7.4 Cooling Problem

[GETTMANN1573]

I have a 1996 7.4 merc bravo3. It has an aftermarket closed cooling system installed. My problem is after it has run at cruising speed for 5-10 minutes and shut off then restarted the alarm goes warning alarm goes off. All the guages read good, water temp about 180, oil pressure 40-50 and oil in the out drive can. I took it to the local boat shop and he replaced the oil pressure switch and installed a new raw water impeller. Still didn't fix the problem. I have noticed that the dischrge line from the heat exchanger to the exhaust manifolds is cool. Shouldn't this line be some what warm? Oh I just got the boat and the previous owner said he had no problems. He did have a shop replace the exhuast risers. What direction should I go?

 

[HT32BSX115]

Re: 7.4 Cooling Problem

It has an aftermarket closed cooling system installed. My problem is after it has run at cruising speed for 5-10 minutes and shut off then restarted the alarm goes warning alarm goes off.

Howdy,

Well, you should determine which sensor is causing the alarm before you replace "things"

You have 3 switches that simply "go" to ground on the alarm wire.
1. the drive lube level switch.
2. an oil pressure switch
3. temp switch.

You can easily disconnect the wire from each switch when the alarm activates and determine which one is (or isn't) causing the problem. If none of them are the problem, then you have a frayed wire touching ground somewhere.

If he replaced the raw water impeller did he find all the pieces of previous impeller failures? (that would be crammed in the oil/PS cooler)

I have noticed that the dischrge line from the heat exchanger to the exhaust manifolds is cool. Shouldn't this line be some what warm?

Maybe. but not very warm....

the previous owner said he had no problems.

They always say that.

Cheers,

Rick

 

[GETTMANN1573]

Re: 7.4 Cooling Problem

I unhooked the temp alarm wire and that is what is setting the alarm off. Since then i have back flushed the oil cooler, replaced the t-stat. At idle the guage reads 175. On plane the temp goes up to 200-220. When I come off plane and stop and run at 2000 RPM the temp comes down to 175. I have a closed system and have inspected the ends of the heat exchanger and it doesn't look restricted. When I took the t-stat housing off the hoses to the risers were before the t-stat. I am thinking that the t-stat should be first then the riser hoses and heater? My mechanic said to get an IR temp gun and check the actual temp of the engine. It seems to me there is a blockage somewhere. What direction should I take next?

 

[GETTMANN1573]

Re: 7.4 Cooling Problem

Ok I checked the temps with an IR temp gun and it was running hot. I rechecked the oil cooler for impellor chucks and again found nothing. I checked the heat exchanger and blew out the tubes with water, everything OK. I ran the raw water lines to the exhuast instead of having them cooled by the enclosed system and presto it runs at 175 all day long. I think that the manifolds maybe restricted but they run cool to the touch and the motor likes it with out them. I think I can live with it this way.