Diagnostic Path for Engine Overtemp?

[John_S]

'96.5 Merc 5.7L, Alpha 1 Gen II

I thought I remembered seeing one, but a scan through the "Adults Only" and both the engine and drive service manuals, did not find it. I remember something about attaching clear hose, to look for air bubbles, etc. Maybe it was just info buried in a thread? While I have my suspicians of what may be wrong, I'd rather go through a logical diagnosis.




If you like to read here are the chronalogical sequence of events. It is meant to be all inclusive, not only the things that may be related to the issue.

- No overtemp issue prior, since boat was owned. This includes lake or running on muffs.
- Replaced water pump in drive (removed for winterization) over winter. It had been 3 years since the last one I did. This is the full kit with SS housing. Old impeller was all in tack, and after sitting the blades are almost straight again.
- Checked engine alignment - good
- Checked U-joint and exhaust bellows for any cracking - no sign of any and still pliable.
- Installed drive with new seal kit.
- Drained and stored the specialty stearn drive anti-freeze.
- Put muffs on, and fired engine up ran to operting temp (175 = normal), shut off.
- Checked for any oil or water leaks - none.
- Restarted and ran for about a half hour. Wanted to burn off winterization oil, and drive any moisture out. Mostly at idle, with some engines revs. NO OVER TEMP ALARMS. Warm water exiting via exhaust relief ports.
- Engine sat during lunch.
- Hooked up vacuum gauge, shop tach, and timing light.
- Ran again on muffs just for a quick check out. - looked OK.
- Vacuum at idle = 17 - good, timing looked within reason -plan to redo latter, idle rpm 600.
- Engine temp alarm goes off, and I immediatly shut-off. This jogs my memory that I have to change the engine water circulating pump. I was get a slow drip out of the weep hole, at the end of last season. While I hoped that was it, in the back of my mind, didn't really thing so.
- Ran yearly compression test: 170-180 on all cylinders.
- Changed oil and filter.
- Changed plugs.
- Changed engine water circ pump.
- Changed dist cap and rotor.
- Changed power steering pulley and serpentine belt. (service bulletin).
- Checked muff position - good.
- Fired up and closely monitored temp. Engine block filled with water, and water started exiting exhaust relief. Temp climbed to normal, and stabilized. Reving engine 1500-2000 rpms, lowered temp slightly. Returned to idle. Temp slowly climbing. Then quickly jumped to 200, and alarm went off. Shut off immediatly.

I suspect an air leak or not enough water volume coming from drive pump.

Absent a diagnostic path, plan to do a retest tomarrow, with water tub vs muffs. Also to add a clear hose at inlet of thermo housing to see if there are air bubbles. Is there any water volume at idle tests?


Update: 180 changed to 200 degrees.

 

[Bt Doctur]

Re: Diagnostic Path for Engine Overtemp?

Depending on the amount of water pressure available, reving the engine may cause insufficiant water to reach the impeller and damage it.Ever replace the risers?

 

[Don S]

Re: Diagnostic Path for Engine Overtemp?

You may remember the Volvo overheat diagnosis file. But it's for an engine mounted raw water pump, not a drive mounted Alpha pump. There was also a service bulletin from Mercruiser (95-13) but that was overheat over 3000 rpm.
I would suspect you may have the gasket between the housing and plate in wrong. It should only fit one way, but it will just fit the wrong way, but the housing will leak off water at low rpm.

 

[John_S]

Re: Diagnostic Path for Engine Overtemp?

Depending on the amount of water pressure available, reving the engine may cause insufficiant water to reach the impeller and damage it.Ever replace the risers?

Water is from a cold water well with 20-40 regulator. Water is always squirting out around edges. Has never been an issue in the past, on this I/O or outboards. I have reved higher and longer, on prior years. Risers are original, but have never seen the salt. They were tore down and inspected a couple of years ago.

I am looking to isolate drive/water flow from engine/exhaust.

 

[John_S]

Re: Diagnostic Path for Engine Overtemp?

