Mercruiser 3.7 Overheating Troubleshooting

[Old Yeller Boater]

I've got an overheating problem with my 3.7. Last fall the outdrive impeller was replaced as the old one had the ends broken off. Loose pieces were recovered from the heat exchanger, but no guarentee that it's all of them. Blowing air through the system seems to be good, and I've removed the hose going up to the exhaust elbow while the boat is running and there is water bubbling out of the hose. I'm not sure if it's full flow or not, but seems to be moving water well. I've also removed the hose that comes from the outdrive into the heat exchanger and it seems clear and good as well. Motor stays at reasonable temps at lower RPMs, but once on the gas, temps rise and eventually overheats.

I'm going to replace the thermostat since it's a cheap part, but how could I determine if the water circulater is having any problems? There's no leaking or other indications around the housing.

Any other troubleshooting tips or ideas welcome.


thanks.....

 

[Old Yeller Boater]

Re: Mercruiser 3.7 Overheating Troubleshooting

Anyone have any troubleshooting suggestions other than what I'm trying?

 

[Don S]

Re: Mercruiser 3.7 Overheating Troubleshooting

Remove the exhaust elbow and inspect the water outlets out of the elbow. If plugged up with rust (Or impeller pieces), water can't flow out and things heat up.

 

[stonyloam]

Re: Mercruiser 3.7 Overheating Troubleshooting

Don+, and that is a good time to check the shutters.

 

[Old Yeller Boater]

Re: Mercruiser 3.7 Overheating Troubleshooting

Looks like I need to take the entire thing apart to check. I'll check the water shutter while I'm there. I'm going to pull the water inlet hose at the regulator and check for flow from the muffs. I was reading another thread discussing water pocket cover and likelihood of an overheat causing problems there. Would that be a good easy replacement to do as well?

 

[Don S]

Re: Mercruiser 3.7 Overheating Troubleshooting

Would that be a good easy replacement to do as well?

That is something you don't want to get into unless it's the last resort. Look at it first, if it isn't all bubbled and distorted from heat, it's not likely your problem.

How much of an overheat did you have? Or are you having now? You replaced the impeller, but what did the pump housing look like? Did you replace the gaskets as well, or just the impeller?

 

[Old Yeller Boater]

Re: Mercruiser 3.7 Overheating Troubleshooting

the problems occurred last fall prior to me owning the boat, but I do know the previous owner. Initially, the bellows split open. While replacing that, the impeller was checked and replaced. I don't recall about the gaskets though because I was not doing the repair and not familiar with the procedure. The overheating was enough that it over heated the motor to the point of antifreeze boiling over. There was smoke coming from the engine compartment where the insulation burned. Since buying the boat last month, the overheating has been antifreeze boiling over. The temp gauge is not working, but I'm working on getting that fixed too.

if I remove the lower unit, I should be able to see the water pocket cover, correct?

 

[Don S]

Re: Mercruiser 3.7 Overheating Troubleshooting

You might just as well get dug in for some work. The 3.7 is noted for blowing the head gasket when overheated, due to the open deck design to make it cheap to build. Pull the drive and split it and look at the housing for the pump and the water pocket.


Any chance of getting your money back?

 

[Old Yeller Boater]

Re: Mercruiser 3.7 Overheating Troubleshooting

compression check should show if the head gasket is bad, correct?

by "split it" you mean remove the lower, right?

by the way, thanks so much for the input. I feel like I'm learning mechanics all over again. There's lots to learn about these outdrives.

 

[stonyloam]

Re: Mercruiser 3.7 Overheating Troubleshooting

The compression test will tell a lot, some of the "boiling over " may be the engine pushing gasses into the coolant through a bad head gasket. You can borrow tools at Autozone to do a cooling system pressure check, which will help diagnose the problem.

 

[Old Yeller Boater]

Re: Mercruiser 3.7 Overheating Troubleshooting

I do know that there's no contamination in the oil, so if its the head, it would only be into the water jacket. I'll do cooling system check along with dropping the lower and also going to to remove exhaust elbow and check for foreign objects there.

thanks for the input and stay tuned!

 

[Don S]

Re: Mercruiser 3.7 Overheating Troubleshooting

compression check should show if the head gasket is bad, correct?

\

I just replied to another 470 thread on the same problem, have a read here http://forums.iboats.com/showthread.php?t=562001&p=3860066&viewfull=1#post3860066

Yes, splitting the drive means removing the drive from the boat and separating the upper from the lower. Don't leave the upper on the boat, get it off of there so you can look at other things.

 

[Old Yeller Boater]

Re: Mercruiser 3.7 Overheating Troubleshooting

thanks Don. I have not seen a pressure surge like the other thread discussed, but of course, that doesn't mean I don't have a problem. I got a chance to remove the lower today and found a bad impeller along with more pieces of the previous impeller there too. I back flushed from the regulator with a hose and flow was good there. Also removed the exhaust bellows and things looked good there. Also flushed water through from the back end of the heat exchanger out the outdrive.. Tomorrow I'm picking up a new impeller and will also pressure test the cooling system for any signs of head gasket problems. I've got my fingers crossed there's no problems with it!

oh...and water pocket cover looked in good shape too.

 

[stonyloam]

Re: Mercruiser 3.7 Overheating Troubleshooting

You should take the ends off of the exchanger too. If there are impeller pieces in there they will probably be under the front endcap.

 

[Old Yeller Boater]

Re: Mercruiser 3.7 Overheating Troubleshooting

Thanks Terry...I did do that. I found a small piece in the front and none at the rear cover.

 

[Old Yeller Boater]

Re: Mercruiser 3.7 Overheating Troubleshooting

bad news.....after replacing the impeller and thermostat, I got the lower back on and took the boat to the lake. As soon as I revved the engine, I began to see bubbling coming out of the overflow reservoir. I took it down the lake about 1/4 mile and at high RPMs, the water was bubbling out of the reservoir about 4 to 6 inches. Time to take the head off.

 

[stonyloam]

Re: Mercruiser 3.7 Overheating Troubleshooting

Yeah, but it is not too bad of a job. Replace the head gasket with a OEM Mercruiser gasket, it is designed specifically for the 470 with some extra sealing features that the aftermarket gaskets do not have. Good luck.

 

[Old Yeller Boater]

Re: Mercruiser 3.7 Overheating Troubleshooting

Just to circle back and close the loop on this thread, I did eventually pull the head and replace the gasket. Looking at the head and gasket, it did not appear to be bad from my untrained eye. I had the guy at the machine shop look as well and he said he did not see any specific indicator of a blow. However, he cleaned up and magnafluxed the head to be sure it was good. he also checked the block to be sure it was level. After I got it all back together, it ran fine and was not overheating. Now on to the other problems!