be something wrong with either fuel pump or injector pump, not sure how to diagnose that without guages to measure pressure in the line. I think I'm going to check compression next, I must know someone with a guage and adapters I can borrow.

You could try putting a small (Zero to 35 psi) pressure gauge on the line that goes into the injection pump, turn on the key so the electric pump runs, and see what pressure is there. If less than 5 psi, replace electric pump. If more than 5 psi, put line back on injection pump and try putting about 6 or 8 oz of fuel injector cleaner into fuel tank. Then run it hard for a while and see if that helps.:2cents:

Dave

 

[BeezFun]

..., turn on the key so the electric pump runs, and see what pressure is there...

Pump is mechanical, not electric. I disconnected line at injector pump and cranked engine, output from fuel pump was quite impressive, so I don't think there's a problem with fuel pump. I don't think a mechanical pump fails the same way an electric pump fails, it would either work or not work, but wouldn't exhibit intermittent problems.