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V6 overheating at low rpm

"Hey guys, I've reading yo

"Hey guys, I've reading your postings hoping to get a clue for my concern...
I have a 20ft RINKER with a 6cyl. 1994 Mercruiser that was just repaired for overheating (I 'm not a mechanic and I wish I was). I took the boat out this weekend and noticed it operating at almost 200 (maybe 195+). The man that repaired the engine said that it was high but OK if temp. didn't go over 200. The temp gauge wire was replaced because it was shorted. I feel a little bit uncomfortable although I trust and recognize the knowledge of this man.
Do you think he's right?"
 
"Carlos:

Start a new postin

"Carlos:

Start a new posting for the most answers. Click on topics above, then Mercruiser, then start a new... in the solid blue. I think you should check the engine temp w/a laser temp gun to verify the boat gauge is working. If it is, then he is FOS."
 
"Okay Guys, I'm nearing th

"Okay Guys, I'm nearing the end of my love for Mercruisers. I think I need a stick of dynamite or something.Anyway, I put everything back together including the beautiful new raw water hose and this engine overheats at idle just like before. The only thing that I didn't check is the power steering cooler. Is it possible that water could be resticted here? By feeling the hose that goes to the circ. pump when the engine is running, It still feels that the hose is almost empty with water and hot. It only looks like 1/4 full though. There is a nice spray coming out of the ports in the transom so The exhaust risers are working .This might be an off the wall question but could the engine block be vapor locked? I think that I am going to take off the water hose from the raw water to the t-stat let it just flow into the bilge, and put a hose right into the the same inlet and then see if the engine still overheats. This would at least let me know if it is the power steering cooler. What do you think ? I just hope that it isn't the engine block.."
 
"CRAP! Sorry for your agony.

"CRAP! Sorry for your agony.

If there is water coming out the idle ports on the transom, then there is certainly water going in... knowing that, power steering cooler blockage at this stage of the game is not likely.
Not enough of the water going in is making it to the engine though.

Is this the new style T-stat housing with the check balls in the lines going to the risers (middle hoses)? If so, are the check-balls there?"
 
Yes Troy The check balls are t

Yes Troy The check balls are there in the hose that go to the risers.
 
"Troy, If the t-stat housing i

"Troy, If the t-stat housing is corroded to the point where the poppet balls do not seat well, could this put too much water to the risers instead of to the engine itself?"
 
"Rich,

That could entirely

"Rich,

That could entirely be the case. The other thing you may want to try is tightening the screw/nut assembly that keeps tension on the springs to help keep water out of the risers and in the engine block. I had to make that adjustment on my 260hp to keep it from overheating in idle and it's working great now.

If the housing is corroded, then a grinder or Dremel can be used to smooth out the edges so you get a good seal.

Mike"
 
"power steering cooler. Is

"power steering cooler. Is it possible that water could be resticted here?"

Troy:

I feel it is worth a look at the PS cooler. That would be the best place for impeller chunks to lodge and still let some water by it.

Rich:

You have nothing to lose by pulling off the front and rear hoses from the cooler and then back flushing it.
 
"Guy and Rich,
It's certa

"Guy and Rich,
It's certainly worth taking a look at the PS cooler... nothing to loose by doing so.
It supports the adage:
"Expect what you inspect."
And, once done, we can move it to the "What we know" list.

Mike,
Welcome aboard and thanks for sharing your experience!

One thing I haven't figured out yet WRT these check-balls is that my book says that on later models with the new style (6 port) T-stat housing, the check-balls aren't installed... wondering what is different that allowed them not to be needed."
 
"The check balls on these olde

"The check balls on these older models are supposed to prevent exhaust gases from entering the cooling water stream due to exhaust back pressure. If exhaust enters the cooling water it displaces it and prevents cooling the parts that the gases touch.

A new shaft & springs w/check balls is available separately from the hsg. But if the hsg. is rusted away you're defeating the purpose of them since they need to seal off gases.

This page will show the part numbers for both old and new designs.

http://www.mercruiserparts.com/selectDocs.asp?doc_nbr=17468

old design: 87290A4
new design: 16413A3

Both are available from this site.

