1990 5.7. 260 Overcooling at idle.

[Mkos1989]

I've had this issue for about 10 years now and although it's been fine and havent given me any issues I figure in should figure out whats wrong as I'm using the boat more and more this year. I have a 143 stat in there, gauge works fine. But after ideling around the river it'll drop to 100 and then lower. Give some RPM. To about 1000 and it works it's way up to 145. Anything above 1000 to WOT for minutes on end, zero issues. Bring back to idle and itll be there for 10 minutes then drop off to 100 again. Never ever gets off 150. And 150 is after a good hard run. The cooling system is only the 4 port model. 2 hoses to the manifold elbows, 1 to motor and 1 incoming. Also would it be wise to put a 160 stat in? Seems unless I run it hard for a long time I usually have a ton of condensation.

 

[Scott06]

Have you tried replacing the 140 thermostat in there? Seems like it is not fully closing. in fresh that isn’t too heavy in minerals you can runa 160 but i think yours is not closing or has debris stuck in ti

 

[Mkos1989]

I was thinking that as well. I put a new mercruiser 143* in there back in 2007 when I got the boat. I'll check. yea no saltwater here. Usually inland lake or Lake Erie and most times that waters 40* -60* degrees. I'm going to pull it off and change it. See what's up. We were out today and honestly I dont think it even gets to 140-150 now. One is at idle. One pic is underway.

 

[Mkos1989]

On another note, the engine came with the 2nd design 6 port thermostat housing but was cracked and rusting. I now have the 3rd design housing if that makes any difference .

 

[Scott06]

On another note, the engine came with the 2nd design 6 port thermostat housing but was cracked and rusting. I now have the 3rd design housing if that makes any difference .

Dont know your housing mine is a 6 port that can cause issues if the restrictors that feed raw water to the elbows hang up, but that causes overheating not over cooling so i think youre on the right path with the thermostat

 

[crazy charlie]

you may be lacking a restrictor between the riser and manifold.Some gaskets allow for 2 areas of flow to the riser and some have all 4 open.I will bet you have all 4 open.Charlie

 

[Mkos1989]

you may be lacking a restrictor between the riser and manifold.Some gaskets allow for 2 areas of flow to the riser and some have all 4 open.I will bet you have all 4 open.Charlie

I thought about this a few years ago when I had one leak into the motor over winter. Pulled my plugs to check before starting up and antifreeze was on all the plugs on the port side and in the oil. Pulled riser and there was a puddle in there. I cant remember what I used. Should prob pull them off for good measure and put new gaskets.

 

[Scott06]

I thought about this a few years ago when I had one leak into the motor over winter. Pulled my plugs to check before starting up and antifreeze was on all the plugs on the port side and in the oil. Pulled riser and there was a puddle in there. I cant remember what I used. Should prob pull them off for good measure and put new gaskets.

I believe on a cold manifold set up you needs 4 open holes not the restrictor @achris will probably know

 

[poconojoe]

Is that air filter marine approved?
It's supposed to be a spark arrestor, not an automotive air cleaner.
Correct me (and I apologize) if I'm wrong.

 

[Paintman1960]

Is that air filter marine approved?
It's supposed to be a spark arrestor, not an automotive air cleaner.
Correct me (and I apologize) if I'm wrong.

I wondered same thing, Also don't see the Tell Tale hose from FP

 

[alldodge]

Suggest don't look for marine approved or local auto supply house

Cooling issue is either the thermostat or how the thermostat was installed. Were all the parts installed and installed correctly

 

[Mkos1989]

Yup, it's the K&N FA. USCG number on the steel lid and the FP is a new MerCruiser with line to the 1409 Edelbrock. Just had to get a new FP as the other one had fuel in the clean line.

When ingrt home from my trip ,I'm going to pull it apart and see what's up.

 

[Mkos1989]

 

[alldodge]

The K&N looks good, but is more restrictive then the standard flame arrestor. Not that the motor will pull in enough air to make a difference

 

[Scott06]

Yup, it's the K&N FA. USCG number on the steel lid and the FP is a new MerCruiser with line to the 1409 Edelbrock. Just had to get a new FP as the other one had fuel in the clean line.

When ingrt home from my trip ,I'm going to pull it apart and see what's up.

Make sure you get the spacer for the thermostat. Ive never been able to get one out without destroying it

 

[poconojoe]

Yup, it's the K&N FA. USCG number on the steel lid and the FP is a new MerCruiser with line to the 1409 Edelbrock. Just had to get a new FP as the other one had fuel in the clean line.

When ingrt home from my trip ,I'm going to pull it apart and see what's up.

That's great!
Looks real nice!
I would have bet it wasn't approved!
Sorry for questioning it. It's just that I've seen what a boat in flames looks like.

 

[Lou C]

All it takes is a small flake of rust to hold them open, it happens all the time with raw water cooling/cast iron engines.

 

[Mkos1989]

Bringing this back up. Replaced thermostat with OEM Merc along with plastic spacer and I have identical results. Again, over 1000rpms. PERFECT cooling. Trolling or idling around the river itll drop to below 100 like its over cooling. I have the non restrictive manifold gaskets with 2 hose cooling like the merc bulletin says but I am half tempted to try the restricted. Thoughts?

 

[jimmbo]

The Lid and Base Stampings are USCG approved, but the Media... That oily media should burn well when the Flames hit it

 

[Scott06]

Bringing this back up. Replaced thermostat with OEM Merc along with plastic spacer and I have identical results. Again, over 1000rpms. PERFECT cooling. Trolling or idling around the river itll drop to below 100 like its over cooling. I have the non restrictive manifold gaskets with 2 hose cooling like the merc bulletin says but I am half tempted to try the restricted. Thoughts?

Couple of thoughts-
is engine actually getting that cold? Have you measured the water cross over in intake at idle to see if it matches indicated temp?
if indicted temp is not accurate is the temp sending unit in correct spot in thermostat housing? Is it somehow just seeing the raw water comming into it from the drive at idle where increased water flow at speed creates more mixing with outlet off thermostat?
also if indicated temp is lower than actual, wondering if low idle speed causes voltage to drop maybe screwing up gauge? Had a situation on my 3.0 where choke wouldn’t open, found idle speed was just low enough that alternator was not really putting out voltage, bumped idle speed up and seemed to work again… wouldn’t have believed it until i saw it