5.0 mpi overheat at higher rpm.

[woodguy81]

Finally got the heads back and on the boat. Runs great but at higher rpm it will overheat and wont cool off unless I shut the motor off for about 2 minutes and restart. Then the temp drops rapidly. Ive noticed if you are looking at the motor the left riser on the exhaust gets pretty warm while the right one stays cool to the touch. Also the dash guage doesnt always match the vesselview temp. I also removed the thermostat and I get the same results. Where should I look first? If im running at 3700 rpm it did rise in temp but not to overheating level. WOT is 4700 with the prop im running. Also worth noting I did change the impeller already. Had the issue before and after I changed it. Will not overheat on the hose at home.

 

[alldodge]

Have a serial number, and is this a Alpha or Bravo?

Also why where the heads removed?

 

[woodguy81]

Cant find the motor cover with the serial number. Alpha. Intake valves were shot and both heads were cracked from a lean condition which I believe was a combination of bad fuel and a completely corroded fuel vent. I posted about these issues previously. The overheat issue is not a new problem but It wouldnt run very well long enough for me to overheat the motor most days. Now its running like it should except for this issue.

 

[alldodge]

Need to know what you have to start troubleshooting. Need at least a year and if it has multi or single point drain system. Will assume its open cooled from first comment

A lean running motor also runs hotter.
Get a piece of clear hose and connect at the thermostat housing from the drive. Need to see if there is bubbles

 

[scoflaw]

Plugged/corroded manifolds and elbows can do it. The balls in the T-stat housing have been known to cause issues. Circulating pump possible. Retarded timing maybe.

 

[woodguy81]

2003 5.0 mpi. Open cooled. There’s a connection under the motor with 4 small hoses and 2 big ones in the cooling system. I also replaced the water pump on the motor already. I’m thinking a blockage somewhere at the moment. Just gotta track it down. Probably worth noting that when it overheated I shut it down and waited a couple minutes and opened the thermostat housing to try running without it and there wasn’t any visible water in the block at the top of the housing. When I restarted it started circulating ok again bc water started leaking from the housing bc i damaged the gasket. Then the temp came down again til I opened it up again.

 

[tpenfield]

Wrong type of head gasket ?

What head gaskets were used for the re-installation?

 

[woodguy81]

Felpro marine gaskets.

 

[alldodge]

Ok you have the single point drain system with the Alpha drive.
https://www.mercruiserparts.com/bam/subassembly/31849/8093/200

Place a clear hose between item g (cool fuel) and f (water dist housing). Then see will in the water if there are any bubbles.


Also use the pic to make sure the hoses are routed correctly

 

[woodguy81]

Thanks. What is the concern here? Bubbles vs no bubbles. Also had the left riser off today and the restricted hole on the gasket was plugged with debris. Flushed it out real good and reassembled. Also had all the front hoses and water pump off the see if it was clear and flushed back through every hose. Nothing seemed blocked.

 

[alldodge]

If you see bubbles then your sucking air.

If you notice your cooling during normal operation is, thru the engine water pump, thru block, out thru thermostat housing, to exhaust. When thermostat is closed there would be a small amount of water coming from the block and out the exhaust. There needs to be a small amount to ensure air is bleed off.

In pic below, note the blue, red and green arrows. Normal flow is red arrows and with the thermostat open. When thermostat closes more water flows thru the blue arrow feed and out the exhaust. The green should stay full all the time and are only used when draining for winterization

There is a restriction in the Blue Tee fitting to make sure the water flows more easy thru the red side. If there is a greater restriction elsewhere, like in the exhaust manifold to elbow, then not enough water makes it to the block.

This pic is for a V6 and bravo drive but otherwise the same

 

[woodguy81]

Also wondering if the restricted flow gasket in the riser could be my problem. I see there’s a full flow option.

 

[woodguy81]

Found the serial number. Om670725 2003 5.0 mpi

 

[alldodge]

Found the serial number. Om670725 2003 5.0 mpi

Thanks, that helps ensure were not going down the wrong path

Also wondering if the restricted flow gasket in the riser could be my problem. I see there’s a full flow option.

You don't want the full flow (both passages full open), but the restricted type.
So the side with the full open hole is positioned toward the rear of the motor, and the small hole faces front

 

[woodguy81]

If I’m understanding your explanation correctly then perhaps my riser that stays cold is experiencing a blockage so the water from the block is not flowing through it. When running on the hose the exhaust water from that side is cold and it’s warm on the other side.

 

[woodguy81]

So mine have the small hole positioned at the rear of the motor. Toward the rear of the boat.

 

[woodguy81]

So my gaskets have the small hole near the rear of the motor. Also the rear of the boat to be sure we are Looking at things the same way.

 

[alldodge]

There is always one riser (elbow) that will be cooler then the other. This is because water flows the path of least resistance. If you didn't have the 1 point drain and had the 7 point (much better), one side is always cooler then the other.

Now neither should get hot enough to burn you, but one side may make you remove your hand faster. If one side is to hot to touch then there is a restriction on that side (most likely) or not enough on the other.

The small hole goes to the bow, larger hole toward the bow

 

[alldodge]

Pic to help

 

[woodguy81]

Ok. My gaskets are backwards. Could that alone cause my issue?