3.7L Mercruiser Overheating---Help!

[NuBoaters]

Fairly new to boating and boat engines and need some help. I have a 3.7L Mercruiser with Alpha 1 heat exchanger and closed cooling system. Last year boat ran great, but some idling problems. This year engine wouldn't even start and sounded like wasn't getting fuel. I replaced all the fuel lines, cleaned out the mechanical fuel pump, rebuilt the 2 barrel carb, and new spark plugs. Also replaced the torn exhaust bellows with the slip on kind. Turned the engine over in the garage and sounded great, idled beautifully. So we took it out to the lake...within 2 minutes of WOT, smoke begins to billow from the dog box. Engine was seriously overheating and coolant spewing from the overflow and out of the closed system resevoir. Needless to say we had to be towed back to the boat ramp.

Any ideas would be greatly appreciated? Thermostat? Water pump? (it has one I assume). It has never overheated before so I'm not even sure where to start. Thanks.

 

[Eric 3.7LX Alpha 1]

Re: 3.7L Mercruiser Overheating---Help!

When you fired it up in the driveway/garage did you have water feeding the drive via a garden hose and muffs connected at the inlet? if not you definately toasted the water impeller in the outdrive. It is made of rubber and the friction without water will cause it to be NG.

 

[stonyloam]

Re: 3.7L Mercruiser Overheating---Help!

When you fired it up in the driveway/garage did you have water feeding the drive via a garden hose and muffs connected at the inlet? if not you definately toasted the water impeller in the outdrive. It is made of rubber and the friction without water will cause it to be NG.

Agree, no water = new impeller.

 

[NuBoaters]

Re: 3.7L Mercruiser Overheating---Help!

I did not have the drive engaged, just ran the engine briefly. Would that make a difference?

 

[Eric 3.7LX Alpha 1]

Re: 3.7L Mercruiser Overheating---Help!

The outdrive is connected to the engine and rotates as long as the engine is running whether in gear or not, shifting is in the lower part of the drive. The impeller is cooked. These engines are VERY sensitive to overheating, you can replace the impeller and go from there hopefully no other damage to engine.

There are Mercruiser manuals for free download in the "Adults Only" section at the top of the fourm. The engine is #8, drive is #6. I would start by downloading them.

 

[stonyloam]

Re: 3.7L Mercruiser Overheating---Help!

No, it would not. You will need a new impeller, maybe a housing too.

 

[NuBoaters]

Re: 3.7L Mercruiser Overheating---Help!

Thanks guys. Ignorance is expensive. Anything else I should look for/check? Once I replace, is there a way to adequately test without putting on the water? Will muffs and garden hose be enough? Did not run on the lake long. Once it overheated, shut everything down and got a tow.

 

[Eric 3.7LX Alpha 1]

Re: 3.7L Mercruiser Overheating---Help!

Hose and muffs are adequate, keep RPM's under 2k,

I would pull complete drive so you can check the bellows, check/grease gimbal bearing and u-joints. Shifter must be in foreward to remove and reinstall the drive off the boat. If there are any fins missing off the impeller they might be in the passages somewhere, just to be aware. Remove prop check for any line behind it, grease prop shaft splines so prop removes easy in future.

Here are couple of youtube video's to check out:
http://www.youtube.com/user/TcElectronicsMarine#p/u/7/kfpasFN_g8w

http://www.youtube.com/user/TcElectronicsMarine#p/u/6/0QC6_j2_wmA

 

[stonyloam]

Re: 3.7L Mercruiser Overheating---Help!

For the engine, I would do a compression check (when you get it all fixed) to make sure the head gasket is OK. I would drain outdrive oil, pull the whole outdrive, check the gimbal bearing, and lube the u-joints. Easiest to split the outdrive on the bench. Replace the impeller and body, and you should be good. The manuals are gone, but if you don't have one here is a link on impeller replacement: http://www.sterndrives.com/supplies/pumpkits2.html . Running on muffs is fine, just keep the rpm low. Any questions, just ask

 

[NuBoaters]

Re: 3.7L Mercruiser Overheating---Help!

Thanks all. Really appreciate the help. Will let you know how it goes. Hopefully didn't do any more damage. Thanks again.

 

[NuBoaters]

Re: 3.7L Mercruiser Overheating---Help!

Another question - Is it possible to do this without removing the drive?

 

[Eric 3.7LX Alpha 1]

Re: 3.7L Mercruiser Overheating---Help!

Yes, you can just drop the lower half while the drive is in place, Make sure the remote shifter is in forward for removal and reinstallation. Also when reinstalling the lower back to the upper you have to hold it in forward by placing in forward and keeping pressure on the prop is it does not slip out.

Much easier with 2-people

You should pressure test the drive before refilling it with lube after the job is done.

In the "Adults Only sticky at top of this forum there is a section on how to make a pressure tester for the drive.

 

[stonyloam]

Re: 3.7L Mercruiser Overheating---Help!

Yes, you can just drop the lower unit.

 

[twister216]

Re: 3.7L Mercruiser Overheating---Help!

Check the inside of the upper impeller housing also. If there is a bunch of burn rubber on it, I would change that also, unless you can get it out.

 

[NuBoaters]

Re: 3.7L Mercruiser Overheating---Help!

Thanks. If I do decide to take the drive off, does it go back on in the reverse procedure--shifter in forward? Any other special procedures? I have seen youtube video of removing, but couldn't find anything showing how to re-install.

 

[Eric 3.7LX Alpha 1]

Re: 3.7L Mercruiser Overheating---Help!

Everything needs to be in forward to reinstall. Here is a photo from Ziggy showing shift shaft on the front side of the drive:

In this photo it is either in neurtal or reverse forward is straight ahead.

 

[NuBoaters]

Re: 3.7L Mercruiser Overheating---Help!

Well I finally got to this. Pulled the outdrive and lower unit. Water pump and impellar were toast. Literally. pick up tube was melted on the pump. Checked everything over and replaced the impellar and both pump housings. Check the pickup tube in the upper unit and didn't appear to have an issue. Found the exhaust shutter in pieces, so planning to replace that. Looks to be pretty simple.

The water hose looks bad, so I am going to replace that as well. Thought I would go ahead and replace all the bellows while I am this far. Maybe even the gimball bearing? There is coolant in the u joint bellows. What does this tell me? Any other suggestions, things I should look for?

 

[NuBoaters]

Re: 3.7L Mercruiser Overheating---Help!

There is coolant in the u joint bellows. What does that tell me???

 

[Blk-n-Blu]

Re: 3.7L Mercruiser Overheating---Help!

Here is some Stuff that will help you out:
http://www.mercstuff.com/howdoi.htm