1989 Mercruiser 5.7 slight overheating at lower RPMs

[vetting]

Raw water cooled. Fresh water only. Alpha 1 Gen 1 drive. Drive is a newer OEM merc with 2 years on it. Motor is 1989 5.7 - unsure on serial number.
Use the boat for trolling on Lake Michigan. Was out in fairly rough water last Sunday and trolled on the main. Looked down and the temp gauge was around 195. Normally the boat is at 160 no matter the RPM. Finished trolling on the kicker and then powered back in for almost 45 min at 3k rpms with the temp never getting above 160. Going through the harbor, the temps came back up to 180 again. Next replaced the thermostat (160F quicksilver) and redid all of the gaskets. My thermostat housing was corroded so I hot tanked it. The ball and spring assembly seem to be a little worn, but not sure if that matters. Took the boat out of the water today and replaced the impeller and housing. Temp still gets up to the 175 to 180 range at lower RPMs. The actual risers are cool to the touch at idle in the slip. So it only seems to get hot at lower RPMs and cant really makes sense of what hoses should be hot vs cold. Attached is a picture. The hose coming off of the water circulation (front of the motor) seems to be really hot.

For the merc water test to see how much water is pumping through, which of the 6 hoses do you remove? Any other suggestions?

 

[Scott Danforth]

Start with a new impeller check raw water flow

 

[achris]

This is the hose from the drive...

 

[vetting]

Judging by the diagram, I almost want to say the power steering cooler is probably junked up. Whats the official flow rate that is supposed to be making it to the housing from the inlet for a 5.7? Thanks for the picture edit and the diagram. You guys are the best!

 

[vetting]

Start with a new impeller check raw water flow

Impeller and complete housing was replaced. Old impeller that came out was still perfect.

 

[achris]

Whats the official flow rate that is supposed to be making it to the housing from the inlet for a 5.7?

See attached Service bulletin (82-4).

 

[Scott Danforth]

Judging by the diagram, I almost want to say the power steering cooler is probably junked up. Whats the official flow rate that is supposed to be making it to the housing from the inlet for a 5.7? Thanks for the picture edit and the diagram. You guys are the best!

the power steering cooler is the first "filter" in the system with the smallest holes. it will get fowled easy. back-flush it

 

[vetting]

I also just ordered a new PS cooler from Mr Cool and water recirculation pump - they are the only original accessories left on the motor so I thought I might as well.
If I want to replace the thermostat housing, is 87290a23 really the right way to go, or do some type of conversion?

 

[achris]

If I want to replace the thermostat housing, is -87290A23 really the right way to go, ...?

If it was me, yes.

 

[vetting]

Looks like you have to cobble together/hack random hoses and the sensor to get 87290A23 to fit. So I'll see if the other items and backflushing clear the issue up before I get a new thermostat housing. Hate that they just stop making parts.

 

[achris]

Motor is 1989

.... Hate that they just stop making parts.

Yeah, it is only 33 years old after all.

 

[Scott Danforth]

Yeah, it is only 33 years old after all.

34, its now M/Y 2023 (or will be Friday)

 

[vetting]

So just finished replacing the power steering cooler - man that was a pain. No blockage at all in there and it was the straight through design anyways. You would have to have something the size of a golfball to block it. Then replaced the water pump. Still got up to 175 to 180 at idle in the slip.

Did the hose test and got around 3.5 quarts of water at 15 seconds. Only did the test once though since it was about to pour out.

Both of the exhaust risers were barely warm at all to the touch the entire time.

 

[achris]

Now it's time to start looking at 'unlikely' things. I'd be pulling the water pump off the engine and making sure the impeller's still there. That would be my next step.

 

[vetting]

Now it's time to start looking at 'unlikely' things. I'd be pulling the water pump off the engine and making sure the impeller's still there. That would be my next step.

When I said I replaced the water pump in the prior post, I meant that I replaced the entire water (circulation) pump on the front of the engine.

The raw water impeller and housing in the drive was the first thing I replaced when I started the investigation. The impeller was the original from the brand new OEM drive from 2 years ago. The one we pulled out looked perfect. I replaced the impeller and the full housing and tube.

How much water should I be getting in the water test with a Alpha One Gen 1 with a V8? The ratios on the chart dont seem to line up with anything that I know of.

 

[vetting]

My next step is to replace the rod/balls/springs and sand down the opening they are in on the tstat housing.

 

[achris]

You're drive ratio should be 1.5:1 (or 1.47).. you should be getting 3.8L in that 15 second at 1000rpm.

Have you checked the water pocket cover?

 

[vetting]

You're drive ratio should be 1.5:1 (or 1.47).. you should be getting 3.8L in that 15 second at 1000rpm.

Have you checked the water pocket cover?

Is the water pocket cover in the drive? If yes, the entire drive is only 2 years old.

 

[vetting]

Since my risers never get hot at all (in fact they are usually cool to the touch), maybe Im just getting air in the system and the Tstat housing is just really hot? The surface temps in Lake Michigan rarely get over 75f.

 

[vetting]

So just looked at my old thermostat that I had laying around. Looks like its a 140 and I replaced it with a 160. So it could just be that my old thermostat was a 140 and 160 was my normal operating temp. So going to go back to a 140 tomorrow and replace the ball/springs/rod.