Overheating issue

[rubherchicken]

ok so I have read a ton about the low speed/ idle overheating that alot of people have had here on this site, and I too seem to be in the same situation. I hook up the garden hose and start the boat, let idle and before long it goes to 200 deg. I hit the gas and bring the idle up to around 1200 and the temp drops. I then lowered my drive unit into a trash can full of water, removed the hose from the drive to the engine to check for water flow, and I had none. Took off the lower drive and removed the impeller. The plastic tube that connect to the copper tube was melted and there was some obvious wear in the impeller housing and impeller fins, but none of the fins were missing. thinking the new impeller and housing with related parts will fix my overheating? was a small amount of blockage from the housing that was worn aswell. Wondering if that would be enough to restrict water flow from my garden hose? what do you all think???

 

[88dancer300]

Sometimes a leaking impeller pump housing base gasket will cause air bubbles, and decrease the waters ability to cool the motor. I would relplace the impeller, plastic stand pipe, complete pump housing and all gaskets. Worked on my Alpha. Runs at 130 degrees on the hose, 130 in the trach can.

 

[SCORPIO]

I've got the same problem on my '89 300DA 5.7s. I'm going to do what 88dancer300 says and replace the entire pump, pipe and gaskets.

 

[andyr]

If you have the thermostat housing with the balls and it overheats at idle but cools down soon as you get off idle like 1500 rpm, then it is the springs by the balls in the thermostat housing. These balls are designed to bypass water to the risers at higher waterpump pressure. When the springs are weak, water bypasses at idle and doesn't get to the engine water pump. A quick test is to stretch the spring. Also check the thermostat housing. If the balls don't seat well, it will overheat at idle. There is also an internal passage from one side to the other in the housing that scales up. It is only 1/2" dia.

 

[SCORPIO]

Andyr, thank you for the explaination of the function of those blasted balls! I've wondered about those since I first saw them. I relaced my thermostat housing last summer after the heating problem began because the old housing was very corroded and the ball seats were half gone. The new housing didn't fix the problem, but I wondered if I tightened the ball retainer nuts enough to properly tension the springs. I'll check that this spring.
Here is a little background on this problem; this engine, a 5.7l (port) is a reman installed two years before we bought the boat. It has about 100 hrs on it. The other engine (starboard) is a 5.7l from 1989 with about 390 hrs on it. At the start of last season, i had all new waterpumps and bellows and gimble bearins installed on both engines by a reputable shop. I had replace all the risers and manifolds on both engines the previous year. Both engines ran rock steady at 145 (raw water cooled in salt water) until about the middle of August when the port engine began to experience low speed heating issues. I tried flushing, back flushed the power steering cooler, pulled a riser and checked for blockage, nothing. I replaced the thermostat and then the housing after seeing how corroded it was, again nothing. I think I may have sucked up a plastic bag or large jelly fish that damaged my impeller. The interesting thing is the starboard engine thermostat housing is in the same bad shape as the old one on the port engine but it doesn't over heat, thats why I'm leaning towards a pump issue.
I think I'll replace the pump while the boat in out of the water and also see if there are any blockages in the hoses or pickups that could cause restricted flow.

I noticed something intesting, one day I pulled the t-stat and took the boat for a run and it didn't overheat at idle, actually ran about 120, and ran 145 at 3200 rpm cruise.
I'm going to look at anything in the drive that could cause this prior to launch then see what happens. I'll report with my findings.

Sorry to the origional poster for the hijack of the thread.
Thanks

 

[andyr]

Here are some things to check: Did you replace the springs when you replaced the thermostat housing? Yes, tighten the screw down. If your overheat problem is only at low speed, the problem is not the waterpump in the sterndrive. When the engine is operating 80% power or more, it generates a lot more heat than idle. If the temps are good at high speed then the pump and risers are okay because that is when they are working hardest. Here is a test: loosen the clamp to the raw water hose at the thermostat, start a cold engine, disconnect the hose. If the water shoots out about 6 in. at idle, the pump is fine. You may have had a piece of scale break off and prevent a ball from closing, or blocking that internal passage. I have seen some new springs that look the same but are weaker. The fact that you said that running without a thermostat gives acceptable temps is a clue that means the pump is okay. It is possible during the season you got some seaweed or bag around the drive. Happens often. Just run in reverse for a few seconds and it usually clears. Perhaps the new impellers and plastic water tube overheated and were damaged when the engine overheated. You could have barnacles or something in the drive intake. Not real likely. The pull the hose test will tell. Can also pull the hose to the riser. Less likely is an engine water pump problem or blocked internal block passage - but if temps are okay at cruise then that is not it. My guess, bad springs and seaweed on the drive.

 

[stuckinforward]

last overheat problem i had on the 5.7' after all the checks, new pump, clean system, no kinks....comp test and found a cracked head... it was the worst case scenario. it was a customers boat. it heated quick, i went thru the motions found a broken pump bolt/screw and figured it was pulling air, it was and may have attributed to the engine running hot and no gauge watching. ????

 

[rubherchicken]

Ok after replacing the t stat gaskets impeller it seems to be good, ran it with wht drive in a trash can full of water, it pulled water up from the drive and shot it out the hose that goes to the p/s cooler/ engine. At idle it float around 150 deg, and fluctuates between 140/ 160 according to my gauge. I am going to use my IR gun to test temps per and post t stat, block temp and riser temp to be sure. I had alot of sand and grit it the impeller housing, it was pretty worn out, not shot but it was on its way. so far so good.

 

[Bumpo64]

If you have the thermostat housing with the balls and it overheats at idle but cools down soon as you get off idle like 1500 rpm, then it is the springs by the balls in the thermostat housing. These balls are designed to bypass water to the risers at higher waterpump pressure. When the springs are weak, water bypasses at idle and doesn't get to the engine water pump. A quick test is to stretch the spring. Also check the thermostat housing. If the balls don't seat well, it will overheat at idle. There is also an internal passage from one side to the other in the housing that scales up. It is only 1/2" dia.

SO, those springs are not supposed to let water pass at idle? I saw that they are adjustable, should I tighten the nuts (shorten the springs) to increase the pressure on them? I had this housing apart to test the thermostat and don't recall seeing the "internal passage" you've mentioned. Is this passage behind those check balls?