Engine failed to start after serpentine belt came off.

[Poloperson57]

Hi,

I have a mercruiser 350 mag MPI and I have just managed to replace the water circulation pump, after the old one failed spectacularly when the bearing failed. I re-fitted the serpentine belt, but have never done this before (nor the water pump) so wasn’t sure if it was tight enough. Anyway, it started and ran nicely, and once it was up to temp I took it for a test run. All was good for 5 mins, but then the engine stopped, because the serpentine belt came off. So I re-fitted and made sure it was a bit tighter this time, but despite the engine turning over, and the belt being on correctly it just wouldn’t start.

I thought it might be fuel related, so I took off the fuel pipe for the tank isolation valve with a view to putting it in a can of fuel direct (which id done before when I ran out of fuel). It turned out the fuel can I was carrying was too big and I couldn’t do this, so I replaced the fuel hose, and was resigned to calling in a marine engineer to look at it for me.

But I thought I’d try one more go, turned the key, and it fired up immediately! Let it get to temp, and ran the engine (without going anywhere) for 15 mins - Idling and revving up, and running at 1,000 rpm for a while, and everything seems fine.

So my question is why on earth didn’t the engine run when I put the belt back on? Could it have been an air lock in the fuel line which was cleared when I took the pipe off?

I also noticed the engine is running hot (always has since I bought the boat last month) - around 225 F, so I’m going to replace the thermostat next to get her running at the correct temp. Once we were running again, and when it got up to (a high!) temp, I stopped the engine and restarted it a few times, and it restarted fine, so I don’t think the starting issue was temp related at all.

 

[alldodge]

I also noticed the engine is running hot (always has since I bought the boat last month) - around 225 F

If you motor always gets to 225* your entire problem is the motor is to hot and all kinds of bad things can happen. Can blow a head gasket, bend valves, size piston rings. The motor should not go above 180* and raw water cooled should be 160*

 

[Poloperson57]

If you motor always gets to 225* your entire problem is the motor is to hot and all kinds of bad things can happen. Can blow a head gasket, bend valves, size piston rings. The motor should not go above 180* and raw water cooled should be 160*

Yes, I’m aware of that now. But some advice on the things to check to help get the temperature to the correct working level would be helpful?

 

[alldodge]

What is your motor serial number?

Over temp is always lack of water flow, so start with the impeller and pump.

Remove thermostat and test in pot of water on stove with thermometer

Get a short piece of clear hose and place it in between the pump and thermostat housing, at the housing. See if you see bubbles with it in the water

 

[Poloperson57]

What is your motor serial number?

Over temp is always lack of water flow, so start with the impeller and pump.

Remove thermostat and test in pot of water on stove with thermometer

Get a short piece of clear hose and place it in between the pump and thermostat housing, at the housing. See if you see bubbles with it in the water

Thanks for this. I’m going down to the boat later this week - I’ll check the serial number then. I’ve bought a new thermostat and will be replacing that when I’m there. I was told the impeller was replaced last December, But I plan to check that as well. Great suggestion about the clear hose and bubbles. I’ll see if I can rig something up. Thanks!