You may remember the Volvo overheat diagnosis file. But it's for an engine mounted raw water pump, not a drive mounted Alpha pump. There was also a service bulletin from Mercruiser (95-13) but that was overheat over 3000 rpm.
I would suspect you may have the gasket between the housing and plate in wrong. It should only fit one way, but it will just fit the wrong way, but the housing will leak off water at low rpm.

Don,

It was a genuine Merc pump kit. This kit, unlike the one prior, did not have seperate gaskets. The gaskets were rubberized and part of the metal plate. I assumed it is an upgrade over the older one. I was careful on the installation, and even questioned if the impeller was on upside down. A review of the service manual, pump install intructions, and old impeller, cound not find a diff from one side or other. Still, being human, still can F it up.


I would like to isolate the problem to the drive/water flow or the engine/exhaust. Does the following sound plausible:

- Test using a tub, filled with water up to the cav plate. Maintain water at that level. If the problem goes away, indicates probably something with drive pump. If problem is still there, add clear hose at thermo housing inlet. If air bubbles found, points to drive/pump. If no air bubbles, volume test.

Not sure how to do a volume test, but I could feed the engine direct with a hose, and seperatly collect the water that was being pumped, say into a 5 gal pail, timed running at idle. Only, I don't know a good number from a bad one. If all of this passes, than it is probably on the motor side.

 

[John_S]

Re: Diagnostic Path for Engine Overtemp?

The small local hardware store did not have any 1 1/2" dia hose, let alone clear. I made a tub by cutting down a plastic 55gal garbage can, about 6". The cav plate set on the top edge of the tub, and put hose in bottom. Filled with water and let run over. Started engine and let run for over 1/2 hour, all at idle. Temp rose to the normal 175 and did not go higher. Exhaust elbows never got above air temp (that would be normal to when I have run on muffs prior). When I got the overtemp, they were warm, but not hot enough that I had to remove my hand, until after engine shutdown and heat soak.

I think this means that it is not an engine/exhaust issue. What I'm not sure now, if it is even a drive/supply issue. Either I will have to do another test or just bite the bullet and go back and dis-assemble and see if there is something with the pump.

 

[Bt Doctur]

Re: Diagnostic Path for Engine Overtemp?

May want to try being atleast 6 inches above the plate

 

[John_S]

Re: Diagnostic Path for Engine Overtemp?

May want to try being atleast 6 inches above the plate

While that would closer simulate the boat in water at rest, wouldn't that be easier on the pump? I choose the cav plate, because that would be the lowest point it would need to pump from. I can easily do that with another 55gal tub, but not seeing what this would eliminate/prove.

I do plan to retest again and make sure it is repeatable on the muffs, such that a kink might have been preventing full flow. There was noticeably more water coming out of the relief ports that when I ran on muffs. Also, while it was running on the tub, only a little water was over running, ie the pump was almost keeping up with the water supply. Also a noticeable change once engine was shut-off.

 

[fossill]

Re: Diagnostic Path for Engine Overtemp?

My Merc manuals say never to run the engine above 1500rpm when using muffs.
Pg 97 in Install Manual

 

[John_S]

Re: Diagnostic Path for Engine Overtemp?

Over the holiday weekend, I stopped by a few more H/W stores, but was not able to find clear hose in the size needed. I retested last night. On the hose/tub, everything is fine. On the hose/muffs, it is still climbing in temp and shut-off before alarm. Carefully checked hose for any kinks or soft spots, and make sure muff position is optimal. Water still squirts out from the muffs while engine is running, but the volume of water out the exhaust ports is noticeably less, than when running on tub.

I am still not 100% sure that this means there is a problem, but if I assume good, and there is a problem once it is in the water for season, it will be very inconvienent to work on. I will pull the lower and recheck things. The only thing that I came up with, from reviewing what I did during the pump replacement is with that hard plastic piece that goes in the upper. I believe this helps seperate the water from the exhaust. When I first installed the halves back together, it fell down and I redid. Maybe it fell again and missed it? Only a trickle of water runs out the prop, and would think it would be more if that was not in place.
Also, I did not use perfect seal on the pump screws, but don't believe that is it. Maybe I wasted the impeller on start-up, but the water was absolutly on, and the inside of the housing had been greased with 24C.

