unequal engine acceleration

[Michael Buchanan]

I have a recent problem that I hope someone can help with.  I have twin Cummins 6bta 370hp engines with about 1330 hours on them.  Recently the starboard engine has been accelerating slower than the port side, to the point it will start to take a hard turn before I can get the engines synchronized again.  Once it gets over about 2000 rpm it will spin up to rated RPM of 3000 and maintain speed with no problem.  Both turbos were replaced with brand new, not reman, Holset units because they were leaking oil into the exhaust and have less than 15 hours on them. The engines accelerated equally both before and after the turbos were replaced.  I get no smoke out of either side, the throttle cables and linkages appear to be reacting equally when pushed forward, the fuel filters are new as are the air filters because I just installed ccv systems on each side.  I had a diver come in and thoroughly clean the bottom and running gear and check the props and shafts for damage or binding and no problems were found.  I don’t know where to go next.  This is my first diesel boat and I haven’t been running it that long so I don’t know if there could be a problem in the injection pump or what.  If it was fuel I would think I’d have a problem reaching and maintaining full rpm, but it runs very strong from 2200 rpm up.  Any suggestions will be greatly appreciated.  Thank you.

Topic

[Michael Buchanan]

Thanks for the input, and no you aren’t all wet. I found what you described in an old Cummins troubleshooting manual I found. It essentially says if there’s more than a 3 psi difference between the two gauges the cooler is clogged. So I’ll be busy this weekend trying to get creative with some plumbing to check these things out. Thanks for taking the time to respond. If weather cooperates I’ll post updates next week.

[Clark Leighs]

I don’t have your engines but something came to mind I read some time ago..  Check the aftercoolers for too high pressure drop.     If I remember correctly there are a couple plugs that can be removed and adapted to the installation of low pressure pressure guages.   THat will tell you if the sluggish engine is not getting the pressure needed likely due to clogging.     What you are looking for is excessive pressure drop between the turbo air input to the aftercooler  and the air entry to the intake.

If there is more than a very few lbs of pressure loss then clogging MAY be at least part of the problem.   THis means the engine is not getting the air when needed.

 

Of course I could be all wet.

 

[Michael Buchanan]

Thanks for the insight. I’ll take a hard look at the turbo and also see if I can plumb something into the the other side turbo so I can compare readings at equal RPM points while I’m running. I think that’s as close to swapping the turbos I can get without actually doing it. Thanks again.

[JimmyK]

Aftercoolers have a water side and an air side.  If the water side were to clog up, yes, could cause overheating issues as well as poor performance.  If the air side clogs up it could cause poor air flow/low power/black smoke.

Because you can reach 3000rpm under load, it wouldn’t be my first thought but it is something to look into.  Furthermore, the aftercooler is probably the most maintenance sensitive item on that engine.  The last thing you want to do is wait too long to service them.  They are very expensive to replace–if you can find them, and the longer you wait the harder they are to service as they will become difficult to disassemble.

[Michael Buchanan]

I haven’t had the intercoolers off yet. I’ve had the boat for about two years and done nothing but work on. Now that it’s ready to use, this happens. This is my first diesel boat so not sure what to look for on air side. Only aware of overheating issues if intercooler clogs. I put 165 thermostats in it per suggestion on this site and they stay at 170 with no problem. I’ll check turbos for equal drag this weekend but would really like to avoid swapping them if possible. But you’re right; that would eliminate turbo as a cause. Can’t check readings running on other turbo running because hatch has to be up for me to get to it and I’m not in open water where I can just float while I’m doing something. Intracoastal is very narrow where I am and I found out the hard way you can’t get too far out of the channel before you get stuck in the mud. I’ll update when I can check turbos for anything obvious. Thank you.

 

[JimmyK]

So that was good trouble shooting and you proved it is not lack of fuel causing your problem.  There is a ‘troubleshooting low power’ article on this site.  Go through it in detail.  Some of the stuff is obvious but don’t write anything off, check it all.  When was the last time you had the aftercooler off?

If that doesn’t reveal anything, then I suspect the turbo, even though it’s new.  Personally, I would take a look at both sides of the turbo and see if anything sticks out.  Spin the impeller and compare to the other engine.  If you find nothing I would swap the turbos.  Yes, it’s a pain in the neck but it would definitively rule in or out the turbo being the cause.

Maybe someone else has a less labor intensive idea, but start with the troubleshooting article.

[Michael Buchanan]

More info on problem

I read the string on the aneroid adjustment and gave it a try.  I adjusted the wheel in the direction indicated in the post and it still did the same thing as before but with a noticeable increase in black smoke.  I also fed it 20 psi from a portable air tank with the same results.  What I did notice was that the engine lag seems to be from about 1400-1500 RPM up to about 2000 rpm, or just where the turbo starts to kicks in.  So both engine RPMs will track equally to about the 1400-1500 range, then the right side lags until the turbo starts to whine.  I checked the boost pressure and it would build gradually until 2k rpm and then build quickly to 15 PSI at 2400 and the hose blew off the gauge.  If I hold the port engine back until the starboard side hits 2k they will both build rpm at the same rate from there, and run up to 3k rated max.  So it appears my problem is in the mid range.  I also ran briefly with the air filter off and it ran the same.  I don’t know what to try next.

[JimmyK]

There’s an article on here about the aneroid on your fuel pump, give it a read.

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Michael Buchanan

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