The last thing I would be worried about is timing chain failure of a D4 or D6 Volvo Penta motor. The D4 and D6 engines were design specifically for marine application and will last thousands of hours if properly maintained and operated with-in the recommended parameters. The D4 300hp is a lot of HP for a 3.7 L engine. If operated at 80% load for most of its life span you will experience reduced longevity. That will not be caused from a timing chain failure it will be caused from wear. I use the theory that 1 GPH average fuel burn per liter of displacement will help to achieve a high hour diesel engine. 3.7 L operation with a log book that shows an average fuel burn of 4GPH will last 1000's of hours. This doesn't mean that on occasion operating the engines at 10 gph all day long is going to destroy the engine. It means that operating the engine day after day at 10 gph will reduce the longevity. The power is there to use but use it wisely if you plan on the engine running for years to come. Gallons burned is a better measurement of a diesel engine than hours. If you use the number 40,000 gallons of fuel burn as a general life expectancy of a diesel engine you can get a good idea of how many hours a marine diesel rated at M5 or M4 ( Pleasure or light duty )will last. If operated with an average fuel burn of 10 gph the most I would expect a well maintained engine longevity would be 4000 hours. If operated with an average fuel burn of 4GPH = 10,000 hours.
D3 is a whole different animal. It was not designed specifically for marine application. It is a marininized light duty automotive engine. D3 puts out a lot of HP at high rpm. It is at best a 4000 hour engine if operated with an average fuel burn of 2.5 gph. Its life expectancy is 10,000 gallons of fuel burn. The largest amount of failures are do to timing belt failures. Engines that have been operated in a marine environment for more than 8 years without a belt change. You are running on borrowed time.
Quote: rheimb1
Two years ago, at the recommended belt replacement interval, I had my mechanic check it for wear, cracks, or checking, and he said it still looked really good. So I decided to hold off a couple more years.
My question to your mechanic is why would you not change the belt if you went through the effort to remove the timing belt cover in a R25 ranger tug and do a full inspection. I have been involved in timing belt replacements in R25 and it is a PIA. There is no way I would go through the work of removal of the cover which is about 70% of the job and not change the water pump, tensioner and belt. Why would he take the chance of sending a customer on the water with a belt that should be changed at 8 years and honestly IMO 5 years in a salt water marine environment. It's not the belt that fails as much as it is the tensioner. I was involved in advising a good friend that left on the Loop this year. His boat had twin D3 220 hp Volvos with 650 hours. I told him before you leave Wisconsin have both your timing belts replaced. He runs his engines hard. He cruises most of the time at 3400 rpm, his engines turn 4000 rpm which in my opinion is not enough (D3 200 hp and 220 hp 4150 rpm, D3 150hp 3150 rpm) He did not take my advise and left on the Loop. His timing belt did not break his timing belt tensioner failed and the engine jumped timing. This caused a valve/piston interference and destroyed the engine. This happened when he was on the TennTom. He limped down the Tenntom single engine and had a Volvo pent dealer in Pensacola Florida find him a long block. His block was destroyed. The dealer has a contract with the USCG. USCG uses D3's in their smaller inflatable vessels. The dealer had resources to get the block. This cost my friend $36000.00 for the total repair and 4 weeks of down time from his loop trip. Part of the cost was replacing the belt, tensioner and water pump of the original engine that was still running. My advise to D3 owners 5 years or 1000 hours if you run your engines at "automotive" rpm 1500 to 2200 rpm which is 60mph to 80 mph in a car. If you run your D3 at 3000 rpm plus like my friend. In a car would be autobahn speed 110 mph+. I would replace the 5 years or 500 hours.
Volvo recommends D5 automotive engines replace the belt and tensioner at 80,000 to 100000 miles or 7 years. Automotive engines are not used in a saltwater marine environment and not run as hard or under the load that a marine engine is operated. Yet Volvo Punta recommends 8 years or 1400 hours for D3 which is the same engine as a D5. Volvo is basically saying it should last as long in a car as it does in a boat. If you research the D5 engine you will find that it was rated as a good reliable engine with excellent performance and reliability except for the timing belt. Replace the timing belt with strict maintenance protocol and the engine should last 200000 miles. That is an automotive recommendation. I would recommend even stricter recommendations for a marine application.
The mechanic that says you don't need to change the belt yet is the mechanic that does not want to change it because it is a PIA in a Ranger Tug !!
