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Old 15-08-2014, 07:56   #16
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Re: rough idle on Caterpillar 3208 Suggestions?

Surging and rough idle are very commonly caused by air leaks on the low-pressure side of the fuel system. These leaks are caused by aging fuel lines/ poor connects, and are exacerbated by increased pressure from bad/ dirty fuel that causes restrictions in the filters, the lines, or even the pick-up tube in your tank. Visual inspection of the filters will not reveal this problem.

Before you start pulling injectors or embarking on other expensive/ invasive diagnostic adventures, you should eliminate the possibility of air leaks by bypassing your tank, your old fuel, and your Racors. Get yourself a plastic, three to five gallon fuel tank from an outboard engine, fill it with clean, new diesel, and connect its hose to your secondary filter (the one on your engine). If you leave the priming bulb in the hose it will make it easy to bleed the system.
The shop guys call this a 'pony tank'.

If this doesn't solve the problem, you can move on to more complicated procedures.


Here is a good thread about how much time and money could be spent overlooking the simple solutions Yanmar Problems
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Old 15-08-2014, 08:11   #17
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Re: rough idle on Caterpillar 3208 Suggestions?

Yukon has some great advice. You can do a lot of the work yourself. It doesn't require a lot of mechanical expertise.

One of the prime areas of air leaking into the system is the Racor filter assembly itself but there could be a leak anywhere from pickup tube to the lift/low pressure fuel pump.

Good luck.
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Old 15-08-2014, 08:35   #18
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Re: rough idle on Caterpillar 3208 Suggestions?

These last 2 posts have given us some hope to avoid extensive financial investment. Yukon Cornelius, your idea of a pony tank jogged my memory of something I did inadvertently that might support the theory of air leak. I have a fuel manifold which enables me to cut off each engine and each fuel tank. The other day I started the Starbrd engine without opening the Strbrd engine fuel manifold. She ran perfectly with the fuel that was already in motor and suddenly I remembered I needed to open that valve to keep from running out of fuel. As soon as I opened the valve to maintain engine running, I started again to have problems running at low rpms. All my lines appear to be steel coming from fuel tanks, I have not chkd those connections. How can I detect an air leak in a coupling that seems to be tight? What is your impression of my inadvertent mistake and your recommendations to follow now? What do you think of the idea of narrowing down to see if its both tanks or just one, to shut off valve to one tank and see if the motors run without the rough idle? All the support is greatly appreciated!
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Old 15-08-2014, 16:11   #19
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Re: rough idle on Caterpillar 3208 Suggestions?

Whenever you change a filter, run out of fuel, or even run the engine with the fuel shut off (as you may have done), you need to fully bleed the fuel system. It's not worth trying to diagnose other problems until you are sure the system has been properly purged.
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Old 15-08-2014, 16:25   #20
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Re: rough idle on Caterpillar 3208 Suggestions?

One of my 3208s does that , especially when cold. I can bump the throttle up a little bit and it smooths out.
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Old 15-08-2014, 18:38   #21
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Re: Rough Idle on Caterpillar 3208 Suggestions?

Yukon I must have left something out. When the valve to the tank was closed and it was running on just what was in the line already, it ran without the rough idle. I shut the motor off and went to open the valve then recranked and the roughness was back. Motion30 our 3208's will not even out, they stay running with the rough idle if I have to reduce rpms. They seem to run fine at higher idle but not at low.
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Old 16-08-2014, 12:49   #22
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Re: Rough Idle on Caterpillar 3208 Suggestions?

No, I understood you, I just don't think that is a smoking gun -engines with air in the fuel lines (especially with multiple petcocks/manifolds and dual tanks) do funny things like that. You still have to bleed the fuel system carefully before you move forward with your diagnostics.

Have you carefully bled the system since all of your recent filter changes? The solution might be as simple as that.
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Old 16-08-2014, 13:04   #23
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Re: Rough Idle on Caterpillar 3208 Suggestions?

