1988 5.7 L overheat

[zellerj]

I have a 1988 5.7 L alpha drive that overheated in a 27 ft Tiara with two mercruisers. I live on Lake Erie so it is fresh water, but raw water cooled. I noticed a hot oily smell and glanced at my temp gauge and the port engine was hot, so I turned it off. I could hear the water boiling in the engine. After limping home using the starboard engine only, and putting it on the trailer, I pulled the outdrive and found four shutter remnants in the exhaust outlet. After splitting the drive the water pump housing and the water tube cover were melted. Upon removing the top impeller housing, I found a intact impeller, with still pliable vanes. I think the impeller was four years old.

I replaced the exhaust shutters, and pulled off the water hose and inspected the power steering cooler for bits of past impeller parts, but it was clean. I removed the thermostat housing and it was clean.

My questions is why did this engine overheat? I ant to be sure I found the root cause so it does not happen again

I have come to two scenarios:
1 - The impeller was weak and stopped pumping water, causing the shutters to burn and drop into the exhaust exit
2 - the shutters where burned during a past overheating event, and just worked their way down to a point where they blocked the exhaust enough to cause an overheat.

Your opinion on this is appreciated.

Another piece of info: Early in the summer the port engine had a overheat issue, but it was temporary only lasting a minute or two - I put the drive in reverse and it cooled down and I did not have another issue in five or so outings. I figured a plastic bag or something wrapped around the drive cutting off the water supply, but I have no evidence of this.

Jim

 

[alldodge]

I would think plastic bag as what may have happened the first time

Do back flush everything to try and get anything out which may have broken loose in the overheat

 

[Rick Stephens]

I agree with AD.

Easy test when you have it reassembled with new unmelted stuff. In the driveway on muffs, pull the water line off the thermostat housing that comes from the transom via the oil cooler. Point it at a 5 gallon bucket and fire up. Should fill the bucket in under 30 seconds. That little test tells you how functional the first half of your cooling system is. As long as the thermostat is still in working order, and I'd probably pull it and test in a water filled pan on the stove before trusting it again, then the rest of the system is only dependent on no blockages in the motor and the circulation pump still working.

 

[zellerj]

OK progress since my first questions:

Take 1: I replaced all of the melted parts - water pump base, impeller, and water pump top. The vertical drive shaft upper seal looked fine, so I did not replace that. I installed the drive on the bell housing and ran it on the hose with rectangular muffs. It ran cool for a few minutes and then got hot, so I shut it down. Removed the drive and split the lower from the upper, and the impeller was destroyed. My only thought on this event was that I installed the impeller with the vanes the wrong way.

Would installing the impeller with the vanes the wrong way destroy it? I had coated the metal insert with a light coating of silicone grease. This was a Siera pump kit, not a Quicksilver part.

Take 2: I put a Quicksilver pump repair kit back in, installing the pump housing as described in Manual #6 (install the impeller on the shaft with the key greased in, and put pressure on the housing while rotating the drive shaft clockwise, which is a very easy way to get the impeller into the housing). Then I installed the drive back on the bell housing and ran it on muffs. It was for the most part cool, the only hot spot I found using a IR temp gun was at the top of the elbow right after the first rubber sleeve. Moving the temp gun a few inches down the elbow indicated 150 F, which is what I would expect. It ran on the muffs for 15 minutes and it seemed fine.
And so hopefully my issue is solved.

Question: should I be concerned that the temp reading at the front of the elbow, right after the rubber sleeve, reads 260 F? but an inch away from that reads 150 F?

Thanks,
Jim

 

[alldodge]

Would installing the impeller with the vanes the wrong way destroy it?

No, it would just roll over. Look at date codes on impellers, there rubber and more then 10 years I would not install, and try to do less then 5

Question: should I be concerned that the temp reading at the front of the elbow, right after the rubber sleeve, reads 260 F? but an inch away from that reads 150 F?

It would bother me but may only be the issue of it being on a hose