Mercruiser 4.3 overheats at idle only

[RCAnderson]

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.

Not doubting you, but how would a cracked head result in no cooling at idle, but only when in the water ?

 

[trendsetter240]

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.

The gaskets that were there were closing off half of the passages. Are these the correct ones?

The manifolds look ok, some rust but the passages appear to be open.

 

[RCAnderson]

Re: Mercruiser 4.3 overheats at idle only

I am pretty sure that you need the flow thru style gasket for a half system. Hopefully someone can confirm this. If I am correct, (and I may be wrong) then your raw water system is restricted, and may be able to flow enough water at above idle, but not while idleing. Either way, get the OEM gaskets, they are better than the Sierra ones on there now.

 

[trendsetter240]

Re: Mercruiser 4.3 overheats at idle only

Thanks I'm going to try and confirm with the manufacturer. That would make sense.

 

[trendsetter240]

Re: Mercruiser 4.3 overheats at idle only

You are right, just found the service bulletin.

http://www.boatfix.com/merc/Bullet/99/99_10.pdf

"Closed Cooled engines with the raw water from the heat exchanger going to the
90-degree fitting on bottom of the exhaust manifold:
Use P/N 27-863726 Gasket – 4 Slot between the exhaust manifold and the exhaust
elbow.
If the engine has 3 in. (76 mm) or 6 in. (152 mm) risers, use P/N 27-863726 Gasket –
4 Slot between the riser and the exhaust manifold and exhaust elbow"

 

[alldodge]

Re: Mercruiser 4.3 overheats at idle only

Was the engine circulating pump ever checked?

 

[trendsetter240]

Re: Mercruiser 4.3 overheats at idle only

Was the engine circulating pump ever checked?

I haven't yet, the pump was "checked and re-sealed" by a merc shop before the boat came to me.

 

[alldodge]

Re: Mercruiser 4.3 overheats at idle only

Since were spitballing here so to speak, Are any of the hoses used when installing the syatem larger than 5/8 ID. There are warnings in the manual about using hoese larger then 5/8 on some connections

 

[RCAnderson]

Re: Mercruiser 4.3 overheats at idle only

I would try the new gaskets first, if they solve the problem that's great. At least you won't have to drain the coolant out of the engine. Or, since you are in there anyway, change to a full system.

 

[trendsetter240]

Re: Mercruiser 4.3 overheats at idle only

Well, just back from a water test and the problem is still there. So removing the risers, cleaning them and installing new gaskets had no effect. I also check the belts and they are tight.

The hoses all appear to be the correct sizes but I will confirm that tomorrow.

Anybody got any ideas? I'll be following up with the manufacturer tomorrow.

Cheers

 

[RCAnderson]

Re: Mercruiser 4.3 overheats at idle only

My next step would be to check the flows. Start on the raw water side pulling hoses off for a second or 2 working your way from the heat exchanger raw water intake, all the way out to the risers. On the engine side, clear hoses from the t-stat housing to the heat exchanger.

Have you looked inside the heat exchanger ? The end caps come off pretty easy, you may want to replace with new, depending on their condition. If there is a blockage in the heat exchanger, i.e. barnacles, then it may not be flowing enough water to cool the engine at idle. You would see this if you pull the hose off the raw water outlet side of the exchanger with the engine running at idle.

 

[trendsetter240]

Re: Mercruiser 4.3 overheats at idle only

Started by pulling the raw water hose off the power steering cooler today. Measured flow was correct at 3.3 quarts in 15 seconds at 1000 RPM but that was on muffs so didn't tell me much.

I tried to backflush though the power steering cooler and there seems to be some restriction. Only a tickle of water would come out the leg and most would just spray back out the hose even tho I had a tight grip on it. Is water supposed to flow freely back through the outdrive?

 

[alldodge]

Re: Mercruiser 4.3 overheats at idle only

I tried to backflush though the power steering cooler and there seems to be some restriction. Only a tickle of water would come out the leg and most would just spray back out the hose even tho I had a tight grip on it. Is water supposed to flow freely back through the outdrive?

There should be no restriction in the PS cooler, it should flow freely as the heat exchanger. The PS cooler is were the pieces of the impeller goes when it comes apart.

 

[RCAnderson]

Re: Mercruiser 4.3 overheats at idle only

And if they make it past the PS cooler, they end up in the heat exchanger tubes.

 

[trendsetter240]

Re: Mercruiser 4.3 overheats at idle only

Well I finally fixed the issue. I pulled off all the raw water hoses and checked them, flushed water back and forth through all the hoses and the heat exchanger. I drained and refilled the engine coolant and bled the system again. Took it to the water today for a test run and it ran perfectly. Temp never budged from 160 the whole time from idle to wot and back again.

Thanks to all for the suggestions offered. Much appreciated.

Cheers