 

[John_S]

Re: Diagnostic Path for Engine Overtemp?

The plot thickens:

I was doing some more engine tuning, running from the tub with freshwater input. Was running fine, but I was keeping a close eye on the temp gauge and the water exiting the exhaust reliefs. Well, noticed the amount of water was going down! Sure enough the elbows started getting warmer, and the temp gauge started to rise. Shut it down and let the water drain out of the drive.

Drained oil and removed lower unit. Pump impeller and everything looks fine! The only thing I notice is some pieces of dark crud on the outside of the metal pump housing. The pieces were dark colored and hard, and about 1/8" X 1/8" irregular by the thickness of half a dime. It could have been rusted pieces of metal or carbon like, but hard to tell. Never had that before.

I am going to do a back flush in the AM. Maybe a bunch of crap was clogged up i the power steering cooler? Outside of that, not sure where to go next.

 

[John_S]

Re: Diagnostic Path for Engine Overtemp?

The start of the day, hasn't been very promising to solve the issue.

- Removed inlet hose from thermo housing, attached hose and back flushed. Nice clear running stream, and no new particles.

- Drain block and exhaust manifold. No particles, and even when I winterized in the past there has only been minimal scale. Yes, I always use a poker.

- Removed the upper half of drive. I didn't like the idea of mating them back up that way. Always have done it with complete drive removed. Water inlet o-ring still in place.

- Attached hose to feed the water inlet hose to thermo housing. Fed water. After block and manifolds filled back up, got good flow out exhaust reliefs and a good trickle out exhaust passage.



My suspisions are leaning toward a kinking water hose at the transom. This might explain the working, not working, depending on drive movement. Would really like to isolate that, before going through what looks to be allot of "fun".

I currently still have the hose attached to feed the thermo housing. I am considering running the engine and letting it warm up to make sure something funky not working with thermostat, somehow shuting of flow to exhaust manifold when hot? Don't really make sense but will be easy to do.

Also feed water at the o-ring inlet at transom. See if I get the same or similiar flow as I saw feedin the thermo housing.

Try to inspect the hose, as best as possible for kinking, but there does not seem to be good access.



Anyway, saga is still ongoing, and always interested in others input.

 

[John_S]

Re: Diagnostic Path for Engine Overtemp?

Still nothing definative.

I currently still have the hose attached to feed the thermo housing. I am considering running the engine and letting it warm up to make sure something funky not working with thermostat, somehow shuting of flow to exhaust manifold when hot? Don't really make sense but will be easy to do.

Outside of taking longer to warm-up, every thing seemed fine. Lots of condisation on manifolds with today's humidity. Water flow from transom looked fine.

Also feed water at the o-ring inlet at transom. See if I get the same or similiar flow as I saw feedin the thermo housing.

Produced a very similar flow.

Try to inspect the hose, as best as possible for kinking, but there does not seem to be good access.

Turned it to the side and could see almost the whole hose. Also ran my fingers along it for what could be reached. No evidence of any kinking.


I am going to take a break for lunch, and think about it some more. Starting to run out of ideas.

 

[John_S]

Re: Diagnostic Path for Engine Overtemp?

BTW, this was a late model merc pump kit. Part number 817275Q05, for Alpha and Vazer drives. Instead of the paper gaskets, the plate has the rubber embossed to it. Otherwise, parts were identicle to what I have had before.

 

[John_S]

Re: Diagnostic Path for Engine Overtemp?

Not any real progress over the rest of the weekend. Re-assembled and installed drive. Sunday was on/off rain with tornado warnings.

Plan for this evening:

- Rerun, and verify problem still exists. (if it goes away now, that would be more concerning, not knowing why)

- Pull thermo housing, check for any obstruction, test thermo stat.

- Hose test each exhaust manifold, see if there is any noticeable flow diffferences.

 

[John_S]

Re: Diagnostic Path for Engine Overtemp?

Well, the problem never returned???

Ran fine on tub and muffs. Was on the lake numerous times over the weekend. Ran right at 175 degrees, like it always had. At this point, I hope it doesn't return, but don't like how it mystererously went away, either.