Varying Engine RPM

[nickr917]

On my last trip out my engine RPM was periodically dropping from cruise speed by between 100 and 200 RPM and then returning to cruise speed on their own. I would say the duration of the drops was about 30 seconds. On the first leg of a 3 hour round trip the issue started after about 45 minutes, then we fished for about 4 hours with some moving between spots but not at cruise speed, and then on the ride home the dropping RPM came back soon after reaching cruise speed. Oil pressure and engine temperature were consistent throughout this time.

The boat is a 2005 28TE (bought this summer) with a Perkins Sabre M300Ti. The max RPM on the engine is 2500 and we cruise at 2200, which I suppose is a bit fast. Could that be the issue? I have not noticed the issue previously. Any ideas?

[TomA]

Replace your RACOR. TomA

[jleonard]

Yes first thing to do is change all of your fuel filters. That is a classic sign of fuel starvation.

[nickr917]

Ok, I'll go through and replace all the fuel filters. Do the clogged filters actually hurt the engine at all or they just cause the varying RPMs?

[RobS]

Ok, I'll go through and replace all the fuel filters. Do the clogged filters actually hurt the engine at all or they just cause the varying RPMs?

Fuel restriction can hurt a diaphragm lift pump. The 6LP does not have an external lift pump, just a pump inside the injection pump which is only serviceable by tearing down the injection pump. Not a DIY item. So change those filters. If you're the type that likes to get it all out of a set of filters (or you have a dirty tank issue) you will want to install a vacuum gauge.

[Graeme K]

Hopefully just filters. Does the engine fire instantly (it should), or do you find it cranking over & over before firing? Underway, throttled right back with engine idling, does the engine run rough?

Your fuel lift pump is externally fitted to the side of the fuel injection pump. The lift pump runs off an accentric on the camshaft of the fuel injection pump.

[jleonard]

Clogged filters can actually shut the engine down if one gets bad enough. Take the rpm surging as a fair warning sign.

[nickr917]

The engine fires instantly and idles fine when throttled back from cruise speed. Good to hear the fuel lift pump is external on the Perkins.

[Graeme K]

Engine sounds good.
The pointers I gave (above), if present, would have shown the presence of the diesel bug - a bacteria which feeds on diesel (being an organic material), eventually producing hydrogen sulphide, slowly destroying the fuel injection pump.

The RPM you're cruising at is sweet for the Perkins and every few hours move it up to 2500 RPM for 10 minutes.

Graeme K.

[nickr917]

Graeme K, thanks for the advice. Do you have any tips for the Perkins based on the experience with yours?

[Graeme K]

In answer to your question - the Perkins Sabre 265TI and 300 TI are the same engine. The 300 was tweaked to produce 35 HP more.
Did you get the Perkins Sabre User's Handbook with the boat?
If you don't know the service history, I would suggest replacing the heat exchanger ''O'' rings & check the front end cap which has an elbow tapped in it for the Tides Seal hose. (Mine was two-thirds blocked.) Also, disconnect hose on Tides Seal nipple and check for water flow. Do service schedules when due; use correct coolant and engine oil.
The engine has no anodes, but a bonding wire from the boat's bonding system should be connected to a stud for this purpose. It's located below starter motor, midway along the engine. Two studs hold a flange pipe. Use the long stud to connect the bonding wire too. (There was no wire and no connection on my engine.)

Graeme K

[Sspdiesel134]

Always start simply ,, check fuel valve make sure it's on all the way , a simply vacuum gauge on inlet of the engine is the best tool, 6 lp yanmar any reading 6 inchs or more at cruise rpm will cause a issue low power , before you do the test do a inspection of the engine , look for any signs of heavy oil coming out of the tube / hose above the air cleaner .
Any way I can help you let me know ,,
Things you will need for fuel restriction test .
3/8 npt pipe tee
3/8npt to hose barb fittings two
One racor vacuum gauge
One piece of 3/8 by 1 foot long fuel hose
Two hose clamps 3/8 hose
Put it right before the engine mounted fuel filter take a ride 3000 rpm record reading , change engine mounted filter before ride .

[SJR]

Could you recomend the correct coolant for a Yanmar 315?

[Cape Codder]

The correct coolant is the orange Prestone Dex-Cool concentrate or premixed 50/50. Just about 3 gallons.
Yanmar recommends YEARLY change due to all the dissimilar metals. About $12/gallon at Walmart.

[SJR]

awesome thanks