94SD370
Member
- Jun 30, 2022
- 43
- Boat Info
- 1994 Sundancer 370
Quicksilver QS-7.5G Generator
Model #9679-0
Serial#1255117
- Engines
- Mercruiser 7.4L “Blue Water Inboard” Thunderbolt IV Ignition
Serial #F215722
I have a 1994 7.4 with about 850hrs. It overheats after about 10-15mins of run time. Engine RPM does not seem to matter. I never let it get over 190 degrees before shutting it down. Sometimes it will climb to 200 degree after shut down. I have put a laser thermometer on several parts. The thermostat housing (sensor) is 190, same as dash gauge, riser elbow 90 degrees, same as water at outlet. Manifold is 220 degrees. This is closed cooling and in fresh water. I have changed the impeller, cleaned the sea strainer, pulled all the hoses, cleaned out the tubes in the heat exchanger. I pressurized the heat exchanger to 19 PSI and it held that until I released it about 15 minutes later. I checked the oil cooler and no obstructions. Water flow out of the side of the boat has a very slight hesitation in flow, but others have said it is normal. I could not pull the large tubes from the riser elbows without removing the riser. I stopped there.
On the coolant side, I replaced the thermostat. Did a coolant flush. I have cracked the bleeder on top of the thermostat housing and verified the circulation pump is moving coolant. I cannot however get it to a point where there are no air bubbles in the coolant. I tightened every coolant hose. I also have the same intermittent bubbles in the heat exchanger. I bought an exhaust gas detection tool and pumped the air out of the heat exchanger while running. It did not detect (change color of the test solution) exhaust gas. I am wondering where the air is coming from and if that is my problem. I suppose it could be not enough exhaust gas to be detected? But still be a head gasket. I am waiting to replace the manifolds because of fear of snapping a stud. If I do that now, likely the heads will have to come off, and that is a season ender for me. My next step is a compression test. After that manifolds. Any other ideas? Or should I put down the shovel.
It’s worth noting that I also have a rattle (the marble sound) at Idle. I changed plugs, cap, wires, etc.. It did not fix the sound. The mechanic at our marina says that noise is normal for a boat of this age and related to worn springs on some type of plate on the transmission.
Thank you in advance.
On the coolant side, I replaced the thermostat. Did a coolant flush. I have cracked the bleeder on top of the thermostat housing and verified the circulation pump is moving coolant. I cannot however get it to a point where there are no air bubbles in the coolant. I tightened every coolant hose. I also have the same intermittent bubbles in the heat exchanger. I bought an exhaust gas detection tool and pumped the air out of the heat exchanger while running. It did not detect (change color of the test solution) exhaust gas. I am wondering where the air is coming from and if that is my problem. I suppose it could be not enough exhaust gas to be detected? But still be a head gasket. I am waiting to replace the manifolds because of fear of snapping a stud. If I do that now, likely the heads will have to come off, and that is a season ender for me. My next step is a compression test. After that manifolds. Any other ideas? Or should I put down the shovel.
It’s worth noting that I also have a rattle (the marble sound) at Idle. I changed plugs, cap, wires, etc.. It did not fix the sound. The mechanic at our marina says that noise is normal for a boat of this age and related to worn springs on some type of plate on the transmission.
Thank you in advance.