4.3 Merc Running too cold at speed

[sr54x4]

1995 Regal Valanti with the 4.3 Merc LX and Alpha Gen II outdrive

Replaced impeller this spring as general maintenance

Last weekend it overheated on me. Got to about 175 F and the alarm went off, shut it down and let it cool. Got towed to the marina and was able to idle it to the trailer before the alarm went off again.

Split the drive, new impeller was still perfect.

Opened T-Stat housing, and the pin in the t-stat came through the top of the t-stat, so there was my problem...stuck closed.

Replaced it with a new 160 F t-stat and plastic housing sleeve.

This weekend went out, and it warmed up to 150-160 at idle and at no wake speeds. As soon as you get up on plane it drops down to 125 F pretty quickly. Slow back down to idle or no wake speeds at it slowly climbs back up to 150-160 where I think it should be.


Any ideas what is causing this issue now?

 

[alldodge]

I take it since you have a 160 thermostat then your serial number is above 0L341000 and looks like the one on the right.

Did you buy a Merc or off brand thermostat?

 

[sr54x4]

I replaced it with a Sierra 18-3647 which cross refrerences to the Mercruiser Part # 807252Q5 and looks like the stainless one on the right. What I pulled out was brass and looked like the one on the left. I didnt realize there was a difference between the two. I will have to check my engine serial number when i get home later.

 

[sr54x4]

0F286155 is my engine serial number. Which doesn't match up to the serial number range in the above parts picture. I looked in my owners manual and while it doesn't specify a part # for the thermostat, it does say in the engine specs it uses a 143F thermostat. Can anyone point me in the right direction for the correct part #?

 

[alldodge]

0F286155 is my engine serial number. Which doesn't match up to the serial number range in the above parts picture. I looked in my owners manual and while it doesn't specify a part # for the thermostat, it does say in the engine specs it uses a 143F thermostat. Can anyone point me in the right direction for the correct part #?

This is the main reason we really want the serial number and not just the year. You have a 1994 engine and does use thermostat which is a 160 degree. This I verified according to the Mercury web site and mercruiserparts.com.
https://www.mercurymarine.com/en/us/parts-and-accessories/parts-catalog
http://www.mercruiserparts.com/Show...r=120&bdesc=STANDARD+COOLING+SYSTEM(DESIGN+I)

While most carb engines do use a 140 degree thermostat, all info leads back to a 160 degree.

Need to find out what the actual temp is, best way to do this is with a infrared temp gauge. The 175 plus temp I can understand, the issue is it goes down to 125 on plane and this should not happen unless the thermostat is defective, or the gauge/sender is the issue

 

[sr54x4]

Ok, so is there still a possibility that using the stainless one on the right instead of the brass one like what was in there originally and on the left in the above photo is causing the problem?

I will try and grab a heat gun from work and do some testing. What areas should I be checking with it?

 

[Fun Times]

Unless this engine is run/used on the Lake Constance/Bodensee then the correct thermostat looks to be the 140. In that year range, merc had 2 designs of T/housings.

One of the best ways for us to know for sure would be to post a photo of the thermostat housing or have a look at the two designs as that may help you know what type you really have,

STANDARD COOLING SYSTEM(DESIGN I) 140
STANDARD COOLING SYSTEM(DESIGN II) 160

Also if you have design 1 and you installed design 2 part, than your temp will fluctuate... Also the bottom gasket with the rivets needs to go on dry or the sensors may not work properly due to lack of ground.

PS, the horn typically won't sound at 175, normally it sounds somewhere between 190 - 200 degrees. Check the accuracy of the gauge against the IR temp gun.

 

[sr54x4]

The boat is used on the Hudson River. Water temps through the season are 60-85.

I have the first design in what you posted above. I think that is my issue. Looks like I used the parts for design II. The kit came with a bunch of different gaskets, one of which had a gasket with the rivets which I did not use. I dont think my design calls for that as what I pulled off did not use it?

I am going to re-order the correct parts and install, then verify temps with a IR temp gun. There is also the possibility the alarm went off a little higher than 175, I heard it go off, looked at my gauges really quickly, kind of panicked and shut it all down and immediately opened the engine hatch to see what was gong on.

Is there anything I should be inspecting for damage that could have been caused by this overheat or possible issues from running the wrong thermostat for a couple of hours?

 

[Fun Times]

I have the first design in what you posted above. I think that is my issue. Looks like I used the parts for design II. The kit came with a bunch of different gaskets, one of which had a gasket with the rivets which I did not use. I don't think my design calls for that as what I pulled off did not use it?

Item number 3 (Merc # 27-530451) is used on both designs.

If the aftermarket kit was ever used then the gasket with the brass rivets punched through it wouldn't have been there as the aftermarket gaskets tend to come without the rivets.

Is there anything I should be inspecting for damage that could have been caused by this overheat or possible issues from running the wrong thermostat for a couple of hours?

Anything rubber is usually the first to start being damaged. Since it was a wet overheat vs dry and you shut the engine down as soon as you heard the horn then you may have escaped any serious damage. The first to go is typically the exhaust shutters located at the top of the exhaust Y pipe. Item number 6, http://www.mercruiserparts.com/Show...8.png&inbr=1599&bnbr=200&bdesc=EXHAUST+SYSTEM

 

[sr54x4]

ok, i re-ordered a new 140 t-stat that matches the part # for design one. I will also re-install with the gasket with rivets to be sure.

tried to pull the rubber tubes from either end of the exhaust elbow so I could see down in the Y pipe and inspect shutters, but couldnt get either one to slip off fully. I will keep trying over the weekend. any tricks to this? i loosed the hose clamps on the riser, both ends of the elbow and the top of the Y pipe and could almost get it to slip off, but still had a little ways to go. then it got dark and i got frustrated so I gave up for the night before something broke. any tricks to getting them off?

 

[alldodge]

Need to loosen the clamps on all the hoses and on the elbow Item 3, they are a pain. Use a hose pic works best to brake them free
http://www.mercruiserparts.com/Show...8.png&inbr=1599&bnbr=200&bdesc=EXHAUST+SYSTEM

 

[sr54x4]

i loosed up all those clamps fully, even took some off. ill try the hose pick to get them free the rest of the way. i got them about 3/4 of the way off, but then just dont have any more wiggle room to get it off the rest of the way.

 

[alldodge]

Really the easiest way is to remove the riser from the manifold. You then will need another gasket.

 

[sr54x4]

dropped the 140 t-stat in this weekend and the temp stayed at a steady 150-155 all day, right where it should be! thanks again for the help!