engine runs hot when idle

Hi everybody, Good day, I am new here, Maybe someone can have a clue. I have a volvo Penta 4.3GL, raw water cooled and used in sea water.
when I run the boat on high rpm (above 3000) the engine works fine and temp stays stable 85 deg celcius. But after 15 minute ride and I lower the revs to 2500 and under, the temp starts going up to 97 deg C. When idle makes it worse, I rev the engine for 5 seconds and the temp start going down to normal And keeps going like this for a while untill it gets stable. But I have to rev the engine say 4 times or more to get it stable. I noticed as well that when driving on plane at 3600rpm, the risers are hotter than normal, but you stiil touch them with you hands. I am thinking that there is a lack of water flow. Checked the impellor and it is in new condition as I changed it begining of last season. Some body told me to start from the thermostat housing to sea if there is any blocking but I was surprised that I made 30minute trip at 3900 rpm and temp stayed stable on 85 deg C, than when I lower revs the problem starts. Thanks for any help.

Hi - I am rather a fan of - https://www.ebay.com.au/sch/i.html?_nkw=engine+watchdog - temp gauge that I have fitted to my thermostat housing. Also have a cheap laser thermostat to wave around the engine. Old senders and gauges are sometimes wonky. Fair bit of pressure in the cooling system I would expect at 3600rpm - I only get up around 2500rp on my old AQ150 which stays around 80C now most of the time after a rebuild. Sound like not enough pressure at low revs - have to bite the bullet and redo the seawater cooling like I did I suspect

Hi, Thanks for the info, I am checking the raw water pump, in fact, I removed it from the engine and I think the cam plate is a little worn, even the cover plate has a sign of wear, I will fix the pump with a new impeller and see the outcome. Thanks again for the information..

A "Ricardo favorite".... check the elbow on the leg where the raw water hose attaches. It is so short lived that one should consider it a wear item. They get pinhole leaks and cause all kinds of wonky cooling issues. Since this is not exactly a new engine, I'd just change out the elbow and be dome with it. replace gasket at the same time. Also... that hose, if original, is another good candidate for pay me now or pay me later avoidance.

Hi, there is no elbow on my VP sx drive, I think you mean the plastic elbow on the vp290. The sx drive has a plastic fitting that tightens up with the drive, and if it was it, it will only do it on plane not while on idle because at idle the whole drive will be submerged in water. My problem is ,low flow of raw water at low rpm(600- 1500rpm) I mouth blown the intake pipe from the water pump inlet hose and the air flows freely through the drive inlet grill, now I am servicing the raw water pump and see the outcome

Hi, Serviced the raw water pump and tested the boat, same problem occurred, Now I removed the inlet hose from the power steering cooler and put 1" suction hose straght to the sea and tie clipped it with platform down pipes and leveled the inlet with the bottom of the boat, tried the engine at standstill and the cooling system was very much better. Risers far colder and Tempriture gauge didn't reach 80deg on standstill at 1500 rpm for 2/3 minutes. Than I pluged the other end which was fitted to the cooler (coming from the drive) and tested the boat. The boat ran fine with temp struggling to reach 80deg and risers slitely warm, as they should be. So I concluded that the problem is in the drive inlet, I think there is a leak between the exhaust and the intake which causes the air in the intake and water flow drops drastically. Now I have to remove the drive to find out, means I have to get the boat out of water. (which I don,t like) or I will leave it with this pipe for this season.

Have you tried backflushing the power steering oil cooler?

The power steering cooler is just a straight pipe with PS oil around it ( you can see trough it), And I did not by pass it with the pipe I did now. I only by passed the drive. I back flushed the system and there were never any blockage as I can easyily mouth blow towards the drive. but I still pumped it with foot pump and air was blown outside the drive inlet grill in the sea. Thanks for the advice.