7.4L Overheating

[CayugaCobalt]

Thanks for the diagram! How would oil cooler plug up over the winter? There were no observed tempearture issues for 6 years. I changed impeller for 3 rd time and pulled bilge pump out from under block. I think the bilge pump hitting or crushing something near the starter is more likely than oil cooler clogging on land?

 

[alldodge]

Stated problem: No water coming from the pump

Need to find out why water isn't flowing. Its either not getting to the pump, or not getting from the pump to the thermostat housing.

Don't know why anything would clog, but the oil cooler and PS cooler are not under the motor, Suggest back flushing everything just to rule out what is a problem and what is not.

 

[CayugaCobalt]

Yes, understood this eliminates another variable and I can remove it easily but will need to take it home to backwash it. I still feel like the problem is somewhere in the transom. I do appreciate your guidance.

So I can just break the hose from bottom of the cooler and see if water makes it that far.

 

[CayugaCobalt]

Removing the hose will work, I would have removed the hose coming from the pump to the thermostat housing.

Don't know your serial number so the pic below is generic. Your motor should have a power steering and oil coolers. They are both inline with the hose coming from the raw water pump. Remove the hoses and back flush everything. Look closely at the oil cooler because they can get clogged

View attachment 346344

My engine serial# is OK151148
It is only 1 line IN and 3 lines OUT of the thermostat??

 

[CayugaCobalt]

That would cause some issues but if it did there would most likely be a leak.

Look at the hose under the bell housing and gimbal ring. It can twist sometimes when it gets old and will cut off the flow

Check the intake hose to make sure its on top of the raw water pump. Comes in on the top and to the motor on the bottom

Stupid question from me. The potentially twisted hose under the bell house & gimbal is in the lake or in the boat? Do you have diagram? Thanks!

 

[Bondo]

Stupid question from me. The potentially twisted hose under the bell house & gimbal is in the lake or in the boat? Do you have diagram? Thanks!

Ayuh,.... Outside the hull,.....

 

[alldodge]

Pump hose is on the right side facing motor the one in front

 

[CayugaCobalt]

No water reaching oil/steering cooler. Woulda been nice if I could have just plugged in a new cooler to solve the problem.

 

[alldodge]

Power steering cooler is before oil

 

[CayugaCobalt]

Pump hose is on the right side facing motor the one in front

Can this hose be replaced without replacing bellows?

 

[alldodge]

Can this hose be replaced without replacing bellows?

All hoses inside the boat can be replaced without getting outside to the bellows

 

[CayugaCobalt]

All hoses inside the boat can be replaced without getting outside to the bellows

Sorry for the confusion Alldodge, I was referring to the potentially twisted hose

MerCruiser Hose Kit 32-8M0090859 which Bondo said was outside the boat on the drive unit.​

This is why I asked if it can be replaced without changing the bellows

 

[alldodge]

You won't need to replace the boots, but will need to disconnect from the bell housing end just to have room

 

[CayugaCobalt]

Removing the hose will work, I would have removed the hose coming from the pump to the thermostat housing.

Don't know your serial number so the pic below is generic. Your motor should have a power steering and oil coolers. They are both inline with the hose coming from the raw water pump. Remove the hoses and back flush everything. Look closely at the oil cooler because they can get clogged

View attachment 346344

My engine has cool fuel system, which has sea water pumped plumbed the opposite of this diagram. Hoses were backwards.

 

[CayugaCobalt]

could be bravoitis

the brown rubber bits are now toast....... literally. you need new exhaust rubber bits including the flappers.

rubber bits are good for 250 degrees F max. exhaust temperatures without water are 1100-1400 degrees. the rubber bits never stood a chance.

Turns out because engine has cool fuel system, raw water pump is plumbed opposite of every other 7.4L without cool fuel. My fault, hoses were backwards.

 

[CayugaCobalt]

tried to make two preventive maintenance steps before launching this year. (1997 233 Boat sits all winter in NY.)
1) SEA WATER (RAW WATER) IMPELLER
2) THERMOSTAT

I have done the impeller twice before so this was my third time. I do it every two years although impeller looks like new after two years. It is always a difficult job to do.

I had not done the thermostat before. Afterwards it seemed easy to complete.

Again did both of these as preventative; had no previous overheating issues.

Launched boat, drove slowly out of marina and after maybe 1/8th of mile noted temp was at 175 (not 160 as rated) and smelled warm. Turned it off. Let it set 5 minutes. Didnt really seem to cool off but wind was blowing me past marina at this point so started up drove 1/8 mile back to marina, got to almost 200 degrees and turned it off again. What is setpoint for overtemp alarm? Mine never went off.

Afterwards, noted the rubber exhaust bellows were brownish. Poured a little water on them and they were boiling hot as was the risers and manifolds.

Now I need to go back to marina and troubleshoot the problem. I think its the sea water pump.

How do I confirm the sea water pump is not working properly?

Should I pull the bottom outlet hose off and see if any water is coming out? Or will that shoot water somewhere electrical or otherwise undesired?

How do I confirm the thermostat is or is not working right other than removing and putting in pot of boiling water?

Hoses to seawater pump were backwards. Operator error. This engine has cool fuel system, which for some reason requires sea water pump to be plumbed opposite of every other Mercruiser.

 

[tpenfield]

Glad you found the issue. My 7.4's had the VST pump instead of cool fuel, so I never had to mess with that part of the cooling system.

 

[CayugaCobalt]

That would cause some issues but if it did there would most likely be a leak.

Look at the hose under the bell housing and gimbal ring. It can twist sometimes when it gets old and will cut off the flow

Check the intake hose to make sure its on top of the raw water pump. Comes in on the top and to the motor on the bottom

Unless one has the cool fuel option like me, then it is the opposite. Raw water on bottom and to the motor on top.

 

[Drcoffee]

Looking at the picture of your impeller, correct me if Im wrong but several of the blades are facing the wrong direction. I could see that as a restriction of flow if only 65% of the impeller is pumping.

 

[alldodge]

I don't see anything wrong with the impeller. Once it comes off the shaft it will distort