Hot engine

[Bad Habit]

It's a new season and I'm still chasing an old problem. My 350, 5.7 mercruiser is running hot AGAIN. It runs hot while trolling but if I kick up the rpms to about 1000 the temp comes down.I had this problem last year and never figured it out. I have changed the impeller and the thermostat. I also hooked up another gauge so I know that's working too. When the engine is winterized the drain plugs are pulled and any sediment that can drain out does. Is there a way to check if enough water is circulating through the engine to cool it? Has anyone had a water circulation pump go bad? I don't hear any sqealing or grinding and it,s not leaking. Thanks for any help. Rick

[Ray]

Just wondering if you thought of...

1. Old piece of impeller stuck somewhere.

2. water hoses bad....collapsing. Happened on my friends boat years ago.

3. New impeller is now bad...pull lower unit and check.

Hope you find an answer soon.

[Capt Vince Pierleoni]

Most of us have "been there." Ray is giving good suggestions, check the tranny cooler for impeller chunks or other debris. This is a common spot for junk to collect and cause a partial blockage. And yes, circulating pumps can go bad, and raw water pumps can go bad wrecking impellers as soon as the begin pumping. I have also personally experienced a clear cigarette wrapper stuck in the intake screen, causing a restriction. Hopefully L&M Hank, or Borderline 350 Bill will see this and help you out. Good luck.

[Big Dave]

The plastic pump housing cover on my out drive warped. It acted the same way . heated up on troll and cooled off when the rpms were kicked up. Replaced the housing cover and impeller and it was good.

[rolmops]

Are there any zebra mussels in your intake?

[Shakemsam]

The plastic pump housing cover on my out drive warped. It acted the same way . heated up on troll and cooled off when the rpms were kicked up. Replaced the housing cover and impeller and it was good.

x2

[Bad Habit]

I have checked ther intake and it is clear of debris and mussles. I also replaced the housing and plastic tube when I replaced the impeller and didn't seem to make a difference. The hoses seem rigid and I didn't find any collapsing. Maybe something is lodged in the system resticting the flow. Any ideas on how to check that? Rick

[lakebound88]

Are there any zebra mussels in your intake?

Been there... check for excessive freeplay in water pump.. I had an efi that would get that problem, come to find out, my one month old impeller sucked up baby zebra mussels....

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[G-Daddy]

We dragged our Sportcraft 252 8 hours from home to Wilson last spring. Son was running the boat as we started setting lines. Said this thing is getting hot what should I do. Shut her down. Checked to make sure raw water pump was working - still getting hot. Got back to ramp, puller her out, went back to hotel. Pulled the thermostat and started her up on water in a bucket from a hose. Engine circulating pump started squealing this time. Dragged the boat back home and changed the engine circulating pump. Boat has been running fine ever since. So the engine circulating pump can go bad, but so can lots of other things that can cause an overheat.

[bigturk_80]

This exact same thing happens with my 4.3 mercruiser every other few seasons. What fixes mine EVERY time is the impeller plate and housing. Not only does the impeller slightly wear, so does the plate it sits against. Many people only change the impeller itself. If you are changing an impeller, always do the entire kit, because the plate wears too, so slightly you can even really see it.

What mine does is this....

It runs at 140 perfectly at any rpm over about 800. If it drops below that rpm, the temp slowly creeps up into the 200 range, maybe even higher if I let it go. After changing the impeller and plate, she trolls at 140 no problem for another 100-200 hours, then time for another impeller job.

I used a little trick to test mine. I disconnected the raw water line at the circulating pump. Hooked a hose up to the circ pump and left the raw line disconnected. Put my outdrive in a big bin and ran it at different rpms. At idle, the water would hardly pump out of the disconnected hose at the engine. At any rpm over 800, out she came.

This test can help you eliminate your recirc pump as part of the problem.

[Knotlost]

1. The first is to do a volume output test on the water pump. To do this you need to remove the pumps output side hose, and if necessary, replace it with a longer one to do the test.

What you need to do is put the boat in the water (this will not work on a flushing device) and with the engine stone cold start it up. Bring the rpms quickly up to 1000 exactly and take the output hose and put it in a bucket to collect the water. After exactly 15 seconds remove the hose and shut the engine off. You do not want to run the engine any more than a minute as it will have no cooling water going to it.

