Volvo MD 2010 sudden power reduction

[yimkin]

I have a 20 yr old Volvo MD2010 in a 26' Victoria 800. After start up it runs under load for 15-20 minutes at about 2,800+ rpm then suddenly the revs drop to about 2,000. The engine sounds fine (no rough or erratic noises) during both phases.
So far I have isolated the tank and run it with fresh fuel from a temporary tank. There is no water in the fuel. The fuel passes through a sedimenter (water trap) followed by a primary filter before going to the engine filter. The filter elements have been renewed recently and I have checked that they are still clean.
Still the same problem.
I conclude it is some problem on the engine. It has been suggested that one of the pistons in the fuel injection pump may be sticking but only occurring after the engine has warmed up.
I would be grateful for any suggestions as to other possible causes. My knowledge of engines could charitably described as 'basic' so I need all the help I can get!

 

[jiris]

I have a 20 yr old Volvo MD2010 in a 26' Victoria 800. After start up it runs under load for 15-20 minutes at about 2,800+ rpm then suddenly the revs drop to about 2,000. The engine sounds fine (no rough or erratic noises) during both phases.
So far I have isolated the tank and run it with fresh fuel from a temporary tank. There is no water in the fuel. The fuel passes through a sedimenter (water trap) followed by a primary filter before going to the engine filter. The filter elements have been renewed recently and I have checked that they are still clean.
Still the same problem.
I conclude it is some problem on the engine. It has been suggested that one of the pistons in the fuel injection pump may be sticking but only occurring after the engine has warmed up.
I would be grateful for any suggestions as to other possible causes. My knowledge of engines could charitably described as 'basic' so I need all the help I can get!

To assist with diagnosis: Please, check the position of the regulator leaver BEFORE the drop and AFTER the drop. Check the density and color of exhaust smoke before and after the drop, the same with coolant temperature.
If you have access to individual cylinders exhaust manifolds, check the temperatures (I never leave home without an infrared thermometer ) and note if any reads substantially lower a few minutes after the revs drop.

 

[bobgoode]

Had identical issue a few years ago. Fine for 20 minutes or so the revs would waver for a few seconds then drop off and hold steady at 1500 or so. Switch off and wait half an hour and all fine for another 20 minutes then it happened again. Clump of diesel bug in tank being drawn on to filter on pick up pipe and reducing fuel flow. When switched off the draw along the pipe stops and clump falls to bottom of tank. Clean tank and it was sorted.

 

[Kinsale373]

I had a similar problem with a 2040 some seasons ago. Fighting the current into Cherbourg I upped the revs to 2800 and after a few minutes the revs suddenly dropped to 800 or so. The revs recovered but this drop in revs happened a few times. I nursed her back to 2000 and eventually limped in. On inspection a mechanic found that one of the seals on the CAV pre filter /seperator was slightly misplaced. He said that this was a common fault, under pressure it begins sucking air instead of diesel and thus the lower revs. He replaced the seals and I have never had the issue again. Im always careful when replacing the filter and seals now. I actually unscrew the filter assembly off the bulkhead to get a good look and see that seals are all well positioned. During the process I found that the small bubble type fuel pumps are excellent for bleeding the system after Filter replacement , I keep one on board with my filters. I hope this is your issue as its a simple fix.
Regards, Kinsale 373

 

[yimkin]

Thanks for the suggestions.
My first thought was floating junk in the tank but I am now running on a temporary tank and the fault still occurs.
I have checked that the filters are clean and closed up with (hopefully) due attention to seals and gaskets
When there is a drop in revs it is not associated with any movement of the throttle lever. In fact I moved the throttle to maximum with no effect on revs.
Today I had to move to a different berth in the marina. The engine performed faultlessly but it hadn't had time to really warm up.
Tomorrow I will take it out and run it at around 85% full power until the rev drop occurs and I will make what temperature checks I can. I have an infrared thermometer.
I will post the outcome. The injectors were serviced recently but the fuel injection pump wasn't so if nothing changes I'll have it stripped and serviced
I too find the bulb fuel pumps make system bleeding much easier.

 

[daverebbettes]

Very similar problem with my 2020. Engineers spent the best part of a year misdiagnosing. Cost me a fortune. It turned out to be a split in the fuel return rail pipe. A relatively inexpensive fix in the end. My symptoms were plenty of revs when under no load. 2,000 revs or so under load but this would drop back to 15-17,000 after 2-15 minutes.

 

[jiris]

........................When there is a drop in revs it is not associated with any movement of the throttle lever. In fact I moved the throttle to maximum with no effect on revs...........

Sorry, I didn't express myself clearly. I am not really interested in the position of the "throttle" (control) lever, but in the position of the rack - the part that transfers the output from the governor to the injector pump. Unlike in a petrol engine, in diesel the "throttle" (actually an incorrect name here) lever doesn't directly control the power, but sets the revs you demand. As results, while the Throttle is, let's say fully open, the rack can be on full (if the engine is loaded) or almost closed (in neutral). So, the position of the rack gives me indication how hard the engine is working. Unfortunately, in most fuel pumps the rack is well hidden and its position can't be detected without removing some covers. I suspect, it will be the case of your engine too - so we may skip this part. This information would be useful to distinguish between performance and control problems - the second one much less likely anyway.