Back pressure found at survey-deal killer?

[Sevo]

While shopping for a boat we found a listing for one that sounds really promising. Our broker called about it for us. It was recently under contract and went to survey where the engine surveyor found “a little back pressure” in both engines. The boat is 2006 vintage. Engines are Cummins QSM 8.3 HO, 540 hp, 1490 hrs with proof of regular maintenance. The boat was on the hard for ~3 months prior to survey but prior to that was used regularly. At this point I don’t know what method they used to detect the back pressure, whether it was measured in any way, etc, I just have the selling broker’s description.
My question is, is this boat worth looking into further, or should I just move on? Is there a benign explanation for the back pressure, or is back pressure always indicative of major problems?
David

 

[SoWhat]

Maximum allowable back pressure is 17kPa.

It could be something simple like restriction in exhaust system or clogged air filters, but it could also be something more complex involving the timing. The exhaust valve has to open at the exact right time, when the cylinder pressure is equal to the exhaust pressure, so that scavenging can take place. If it's off engine would lose efficiency at a minimum. If it's way off thermal transfer would be impacted and now you are in the red zone.

I think having an exact number would help (a little back pressure????) and I would then get an experienced Cummins mechanic involved to get an accurate diagnosis.

 

[DavidM]

A real pro at this sort of stuff and a Trawlerforum member is Ski in NC. He is not that far from you near Wilmington, NC and could consult on the problem and perhaps do the mechanics inspection of the engines if you go ahead.

PM him at Ski in NC.

David

 

[twistedtree]

A real pro at this sort of stuff and a Trawlerforum member is Ski in NC. He is not that far from you near Wilmington, NC and could consult on the problem and perhaps do the mechanics inspection of the engines if you go ahead.

PM him at Ski in NC.

David

And for what it's worth, if it's the 8.3 liter engine, it's a QSC. Or if a QSM, then it's 11 liters.

 

[High Wire]

The only way a surveyor has to quantify the back pressure is to measure it. He should have written the number down in the survey. Ask the broker if the number is in the survey and what is the value.
Also 17 Kpa is about 2.5 psig.

 

[sunchaser]

It sounds unusual to measure back pressure during a survey. There must be more to the story. Possibly he removed the oil filler cap and held his hand over it

 

[Sevo]

Thanks to all. It sounds like its worth further investigation. My (apparently mistaken) understanding had been that back pressure was usually caused by one or more “ leaky” piston rings and so was suggestive of need for overhaul. Good to know that the explanation might be less serious. Will see if listing broker has and will share details of the survey. He may not have it, since the survey was undertaken by the prospective buyer, who aborted the sale.

 

[Ski in NC]

Back pressure is a rather generic term. What exactly was being measured? Exhaust back pressure? Measured where? What were the readings? Crankcase pressure?

 

[BruceK]

Thanks to all. It sounds like its worth further investigation. My (apparently mistaken) understanding had been that back pressure was usually caused by one or more “ leaky” piston rings and so was suggestive of need for overhaul....

That sounds more like "blow by". Hope it gets sorted and the purchase proceeds.

 

[oscar]

It is possible broker translated a number to "a little". Could be a disclosure (at least in his/her eyes) of a defect. Proceed with extreme caution.

 

[Ski in NC]

Cummins has a test for measuring blowby. It requires a few specific (but very basic) tools and results in an actual measurement of blowby flow. Survey should state what the flow rate is.

Basically you run the blowby through a measured orifice and use a manometer to measure differential pressure across the orifice. They supply a chart converting that differential pressure to a flow rate. Different engines have an acceptable blow by flow rate.