The ability of this test to detect a problem is dependent upon the accuracy in its preformed use a shop tachometer with an error of less than 5 % . The new digital timeing gauges/lights will do this as well. The boat tach is not accurate normaly. A stop watch should be used as well.

Now you need to measure the water in the bucket,

Alpha stern drive pump output for 15 second period

Drive unit 1.98:1 is 3 us qts, 1.84:1 is 3.3 qts 1.65:1 is 3.6 qts 1.50:1 is 4 qts and 1.23:1 is 4.5 qts

If it is under, the pump, and everything on the inlet side of the pump needs to be checked.

Through hull fittings, seacocks, and sea strainers can all clog up an limit water input to the seawater pump.

[Knotlost]

The next step would be...

2. Water is not circulating. Go ahead and replace the tstat. You will also need to pull every cooling hose off the engine 1 by 1 and check to see if there are any restrictions in the fittings. Common items would be broken impellar blades from a failed water pump, rust, sand, dirt, ect. It is rare, but the water circulating pump on the front of the engine can lose its own internal impellar.

3. Water is not getting out. If water gets in the engine, but can not get out, it can not take the heat with it. Water exits through the exhaust risers. What you would have to do is pull the risers and inspect the cooling passages in the risers and inspect the discharge side fo the risers. They do get clogged up, and they are a maintainence item. In saltwater, most risers last 3 to 5 years before they need to be replaced.

4 . Heat is not exchanging - This is if you have a closed cooling system with a heat exchanger. Heat exchangers can and do get clogged up. If you have one, pull the end caps off and make sure the tubes are clear. Also, in rarer circumstances. If you boat in a muddy area engine blocks can get filled with silt/mud.

5. Cooling system is getting airbound. If you have gotten to this point and have not solved it yet. Time to see if the system is getting airbound. This would happen if the seals on the water pump are leaking air into the system. It will also happen if the engine has a bad head gasket.

[Bad Habit]

Thanks for all your advice so far. Spent the day today working on it. I pulled up the deck for easier access. Opened and removed all drainplugs looking for any debris, sand, dirt or anything else that might be restricting flow. Used a coat hanger to dig around. Pulled all the cooling hoses looking for the same thing. Didn't really find much, just a little sediment and rust. Flushed out everything while it was disconnected with a garden hose and nozzle. Put everything back together and still running hot. I even tried running it without the thermostat......Still hot. I did hook up another temp gauge a couple years ago to see if mine was accurate but is there another easy way to check the temp without buying an expensive temp gun?

[Knotlost]

If your time is worth more than $1.00 and hour they are not that expencive any more. Check harbor freight I believe they have a cheep one that will last a while for less than $20. $60 will buy you a pretty good one now.

[Bad Habit]

Checked Harbor freight for an infared temp gauge but the didn't have one. Decided to run the motor until it got hot and check the water temp with a candy making thermometer. Tested it on boiling water and it read 210. While the motor was running I opened a drain valve enough to let the water run over the tip until the reading stopped rising for a couple of minutes. Thermometer read 155 degrees. It seems like this should be fairly close to the actual temp. Not sure what it should be with a 140 degree thermostat but it wasn't as bad as I thought. Now I need to find out what it is for sure.

[buckboardjr]

If the drain plug that you pulled was down on the block and not on the head in the area of your actual engine temp sensor then I feel this is not a true indication of your temp. The highest temps are in the heads of the engine. You can actually fill the water jackets in the block with concrete and just flow the water through the heads on intermittent use drag engines as 80% of the heat is disipated into heads water jacket. Just want to insure your having an apples to apples comparison. I like your experiment providing your getting the water sample from the head.

[Bad Habit]

Well I thought I had the problem fixed so I splashed the boat Sunday. Unfortunately I was wrong again. While trolling the engine ran hot again. So far this spring I have Changed the impeller, changed the thermostat, pulled and flushed all the cooling hoses, pulled and cleaned all the drain plugs, new water circulating pump, new belts and a new temp sensor and it's still running hot. The only thing left that I can think of is something clogging a water jacket somewhere. Any suggestions on where to start the proccess to try and find it?

