Weak turbo on my Cummins (?) 6BTA

[flyboyjack]

A couple weeks ago, after getting the boat out of the paint shop, I took her out for a shakedown cruise. I ran about 2500 RPM +/- 100 RPM for about 45 minutes. Everything looked great. Then I felt the boat sorta do an abrupt deceleration. (Like I had skimmed a sand bar,... I was in 15 feet of water), and I lost about 200 RPM. All temps and pressures looked normal, but I turned for home port. As I always do, about 5 miles from home I poured the coals to her and nuttin'. I couldn't feel the turbo come in, nor could I get my usual
21+ knots. I went to idle and tried again, still no turbo. Tried again, nothing. After 3-4 more tries with no turbo kicking in. I pulled into port and called a mechanic. There was no black smoke indicating a frozen turbo, but I could not get full RPM, (still about 100-150 low), and of course WOT was still 2-3 knots slow. After doing a sea trial with the mechanic, who found another serious/scuttling malfunction that should have been caught on the survey,...another story, another time,....the turbo started coming online again albeit at a much later/higher RPM than before and still without previous top speed. Repaired the remaining scuttling gripe and ran her again. Same thing, turbo in late and still a perceived loss of power. Again, all temps/pressures normal. RPM's are still off a little.

Anyone out there ever experience anything like this or know of someone who did? It's driving me nuts and the mechanic has no idea. So short of pulling the turbo, (which almost requires pulling the motor), I'm perplexed.

Jack

[rcwhite]

If you are only down a couple hundred RPM, it sounds like the turbo itself
is working. First thoughts would be fuel delivery (fuel filters, air in fuel, etc) or a leak after
the turbo somewhere. (hoses, clamps, aftercooler, etc)

Going a bit deeper, other possibilities would be intake air restriction, exhaust restriction, problem
with the fuel lift pump or shutoff, AFC related (sticky/leak) depending on your injection pump. Injector issue
is possible as well and of course it can get deeper from there but I would be looking at the easy stuff first:)

Surveys are to some extent a necessary evil. Required by the insurance company
but limited in value otherwise. You have to expect them to overlook plenty and
inspect everything yourself.

[jleonard]

Sounds like a fuel delivery issue to me. No black smoke indicates turbo is working at first blush. Injectors would be way down on the list.
I would check fuel delivery then maybe do a turbo wash to clean the intake plumbing

[rcwhite]

One reason I mentioned an air leak is that this came on suddenly by
the description. Obviously a restricted filter or similar would be the
first thought on this but in many cases the Cummins injection is
setup to deliver fuel under boost based on the boost pressure. An air leak
after the turbo can cause a reduction in pressure causing a delay in response
and low top end without smoke as the pump will not deliver additional fuel
until it sees the pressure come up. This is done though the Air Fuel Control
assembly on the back of the pump which is connected with a small hose
to the intake manifold.

[RobS]

Would be nice to see some good shots of the turbo, exterior and exhaust side, and riser, but I know that is nearly impossible on that install.

[Marcy K]

This may really dumb - but could you have picked up a piece of plastic or line around your shaft/prop? We snagged a mesh onion bag once and had a sudden decrease in power with no other symptoms. The bag was discovered when we arrived at our anchorage. Just a thought.

Marcy K
A 34 Paumaonk 1986
Cataumet, MA

[flyboyjack]

Thanks for the thoughts. I'm having the boat hauled in a week or so. That'll be one of the first inspection areas.

Jack

[rcwhite]

I just finished the process of troubleshooting a similar problem on my port engine.
The symptom could be best expressed as a lazy turbo/excessive lag. Having seen
recent posts of bad turbos due to poor exhaust configurations (moving up on my project list) I
was expecting the worst but figured I should start with the basics just the same.

After a good deal of head scratching, filters, and going over everything a few times
I replaced the overflow valve on the injection pump. The valve had a broken spring which
caused the low side fuel pressure to drop enough to cause the engine to be slow to spin up.
Need fuel for boost and with the emissions, you also need boost for fuel so it can be fun:)

Best diagnosed with a pressure guage but you should hear this valve opening
when operating the primer. If you do not hear it "squeak" then it is likely bad.

This is a picture of the valve location.
http://www.sbmar.com/articles/bosch_p71 ... e_manifold

Bosch number was 2 417 413 093 which appears to have been superseded.

I went with an alternate.

http://www.torkteknology.com/products/T ... FV020.html

[RobS]

rcwhite,

Great info, thanks for posting and the links too.

[TomA]

Flyboy what size Albin do you have? Cummins HP etc.? Tom A

[filer]

<moved to maintenance forum>