Hello Fellow cruisers!
I want to share a problem I had with my Volvo Penta
TMD 31 that did chase up and down on idle RPM
When going down from high rpm
directly to Idle it could even stall and stop but usually just did go up and down for a minute or so and then back to steady idle.
If I did run the engine
under load for about 30 minutes the problem would be gone just to reappear ir the engine
was not used for a week or two.
Strange?? Consulting experts they suggested many possibilities.
2) Weak lift
in the High pressure pump ( one smart expert said that he really did not believe in this since the engine was normal after the running under load)
4) blocked filters or blockage in tank
5) injectors faulty
So what did I do.
The injectors are changed 2 years ago so I really did not think that was the problem.
I changed the lift
pump and the hoses from the fuel tank
( Of course the filters as well) doing so I could determine that there was really no blockage in the tank.
Found the liftpump (after 4000 hours of service) to be very much a suspect. Not much action there.
Getting air out of the whole system I could se that there was air bubbles coming out of the HP tubes at the injectors.
Everything got a lot better but after running the engine with the new lift pump ( situated before the fine filter and pressuring the fine filter and HP pump) I could se a small leak from the fine filter connection.
On this engine it is banjo with cupper washers.
So I did have an air leak that was not visible with the weak lift pump. The HP pump was together with whatever action there was left in the old lift pump able to get fuel
but only at higher RPM. THe system must also had a small under pressure in this situation allowing air to be sucked in when at idle but I think that the lift pump did " kick in" at higher RPM reducing this air leak.
My main point is to give you an Idea that an air leak can be the problem if you are experiencing idle problems on your Diesel