The old design is readily available on eBay."
 
"Well, I backflushed the ps co

"Well, I backflushed the ps cooler and I tightened up the check ball assembly and got the same results...shutting down the engine at 200 degrees.And I thought we had it this time.But I'm still hanging in there with what little hope I have left..I'll even try some pixie dust at this point.."
 
What was the condition of the

What was the condition of the housing where the the check balls seat?

Something is not allowing enough inlet water into the engine... but at higher RPM you're fine right?

I wonder if the gaskets between the exhaust manifold and risers should be turned 90°?
 
"There was small area that was

"There was small area that was gone from corrosion on just one of the stems where the ball seats, but it wasn't significant enough to me. The balls seated pretty good the way it looked . As far as the riser gaskets, they can't be turned 90 because of the design. They are rectangular."
 
"Troy and Guy, I think that I

"Troy and Guy, I think that I will try putting water in directly from my garden hose into the t-stat housing at a suitable rate and then see if overheating still occurs. I f you guys have a better idea, please let me know.. Thanks"
 
"I wonder what it would do if

"I wonder what it would do if you put the boat in the water and ran it. (We usually hear about just the opposite though... runs fine on the muffs, overheats in the water.)

Another thought is backflushing the eninge:
Remove the circ water pump and t-stat housing, flush water in through the top of the engine below where the t-stat goes, and out through the two ports on the front of the block where the the circ water pump discharges."
 
"Rich,
I had a similar prob

"Rich,
I had a similar problem, issue was the hose in the bell housing. Are you running this engine on the ear muffs?, if so you may wish to try removing the outdrive, and purchasing a tool that Mercruiser sells so that you can deliver water from the garden hose directly to the block. The tool ataches to a lug on the bell housing. see the following cite below this tool will do what you want instead of directly attaching the garden hose to the T stat assembly. If the boat sill over heats then you know the problem is somewhere in the engine if not, then it lies within the drive. If you had an over heat problem, you may have an issue with the lower water pump housing, or yoy may have melted the water pocket assembly within the upper part of the drive. http://www.mercstuff.com/page02.htm"
 
"OK Rich, this is starting to

"OK Rich, this is starting to keep me up at night!

What (we think) we know:
- overheats at idle, not at higher RPM.
- impeller OK.
- water hose between bell housing and transom OK.
- PS cooler OK.
- raw water flow to t-stat housing OK.
- water flow leaving t-stat housing to elbows OK (or too much).
- raw water flow to circ water pump suction... insufficient at low RPM.
- Water pump OK (new).
- T-stat OK.
- water leaving engine through manifolds OK.

The data from above:
- Raw water inlet temp - 64F
- Water out to risers - 82F
- Circ water pump inlet - 173F
- Water out of engine - 190F
- Water to exh manifolds 170F

Things that don't make sense:
- water out to the risers hotter than inlet water temp.
- water out to the manifolds cooler than engine outlet temp.


Can you run the engine at the lowest RPM that it doesn't overheat and collect the same temperature data?"
 
"Two suggestions to maybe stim

"Two suggestions to maybe stimulate some more diagnostic thoughts.

I noticed in the orginal post that on an increase in rpm it cools down, then heats up. This would indicate that the circulation pumps are working as the new increase in fluid flow due to the rpm change removes the current heat (from the old power setting), then as the new combustion heat flow from the new power setting reaches steady state, you overtemp again. Which would point to a reduced flow rate of coolant at all rpm, not just low rpm.

On the infrared temp monitors, check the how big the sensor window is at a given distance from the gun to the spot to be measured. You don't want to measure something next to your "target". The better ones have this info molded into the top of the gun or check your manual. Also try and measure similar materials and part thickness as this is getting surface temp not fluid temp inside.

Hope this helps in some way."
 
"Rich one other question, when

"Rich one other question, when you replacedthe hose from the bell housing to the transom assembly, did you notice if the plastic elbow was crushed, if this is crushed because of an over heat you will reduce flow to th engine."
 