[rustyrat]

After all the time and effort you spent for nada, your should have taken it to Hank. We all try to help and most of us are handy but when it comes to MY motor, I take it to a pro. Take it in and get it fixed by a pro. Not try to diss you but just the way to end your problem. Good luck.

RR

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[Xxx]

I had a simular problem with my motor did every thing you did,it ended up being 3 things,one was the exhaust risers were blocked, removed them cleaned them out the best I could, the marina I deal with said to cook them out with muratic acid only for 20 mins. the other things were my exhaust manifold was cracked and 3rd but not least was one of my exhaust shutter was tore and the exhaust from the port side was blowing the other side shut, so the starboard side wasn't letting the hot exhaust out of the manifold, or out at all. To check if it is your exhaust shutters, loosen up the hose clamps on the big rubber boot that connects the riser to the exhaust y pipe loosen them up from the riser, hook up water and then rev up motor if exhaust comes up along rubber boot then you have a blockage in the y pipe either a shutter fell down or one tore, they are located at the bottom of the y pipe or (exhaust pipe) the newer ones are located under the riser. Hope this helps. PAP

[Has Been]

I am not a mechanic & don't claim to be. I can do some stuff but Rusty rat is right, take it to a pro.I learned my lesson. In the long run it may cost you less. And more important, I want it to run when I go fishing,with no problems, & peace of mind.

[A*M]

Your answer is in the previous posts.

Originally you said it happened while trolling, suggesting impeller,plate, etc.

1. Extend the hose right where input hooks into the motor. (thermostat housing) Start it cold and run for 15 seconds at the correct rpm to see if you have enough water coming in.

2. You can also put a clear hose from the cooler to the thermostat housing to check for air bubbles. Air bubbles means restriction.

You said you ran it without thermostat and still had problem that sounds like a water coming in problem, but maybe not.

3. If you have good water flow, then pull the rubber boot and see if a flapper broke and is plugging water going out.

4. Pull the elbow off the exhaust and check for rust. Tap it on a board to see if anything comes out. If you've got blockage here, you probably have blocked passages from the manifold,spacers if you've got them, and the elbows.

There is a service bulletin from merc for the gaskets-make sure you put the right ones in. You haven't said how long you've had the boat, if it's been in salt water or not. If someone replaced the manifolds before and put the wrong gaskets in it, you need to know if you have the restrictor or free flow gasket.

If you suspect the exhaust, before you go any further I'd call Hank and have him LOOK at it.

[Bad Habit]

Thanks for all the replys. The boat is a 1987 with about 4000 hours. I've owned it since it was new. Never in salt and never had the exaust off. I did check the intake flow to the thermostat housing and it looks good. I can hear the flappers clicking probably from the rubber seal worn or chipped off. Maybe that is the problem. Thought it would be an easy fix and it just kind of snowballed from there. I guess it's time to admit defeat and do like Rustyrat says and take it to a pro.

[kingslamon22]

The plastic pump housing cover on my out drive warped. It acted the same way . heated up on troll and cooled off when the rpms were kicked up. Replaced the housing cover and impeller and it was good.

x2

x3 also a brand new impellar will burn up in 30 seconds withough water in the system while running.

[Bad Habit]

After doing some searching on line about what the purpose the "check balls" that are on the thermostat housing serve I came across this on another forum.

(For the "check balls", you simply tighten the little lock nut(s) all the way down, until there's no more threads..........in other words, you bottom-out the nuts. There's no "setting" or "calibration" for the balls.....they're only to provide a slight back pressure to the "extra" flow from the drive, so the cool water doesn't simply go right out to the elbows without circulating through the block and keeping the system topped-off. Issues with the check balls usually cause overheating at low rpm's.)

I decided It would take about 5 minutes to check and I had nothing to lose. I added a couple of washers to tighten it up slightly and it actually fixed the problem. I apparently didn't have enough pressure and it was allowing to much water to go directly to the exaust elbows and not through the motor. I don't know if the springs weaken over time or something else happened. The set up is actually quite touchy. a little to tight and it runs too cool, and a little loose and it runs warm at idle. I still need to dial it in a little but I'm back up and fishing!! Thanks for all the suggestions. Rick

[rustyrat]

Yea!

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