1986 5.7 Mercruiser over heating

[pmore628]

My 1986 22’ Chaparral VIII boat has been running perfect until this spring. Used it six times this spring with no problems as usual and then all of the sudden it started to overheat. The temp gauge would shoot up and down like crazy. I noticed it would stop pumping water (through hull exhaust) when the temp would go down (sound like it runs out of water) on the gauge and it starts back up when the gauge reads higher. Also it seems to pump enough water... I think, except when it’s hot it seems to slow down. Temp would hit 180 at idle with no thermostat and when you put a load on the engine it hits 200+. Then when you come down to a idle is when the gauge bounces from 0 to 200+ F really quickly. This engine is cooled by raw water.<br /><br />So, in order here's what I replaced so far in process of elimination:<br /><br />1.) Thermostat <br />2.) Out drive impeller <br />3.) Risers & Manifolds (existing risers & manifold were 6 years old)<br /><br />It still seems it could be my impeller after reading all other posts so I am going to put another one in this weekend.<br /><br />Any suggestions?

 

[Don S]

Re: 1986 5.7 Mercruiser over heating

Is this an Alpha drive?<br />Is it raw water or fresh water (with heat exchanger) cooled

 

[pmore628]

Re: 1986 5.7 Mercruiser over heating

Yes, Alpha 1 with raw salt water and no exchanger

 

[Don S]

Re: 1986 5.7 Mercruiser over heating

Could be a problem with the pump body, or the gaskets. You did put new gaskets on the housing and plate when you installed the new impeller didn't you.<br />Could also be a blockage in the sharp 90° elbow that goes through the transom shield. Seen that happen a number of times. Or even a kinked hose between the drive and the transom shield.

 

[pmore628]

Re: 1986 5.7 Mercruiser over heating

Yea, I bought the kit with all the gaskets and O rings. <br />Also I have through hull exhaust - no elbows. My plan was to disconnect the hose coming from the transom and see if I get some water flow. We'll see... Thanks

 

[cslaufer]

Re: 1986 5.7 Mercruiser over heating

Have you hit bottom recently??? I have heard of some unluckly people that have kit mud or sand and the impellor sucking it up, the silt fills the water galleries and bakes in there, Pull out a welsh plug and have a look

 

[pmore628]

Re: 1986 5.7 Mercruiser over heating

No, I hauled the boat out and then changed the impeller and it still overheated before I dropped it back into the water.<br />My mechanic is scratching his head as well!

 

[kwoolard]

Re: 1986 5.7 Mercruiser over heating

Try the clear hose trick. I put a clear hose from the transom fitting up to the inlet to the thermostat housing and clear hose going to both risers. Took the boat out and noticed quite a bit of air going through and also a low flow condition. Whatever your case is you can eliminate the pump and any blockage if there is an excessive amount of water running through the hoses, which is visible using clear hose.<br /><br />If there is little flow, which I would expect from your temperature readings start at the transom and disconnect individual hoses to check the flow. Trace the flow up to the risers to find out where the restriction is if you have one.

 

[BaysideMarine]

Re: 1986 5.7 Mercruiser over heating

Firstly, do you know for sure it is overheating? Erratic needle movement from the temp guage would tend to indicate perhaps a guage earthing issue or an intermitent connection of the sensor wire.<br /><br />Try another temp guage (if you have one handy) or use an infra-red heat gun and verify the engine temp. You may be changing parts for no reason.<br /><br />Also is your circulation pump a remote belt driven unit or does the engine use the original waterpump (ie. where the fan would bolt on if it were in a vehicle)?<br /><br />Cheers,<br /><br />Nicholas

 

[pmore628]

Re: 1986 5.7 Mercruiser over heating

Kwoolard,<br /><br />I pulled the hose from the transom and ran the boat and it seemed to pump about a gallon in 30 seconds, roughly. Also pulled each hose and they all had some flow. Not sure how much flow is normal though.<br /><br /><br />Nicholas,<br />The engine is definitely over heating, gets real hot. Usually can touch risers but not anymore. The circulating pump is also belt driven.<br /><br />It seems like it runs out of water when it heats up too, if that means anything.<br /><br />Thanks

 

[kwoolard]

Re: 1986 5.7 Mercruiser over heating

I am not sure what the volume flow rate should be, I had good flow with mine both coming in and going out, yet it turned out to be my impeller. They flow looked good but still wasn't enough to cool the engine. <br /><br />Did you do this test on the muffs or in the water? What about clear hose...that would tell you whether you've got a good seal down in leg water pump body. Even though you replaced it with an entire kit, it is still possible to over torque the nuts and warp the pump body. <br /><br />Try putting some clear hose between the transom and the T-stat housing and go for a spin on the water. About $2 at your local hardware store. Look for bubbles while up on plane. Double check your temp gauge and sender unit and make sure those erratic readings are not a loose ground connection. If not, erratic readings usually would indicate an intermittent blockage. Something floating around in the block or distribution housing. Pull the T-stat housing and flush it out real well.<br /><br />Keep us posted!

 

[crazy charlie]

Re: 1986 5.7 Mercruiser over heating

Disconnect the hoses and back flush the p/s cooler.Change the circulator pump.Did you use any sealant on the risers when connecting them to the manifolds??? Charlie

 

[pmore628]

Re: 1986 5.7 Mercruiser over heating

Pow!!! Went down after work to the marina and took the circulating pump off to inspect it and when I took the back cover off the shaft to the pump snapped!!! Strange... they usally don't break like that from what ive seen. <br />I will replace it this weekend and I think it's safe to say I found the problem.<br /><br />Thanks for all your input it obviously was a success!!!

 

[MFD442]

Re: 1986 5.7 Mercruiser over heating

ok, this thread is a few years old so i am not sure if anyone will see this, but i am experiencing exactly the same problem as this person. it has my marina guy stumped as well as making a substantial hole in my wallet chasing the problem. It will overheat very quickly when up ON THE RPMS. the temp gauge is very erratic and moves up n down very quickly, have done the same tstat, impeller, risers , and manifolds only to have the same problem. the person who started this post ended up finding a problem with the shaft of his impeller? am i reading that right?

i also noticed that when i squeeze the hoses from the tstat housing to the risers , i can feel something crunchy inside( maybe some salt or setiment) is that normal? or could that be 10 yrs of junk finally clogging the hoses?

thanks, John Geldert

 

[Bondo]

Re: 1986 5.7 Mercruiser over heating

Ayuh,... Welcome Aboard John,.....

Did you read the sticky at the top of this forum,.... Probably Not....

Hy-Jacking a thread that is over 3 Years Old, just Isn't the way to get any Answers.......

Sooo,......
Lets say you Start Again by posting a thread of your Own, with Any,+ All the Required Info.....
Then We'll see if We can find Your Problem.......