"Well, again thanks to all for

"Well, again thanks to all for all the great advice.
Yesterday I removed the t-stat housing and the circ. pump like Troy suggested and flushed the engine block the best I could. I put things back together and nothing changed. It took 8 minutes to get from cold temp. to 200 degrees.
As Joe mentioned, when I replaced the water hose in back of bell housing, the plastic stem looked good and was not damaged.The thermostat did open by feeling the hose , but It seems that there is still not enough water going through to remove engine heat.
The only other connection that I haven't yet removed was the one just inside the transom from the raw water hose. I'll check that later today.
Can you guys tell me (visually)how much water flow /or pressure I should be getting into the t-stat housing. The way it looks like to me is as if one took a cup of water and did a slow to medium pour. There sure isn't any pressure ther so to speak.
I will also check the temp. gun setting as Mark mentioned ..
Oh , and Troy, sorry about the gears still turning when you go to sleep, Mine have been doin that since May."
 
"Glad you haven't given up

"Glad you haven't given up on it, I was getting a little worried after not hearing anything for a few days, but figured you probably needed a break from it... I certainly would have.

The slow to medium pour going into the T-stat... is that how much is coming out the idle ports on the transom as well? What comes out should be the same as what's going in.
Anyway, that's not nearly enough... gotta figure, the first thing it has to do is fill the engine up, sounds like that is exactly what's not happening.
You should see more like a medium to heavy pour out of a pitcher, like a garden hose without the nozzle, (with average houshold pressure).

So, if that's what you're seeing going into the t-stat housing, that narrows down the search considerably, something between the impeller and the T-stat housing. Hand over hand every inch, shouldn't have too much more to look at.
If you haven't done so yet, take it apart and look inside. There's a booger in there somewhere.
Another thought is the discharge tube above the impeller housing in the outdrive. Did it line up correctly when the lower unit was put back on? And, are all the peices there? The pipe in the upper unit that slips into it is fairly thin-wall tubing, it could have been crushed or pinched if misaligned."
 
"Okay Troy, I'll drop the

"Okay Troy, I'll drop the lower unit again, check out everything from the impeller to the t-stat with a fine toothed comb, especially the discahrge tube and alignment. One more thing that I failed to mention that might mean something, there is quite alot of water flowing out of the prop hub when running"
 
"Rich,
The connection from

"Rich,
The connection from the inside transom assembly to the hose to the oilcooler is connected to the hose within the bell housing to the transom assembly by the neopreme elbow I was talking about. Here is a cite that will show the connection within the boat as wellas outside the boat. http://www.mercstuff.com/bellowsreplacement1.htm
One other way to check the flow, place a clear rubber hose from the T stat incomming side to the hose from the outgoing side of the oil cooler. If you see air (voids of space where there is no water flowing) thyen the problem lies within the drive, if not the problem lies within the engine area. This may help tell in which direction you need to look."
 
"You're going to have to q

"You're going to have to quantify 'quite alot'. If there's only a slow to medium pour from a cup going in, then 'quite alot' coming out doesn't seem to add up.

At idle you should see the majority of water and exhaust come out the idle ports on the transom. Those are there to reduce the backpressure on the exhaust system since the prop hub is farther below the water line (when the boat is in the water). The engine would have to work harder to push the exhaust that much farther... therefore the idle ports."
 
"Rich,

What condition is th

"Rich,

What condition is the O-ring in from the outdrive to the housing on the left side when you are reinstalling the drive? When I reattached my drive I had to use some Moly to keep it in place. If that's missing or damaged, it could be leaking into the exhaust chamber for the drive and that's why you might see more water coming from the prop hub, and why you're not getting enough water to the engine.

Mike"
 
"Mike, Installed a new one and

"Mike, Installed a new one and I also used some moly so it wouldn't fall off.o ring is good Thanks"
 
"Rich,
Putting your earlier p

"Rich,
Putting your earlier post and Mike's post together may expain why more water is going straight out the prop hub ('quite alot') and not going through the engine ('slow to medium pour').
Is the water coming out the hub warm, or the same temp as the water source?

This makes me really suspect the impeller housing discharge coupling, sleeve, and water tube in the outdrive."
 
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