1986 mercruiser 5.7 overheating !!! help

t_man41782 · May 2, 2010

I have a 1986 wellcraft elite 210 with a mercruiser 260/5.7. The problem is that it overheats to around 220-240. i started from the basics, replaced the impeller in the lower unit, replaces the t-stat, new temp guage and temp sensor. I have done a compression check on all the cylinders and the lowest reading i am getting is 100. I am lost on what to do now, please help!!!!!!

Don S · May 2, 2010

Re: 1986 mercruiser 5.7 overheating !!! help

Have you checked to see if you have water flow from the outdrive to the thermostat housing (assuming raw water cooled engine) if that is ok, it's possible the manifolds and risers are plugged up and in need of replacement.

littlebookworm · May 2, 2010

Re: 1986 mercruiser 5.7 overheating !!! help

Don said it all. Disconnect hoses and check water flow. If it's good, you probably need new manifolds and risers. Start with the hose coming off the transom fitting. Disconnect it at the oil cooler, if you have one, or the thermostat housing. Start your engine and check for flow. (A helper is mighty handy to do this.) Work on through the hoses. Remember hoses do deteriorate from inside out and may have collapsed. Check for overly soft hoses; they'll need replacement. Good luck. Hy

t_man41782 · May 2, 2010

Re: 1986 mercruiser 5.7 overheating !!! help

i have checked all the hoses when the motor is running and there is water running through them all. This is why i am stumped

PitchFork · May 2, 2010

Re: 1986 mercruiser 5.7 overheating !!! help

When does it overheat if at idle for awhile or running on plane? Do you have the correct Tstat in it per the manual? Have you checked the timing? 100psi sounds a little low all cyclinders should be with 20% of 150psi if I remember right.

t_man41782 · May 2, 2010

Re: 1986 mercruiser 5.7 overheating !!! help

it will overheat while on plane, and the t-stat is 140 degree, and all the other cylinders read 125-150 except the one that is reading 100

alien4fish · May 2, 2010

Re: 1986 mercruiser 5.7 overheating !!! help

you have a dead hole, you may get lucky and have a blown head gasket, it may be time for a long block check the oil and see if its getting milkey.
Im right in the middle of a repower on my Merc 260 right now. I had the EXACT problem you are having. And my mechanic explained since the compression is low the rings are tired and the motor is just plain worn out.
Now you need to ask yourself.................is the boat worth it for you to you to take on this project? And there are deals out there right now for long blocks you just have to tip toe thru the minefields if you know what i mean

Tommywalton · May 2, 2010

Re: 1986 mercruiser 5.7 overheating !!! help

A 350 chevy is easy to work on. U can tell lots with a compression tester. On the cylider with low compression remove valve cover, make u an air connection using the compression tester. You want to pressurize that cylinder with air. Rotate engine until both valves are closed. Listen for air leaks in 4 places. 1. Exhaust, if it's there it's a burnt valve or rocker arm too tight. Same on intake listen through carb. Next listen to dipstick tube if it's there with both valves closed it's comp rings. Finally listen to thermostat housing, if there it's head gasket. 100 psi will still run without noticable performance loss. Probobly not your problem. Your not getting water to your engine just like me right now.

alien4fish · May 2, 2010

Re: 1986 mercruiser 5.7 overheating !!! help

if he's got readings from 125-150 on each cylinder to #7 then 100 on #8 theres your problem right there, and just like Tommywalton says now that you isolated that cylinder find out whats going on with it Valve, rings, head gasket, etc.
and yes Chevy's are easy to work on. I pulled the outdrive disconnected the motor,and ive got 2 bolts left and my motor is ready to come out.

t_man41782 · May 2, 2010

Re: 1986 mercruiser 5.7 overheating !!! help

draining the oil as we speak and it is not milky, also ran water throught the intake in the lower unit and water comes out the exhaust. I do have thru hull exhaust, does that make a difference?

Bt Doctur · May 2, 2010

Re: 1986 mercruiser 5.7 overheating !!! help

t_man41782 said:
draining the oil as we speak and it is not milky, also ran water throught the intake in the lower unit and water comes out the exhaust. I do have thru hull exhaust, does that make a difference?

Most of the drives i worked on you cannot just flow the water thru the pump and have it come out the exaust without the motor running to pump the water.I suspect theres a pump issue in the lower unit.

t_man41782 · May 2, 2010

Re: 1986 mercruiser 5.7 overheating !!! help

i have the lower unit off and i just ran the hose to the inlet

Bt Doctur · May 2, 2010

Re: 1986 mercruiser 5.7 overheating !!! help

Then put the muffs on the lower unit and see if any water leaks out from the pump area.
the pump base gasket may be sucking air or someone did this by accident.

Tommywalton · May 2, 2010

Re: 1986 mercruiser 5.7 overheating !!! help

I can remove my 350 from a bayliner Capri from boat with a Volvo 270 drive in 10 minutes bellhousing, engine mounts, wires and hoses. Bam it's out. You should check things like I described before you take the head off.

alien4fish · May 2, 2010

Re: 1986 mercruiser 5.7 overheating !!! help

hey tommy, the transom bolt on the starbord side is frozenI soaked the S%$# and still cant get it to budge. I was thinkin I may have to cut the head bolt off then pull the motor and then try to get at the bolt with pipe wrench.
any suggestions?

Tommywalton · May 2, 2010

Re: 1986 mercruiser 5.7 overheating !!! help

Hey, i would cut it only after I stripped the head. Heat my friend heat. If it's possible heat the bolt with a torch or brazing tip, even a soldering setup u can get at walmart with map gas will prob do it. Heat it and let it cool a couple of times. I'm sure there is not enough room for a 1/2 inch air impact huh. Just remember when u break it it's possible that u will have to drill it out, hog it out with die grinder then chase the threads. Last resort IMO. However it can be done. Why are u pulling engine?

1986 mercruiser 5.7 overheating !!! help

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