1993 Mercruiser 7.4L 454 High Temp/Overheating

[flexall1]

Running idle in driveway with muffs on, temp getting upto 225+ in 5 minutes and setting alarm off. Just replaced impeller.
Water is pumping through motor and out thu hull exaust. Water coming out exhaust is very hot. Any suggestions on
this would be greatly appreciated....

 

[flargin]

Re: 1993 Mercruiser 7.4L 454 High Temp/Overheating

I have a 89, so it may be the same.

if you blew your last impeller, it may have broken the pieces and they are now packed in at the Oil cooler. Those pieces are blocking the water from going through the oil cooler and thus no cooling.

 

[tpenfield]

Re: 1993 Mercruiser 7.4L 454 High Temp/Overheating

Water will come out the exhaust, but not actually go through the motor very well.

The things to check would be on top of the engine at the thermostat housing. I would start with the thermostat itself and check yo see if there are clogs, etc that is preventing flow through the engine block.

 

[flexall1]

Re: 1993 Mercruiser 7.4L 454 High Temp/Overheating

I have replaced the impeller twice this year. Last time I think I installed it wrong. Both times, there were no pieces of the impeller to be found. Is there an easy way to check the oil cooler??? As far as the thermostat, when we replaced the impeller, my boat guy checked the thermostat and completely tore out the center to let water flow....

 

[tpenfield]

Re: 1993 Mercruiser 7.4L 454 High Temp/Overheating

The oil cooler should be located on the port (left) side of the engine down low. You can remove the inlet hose to the cooler to see if 'stuff' is blocking it. You can also back flush the oil cooler by disconnecting the inlet hose at the thermostat housing and forcing water back down towards & thru the oil cooler.

I am curious to what "completely tore out the center to let water flow...." means. There are some spring-loaded restricting devices that are assembled into the thermostat housing, which limit the cold water flow to the exhaust elbows, thus forcing the water to go through the engine and out the hoses leading to the exhaust manifolds.

You might want to refer to the cooling diagram, which is in the maintenance manual for the engine, as water takes various routes through the system. It is not just a single route.

 

[flexall1]

Re: 1993 Mercruiser 7.4L 454 High Temp/Overheating

Thanks for your info. About the thermostat, he just pulled out the spring-loaded device out of middle so it was just a straight hole, no restrictions.

 

[tpenfield]

Re: 1993 Mercruiser 7.4L 454 High Temp/Overheating

Thanks for your info. About the thermostat, he just pulled out the spring-loaded device out of middle so it was just a straight hole, no restrictions.

That would increase flow straight out to the exhaust elbows and reduce flow into the block. It may be part/all of the problem.

 

[flexall1]

Re: 1993 Mercruiser 7.4L 454 High Temp/Overheating

So I should try getting a new thermostat???? I was kind of unsure why he did it that way, but he said he had done it before. Does that just take an 180degree thermostat?? Thank you for you info..

 

[tpenfield]

Re: 1993 Mercruiser 7.4L 454 High Temp/Overheating

143 degree thermostat ( I believe) . . . but that may not be the problem. The flow restrictions were there for a reason. Not all mechanics are geniuses. So, I would get it back to the way Mercruiser meant it to be.

 

[flexall1]

Re: 1993 Mercruiser 7.4L 454 High Temp/Overheating

Thanks for all your help.. I'll let you know what I find out....

 

[ktbarrentine]

Re: 1993 Mercruiser 7.4L 454 High Temp/Overheating

I have replaced the impeller twice this year. Last time I think I installed it wrong. Both times, there were no pieces of the impeller to be found. Is there an easy way to check the oil cooler??? As far as the thermostat, when we replaced the impeller, my boat guy checked the thermostat and completely tore out the center to let water flow....

Ok...this one begs for more info..... if you've replaced it twice this year, and it disintegrated both times such that "no pieces of the impeller to be found", something is VERY wrong....(besides the issue of of basic overheating when you say you have good flow out the exhaust). I'll be interested to hear how this one comes out. I vote for the oil cooler inspection and removing outdrive/backflushing...to start.

 

[HT32BSX115]

Re: 1993 Mercruiser 7.4L 454 High Temp/Overheating

Howdy,

Water coming out exhaust is very hot.

That's because you have very little flow.....


I have a 1997 454/Bravo with an oil cooler.


You MUST remove your oil (and Pwr Steering) cooler and absolutely ensure that they're clear of impeller fragments and other debris.

When I removed my oil cooler, It was so CRAMMED with impeller pieces that I am surprised ANY water got through it!


AND, I had to pull most of the small pieces out with needle nose pliers... So forget about "back-flushing"

 

[flexall1]

Re: 1993 Mercruiser 7.4L 454 High Temp/Overheating

I want to thank everyone for the their help. After inspecting the oil cooler. Feeling the intake hose, felt like there were
2 tennis balls stuck in there. Pulled off house and crammed packed with impeller pieces. Picked them all out and installed
a new thermostat now temp is fine boat running perfect. Again, thanks for everyones help. You helped me do it my self
and learn a little more about my boat....