Mercruiser 4.3 overheats at idle only

[trendsetter240]

Mercruiser 4.3 carb, alpha one gen 2 (1995 model year I believe). Engine was originally raw water cooled but has an aftermarket fresh water cooling system in place.

Hi guys, I'm helping out a neighbor with an overheat issue on his mercruiser. The symptoms are that the engine will run at a normal temp when cruising, pulling tubes, full throttle etc. It will overheat when brought back to idle. This doesn't happen on water muffs only when the boat is in the water.

I have pulled the lower unit and inspected the impeller which appears to be fine. I will replace it regardless. I've also pulled the thermostat housing and it appears normal (no rust or blockages). The thermostat was replaced last year and did not correct the issue. This is not one of the thermostat housings that have the check balls.

Any ideas on what else I should check while I have the boat? I'm kind of at a loss and if no suggestions plan to replace the impeller and take the boat for a water test to see if I can reproduce the issue.

Cheers

 

[alldodge]

Re: Mercruiser 4.3 overheats at idle only

Maybe the engine water pump? Could be the vanes are coming loose and with higher speeds it manages to move enough water, but it idle its not.

 

[trendsetter240]

Re: Mercruiser 4.3 overheats at idle only

Maybe the engine water pump? Could be the vanes are coming loose and with higher speeds it manages to move enough water, but it idle its not.

Thanks for the reply! Yeah, I'm not sure but I will check it out. The recirculation pump was supposed to have been replaced last summer but maybe it was not.

I'm going to put the new impeller in today and go from there.

 

[trendsetter240]

Re: Mercruiser 4.3 overheats at idle only

Ok, just back from a water test and unfortunately the problem is still the same! New impeller, thermostat and gear oil was done yesterday.

The engine runs great at anything over 1400 RPM. Stays nice and cool (150-160f) everywhere from 1400 to 4800 RPM. Pull back to idle and neutral and the temp climbs to 200f and sounds the alarm. Takes about 30 seconds to overheat at idle. Rev the engine back above 1400 RPM and the temp drops back down to 150.

Anybody have any suggestions? The aftermarket heat exchange is made by san juan engineering and manufacturing. Problem started happening about a month after install of this FWC unit.

 

[alldodge]

Re: Mercruiser 4.3 overheats at idle only

Was this boat previously used and/or currently used in salt water prior to the closed cooling install? If so please explain

 

[trendsetter240]

Re: Mercruiser 4.3 overheats at idle only

Was this boat previously used and/or currently used in salt water prior to the closed cooling install? If so please explain

This boat was fresh water use only prior to the install of the FWC cooling system. It is now salt water used and is in the water for 2 months per year. This cooling system was installed in July 2012, issue first occurred in August 2012.

 

[alldodge]

Re: Mercruiser 4.3 overheats at idle only

There is a Merc news release about only doing the change over to closed cooling in the first 100 or 120 hours in it is in salt but not sure about fresh. DonS has the release and hope he can bring in some expertise. This has to be amount of cooling water passing the heat exchanger as it relates to amount of heat disapated. So since this started a month after the insall I'm thinking now that some crub (what ever crud it is) broke loose and is clogging some of the passages in the heat exchanger, or the exchanger is not sized correctly for your engine. You went from raw water to heat exchanger and that is about all there is with this one. Can you contact the manufacture and ask them what they think. Starting to sound like the system was designed right on the edge of the tollerance? Only thing that's strange is your in BC and the water there should be fairly cool.

 

[trendsetter240]

Re: Mercruiser 4.3 overheats at idle only

Thanks for your advice. Yeah, the water here is cool. River or ocean the result is the same.

I could try contacting the manufacturer tomorrow to see what they say. I could also try flushing out the heat exchanger with a hose to see if any crud comes out. I'm thinking the same thing that it must be a restriction somewhere but I haven't found it yet.

 

[trendsetter240]

Re: Mercruiser 4.3 overheats at idle only

Update: Today I tried filling a large plastic tote with water and lowered the outdrive into it. I then ran the motor at idle until temp came up to normal (150). I tried revving the engine up to 2000 RPM for 5 minutes. The temp increased to 160. I then pulled it back to idle.

The temp went back down to 150 and didn't overheat. So this is the same as on muffs but the issue still occurs when in the water. So what's the difference? Back pressure on the exhaust?

Does this give anyone any hints as to what might be happening here?

 

[alldodge]

Re: Mercruiser 4.3 overheats at idle only

I wouldn't expect a difference between the muffs and a tote because the engine has no load on it. Reving the engine wouldn't do the same load. Going to use a WAG from here; Are you sure you got all of the air out of the closed cooling system when refilled? Maybe there is an air pocket in it, but an air pocket would seam to cause the same over heat but just trying a guess.

 

[trendsetter240]

Re: Mercruiser 4.3 overheats at idle only

I wouldn't expect a difference between the muffs and a tote because the engine has no load on it. Reving the engine wouldn't do the same load. Going to use a WAG from here; Are you sure you got all of the air out of the closed cooling system when refilled? Maybe there is an air pocket in it, but an air pocket would seam to cause the same over heat but just trying a guess.

Hey, yeah the reason I was thinking the overheat might happen with the leg submerged is because the engine will overheat at idle in neutral when it's in the lake. No load and it still overheats on the water but I cant create that in the driveway.

The FWC system has been bled for air and is moving water smoothly. Still cools great at cruise and above just not idle.

Frustrating!

 

[RCAnderson]

Re: Mercruiser 4.3 overheats at idle only

Is this a full or half system ?