The only way I know to bleed the system is by pumping the manual pump at the spin on fuel filter on top of motor until fuel spurts thru the relief valve. I have done that numerous times. A local mechanic advised the marina owner that the fuel injector pump be bench tested. Reluctant to do until I satisfy in my mind that the air in system is resolved. Trying to figure out fittings and how to attach the pony tank as you suggested. Spent all morning in the ER and about ready to pull out hair, what there is left of it!
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Old 16-08-2014, 14:32   #24
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Re: Rough Idle on Caterpillar 3208 Suggestions?

If you think you may be sucking air, about the only way you be sure your not is to put an electric pump right at the tank, that way the entire system is under pressure and it can't suck air.

I've had a Duramax Diesel pickup since new in 2001. A week ago it suddenly shut off and wouldn't restart. It's never done this before, more on a whim than anything else, I bled the fuel system and it started and has run fine since. Where did the air come from? I have no idea, but since ULSD is $4 bucks a gl now, the truck sits a lot now. Maybe sitting somehow or another is the cause
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Old 16-08-2014, 14:36   #25
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Re: Rough Idle on Caterpillar 3208 Suggestions?

A lot of great ideas and no sure how to do any of them
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Old 16-08-2014, 15:38   #26
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Re: Rough Idle on Caterpillar 3208 Suggestions?

My brother was a Diesel Mechanic and I ask him about your problem. He said he is 60% sure you have and Air problem plus bad fuel. He said add a fuel stabilizer to the fuel, enough for all the fuel in your tanks. Also add it to your tanks any time you are going to be laid up for long periods of time. He said bleed the entire fuel system and look for any slight fuel leaks anywhere on your fuel lines. If it is a bad fuel problem he said most likely your injectors are gummed up too so they are going to need to be cleaned or replaced. This is all he can tell you without being there on the boat to physically to check out the motors. He said also to double check your fuel caps on the tanks to see if there is any problems with them too. If they are not venting properly the engine will not be getting the fuel from the tank to the engine like it is suppose too. We have had this problem on the our bass boat before too. The motor was running rough and trying to die, Dad looked back and the sides of the fuel cans was being sucked in on the sides from lack of air to the tanks. Removed the caps till we got back to shore and replaced the caps and they worked fine after that. I think only one cap was bad but since the tanks were hooked in a series the one cap that was working was not allowing enough air through to keep up with the fuel demand of the motor. Just something else to check on. Hope it helps. Best of Luck.
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Old 16-08-2014, 15:42   #27
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Re: Rough Idle on Caterpillar 3208 Suggestions?

Hi Whatever,
Yukon is onto something.
Pulling and bench testing pump illogical.
You had said -first one engine started running ruff then the other-2 pumps going south at same time -Hmmmmmm.
steel lines throughout system puts me back on air in lines.Is each engine using independent tank?My thoughts drift to filter mount(s).Possibly filter not tight enough or filter threads faulty.Have old Volvo P1800 and went through 4 filters for it to seal correctly-Factory Flaw!.Is filtering sys independent for each engine?
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Old 16-08-2014, 15:51   #28
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Re: Rough Idle on Caterpillar 3208 Suggestions?

Broken-Sailor mentioned a good point, the tank vent. Check that the vent line is clear by running the engines with the tank filler cap off. While you have it off check the O-ring. Replace it if you haven't done so lately.
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Old 16-08-2014, 16:09   #29
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Re: Rough Idle on Caterpillar 3208 Suggestions?

Forgot to add that once, they had a Truck with bad fuel in it that after cleaning and bleeding the system, and stabilizing the fuel, they went through 3 fuel filters in less than an hour before they got all of the trash out of the fuel lines. So even it you did just change the filters if there is any trash in the fuel system they might be clogged up again.
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Old 16-08-2014, 16:34   #30
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Re: Rough Idle on Caterpillar 3208 Suggestions?

Your Cat thinks its an old Ford. My Ford always smokes till the engine is up to temperature, apparently if they don't there's something wrong.
Maybe not hot enough when on idle?
You got glow plugs?
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