It sounds like you are not flowing enough water at idle, on either the raw water side or the coolant side. Given that you have replaced the raw water impeller, that could almost eliminate the raw water side, unless you have an obstruction between the raw water pump in the leg and the risers where the raw water exits. To completley eliminate any chance of problems on that side, next time you are at the lake, pull off the hose that feeds raw water into the heat exchanger and start the engine for a few seconds. You should see a good stream of water come out of the hose. Continue to check raw water output at the hose connections downstream of the heat exchanger to make sure you have no blockages anywhere. It's not likely, but worth checking anyway.

Next is the engine coolant side of things. Not much to check here, but it could be your engine circulation pump. Get an infared thermometer and point it at the thermostat housing to confirm temp against the gauge.You could try running with no thermostat and see if that duplicates the problem. You could temporarily run a clear section of tubing between the t-stat housing and the heat exchanger to check for flow. Might be airlocked. Is the cap on the heat exchanger spewing coolant out the relief when you overheat ?

Just the fact that you are overheating only at idle, with a load on or not, but only in the water not on muffs, makes me think it's on the raw water side of things, not flowing enough at idle.

 

[trendsetter240]

Re: Mercruiser 4.3 overheats at idle only

Hey RC thanks for the reply.

It's a half system I guess since it's raw water through the exhaust and coolant through the block only. Hopefully I can get it back to the water soon and try the flow test at idle you describe.

In this system the thermostat is below the temp sender so it doesn't read much temp until the thermo opens at 160 then it reads correctly. When it's overheated the cap is definitely spewing coolant back to the reservoir and there are some air bubbles coming up through the reservoir.

The weird thing is with the leg in a tote of water it still won't overheat. It's only when the exhaust is submerged as well. I'm guessing that means the pressure is too low for proper cooling with the back pressure from the exhaust. On muffs or in a tote the exhaust spits out warm water quite well.

 

[alldodge]

Re: Mercruiser 4.3 overheats at idle only

The issue is diffently puzzling. Been looking over the net and have not come up with anyone having the same issue. I'm thinking you may be on to something with the half system. I could not find a San Juan half system. They do talk about if your using higher risers but nothing else. Have you tried calling San Juan Engineering Products, Ph: (360) 734-1910, FAX (360) 734-9683. The issue you have could very well be as you discribed about pressure differences being in and out of water. My additional question would be why was it fine for a month.

 

[RCAnderson]

Re: Mercruiser 4.3 overheats at idle only

The half system just doesn't use engine coolant to cool the manifolds, usually because the heat exchanger does not have the cooling capacity to do so. In my view, that is missing out on half the value of fresh water cooling, especially for salt water use.
Just thinking out loud here, but if the backpressure caused by having the leg submerged is not letting the raw water flow, then you will overheat, just as if you had no raw water pump. I can't see how this would be possible, unless you have almost completley clogged up exhaust manifold water jackets or risers. It can't be the y-pipe or leg, because then the exhaust itself would be restricted.

So I think either your exhaust manifolds and\or risers are just about clogged, or your engine circulation pump is not doing it's job at idle speeds. Has anyone changed the pulley diameter on the water pump ?

Just out of curiosity, what is the diameter and length of the heat exchanger ? Why don't you go to a full system ? It's just a matter of running a few hoses, and changing riser gaskets.

 

[RCAnderson]

Re: Mercruiser 4.3 overheats at idle only

Also wondering why it worked for a month. Was it just not run for the first month ? Can you post a picture that shows how the system is plumbed ?

 

[trendsetter240]

Re: Mercruiser 4.3 overheats at idle only

Thanks guys, I just got off the phone with the manufacturer and they were super helpful tho they hadn't seen this particular issue before.

They recommend that I start by pulling the risers and checking the cooling passages for blockage. Also to check the belt tightness and take some readings with a temp gun. I'll try this late today hopefully.

The boat is owned by my neighbor so I'll have to ask him again why it was working for the first month.

I will check the diameter of the pump pulley, I don't think it has been changed tho. I agree with you that a half system isn't the best choice for salt water as well.

I'll post some pics and an update later today.

Cheers

 

[Volphin]

Re: Mercruiser 4.3 overheats at idle only

What type of coolant is this system using? You need 50/50 Ethylene Glycol... the green coolant. Dex-cool has been known to cause issues. Has the circulation pump been changed? If so, make sure it is the correct rotation. Consider a higher flow circ. pump... Worst case scenario is a cracked head.

 

[trendsetter240]

Re: Mercruiser 4.3 overheats at idle only

What type of coolant is this system using? You need 50/50 Ethylene Glycol... the green coolant. Dex-cool has been known to cause issues. Has the circulation pump been changed? If so, make sure it is the correct rotation. Consider a higher flow circ. pump... Worst case scenario is a cracked head.

I didn't add the coolant but it is the green colored stuff. The circulation pump has not been changed but was "checked and resealed" by a merc service center last summer in an attempt to solve this issue.

I've just pulled both risers, they had some rust but weren't blocked. I used a pressure washer and pushed 5000psi of water through all the passages and nothing that would plug them came out. So all passages are now clear.

While I have the risers off is there anything else I should check? Where are those exhaust flappers located, on the other side of the elbow?

 

[RCAnderson]

Re: Mercruiser 4.3 overheats at idle only

Make sure the correct gaskets are between the riser and the manifold. For a half system you need the coolant passages in the manifold open to the coolant passage in the riser. How do the manifolds themselves look ?

The flappers are in the y pipe. I'm not sure if you can get to them or not with that motor.