How can I tell if my Mercruiser 3.0lx is running hot?

[MikeDobbs]

Hey everyone-

Trying to get my merc 3.0lx ready for the water- boat is new to me and I purchased without motor running, so I have no history of how things used to work.

Started a couple times today and both times the temp gauge just kept slowly climbing while engine ran. I shut it off around 170 each time because I didn't want it to get too hot. I just put a new thermostat in (140 degrees), and the guy I purchased the boat from said it would always run hot- he claimed it would run at 240 on the thermostat all day with no problem. I have no idea if this is true, but if it is, the temp gauge is obviously bad.

How can I tell if the engine is truly running hot or just suffering from a faulty gauge or temp sending unit? If I place my hand on the riser it doesn't feel hot- does that mean the engine is running at an OK temp? Or is that not a reliable indicator?

What else could I do to determine if I have a serious problem or a faulty gauge, etc?

 

[Fun Times]

Sounds like the temp gauge or temp sending unit is defective if it's reading that hot but you can hold your hand on the engine and the warning horn isn't sounding while keeping in mind that some 3.0LX engine models had the warning horn as standard and other 3.0's didn't offer it as standard.

One of the best tools to have is an infrared thermometer temperature gun. http://www.harborfreight.com/catalogsearch/result?q=infrared+thermometer

 

[enginesilo]

I know that some use a 140 thermostat and others a 170(Or right around that). I had a faulty gauge but thought I was running hot because of it. I tested with an IR thermometer. Shoot the laser all over the engine, particularly around the lower block area, exhaust manifold area, riser, thermostat, etc. You shouldn't see more than 175 degrees give or take, maybe less if your thermostat is a lower temp one. Amazon sells an IR for under $15 that will give you an accurate enough reading to either find you are running hot, or put your mind at ease.

Another way to test is to go buy a cheap gauge and hook it up and see what it reads. I ended up finding an exact matching gauge but with a different color bezel for around $10-$15, but they usually aren't more than $30.

Is it sucking and spitting out water from the exhaust ports when run on the hose? If you don't see water exiting you could probably bet it is running hot but you would probably know it.

If it was running at 240 that engine would have been burned up a long time ago. The riser would also be uncomfortably hot to the touch. While you are at it double check if you should be running the 140 degree thermostat or the higher one since I see the 3.0 running both and am unsure why they have one over the other.

 

[UncleWillie]

...If it was running at 240 that engine would have been burned up a long time ago...

+1.
Your block is not pressurized like an automotive block.
At 240 you would be blowing steam like a locomotive.
There is no way to ever get past 212 with any water in the system.

 

[MikeDobbs]

OK- so I just had an idea. On the housing where the thermostat sender screws in (right below the thermostat) there is a blank space- not sure what for. Could I just purchase another gauge (maybe a mechanical one to eliminate one more possible source of error) and run the engine while monitoring that gauge? Then I'll have a mechanical gauge connected directly into the cooling system. Thoughts?

Also, here's some more info from when I was able to run the engine today. I shut it down because the temp gauge rose to about 170, but right afterwards I went around the engine feeling hoses and the exhaust riser. I was able to comfortably hold all hoses (even the large one on the starboard side of the engine that carries water up to the thermostat housing. HOWEVER- while that hose was comfortable warm, the smaller hose coming out of the top of the thermostat housing (the one that only gets flow when the thermostat opens) was COOL to the touch. Which means (if I understand the cooling system on this engine) that the thermostat had not opened. Now, it's brand new (installed yesterday) so it should be operational (but I know that's not a guarantee). So, either the them is defective, or the engine never got hot enough to open it.

Also, can I use the water coming out of the back of the boat as a gauge of engine temp? If that water is warm, but not hot, does that mean the engine should be at an OK temperature?

Thanks!

 

[stonyloam]

If the temperature keeps going up, chances are you ARE overheating. If the impeller in the outdrive has not been changed, do it before you do anything else. The thermostat rating, either 140 or 170 is of no importance if the engine is not getting enough cooling water.

 

[MikeDobbs]

Since the boat is new to me I replaced the impeller before ever trying to start the engine. Just had an interesting experience with the boat. Started the engine on the muffs- and ran for about 20 minutes with the temp reading right around 140 (that's the thermostat temperature). I could place my hand on the hoses and exhaust riser easily- and the water exiting the boat was actually cool (which I thought was a little weird). Then, all of a sudden after about 20 minutes the temp gauge shot up. I looked over and saw it around 175 on the gauge, so I shut the engine down. Could still easily place my hands on all hoses and riser. Interestingly, the small hose where the water flows when the thermostats opens was still cool. I'll try to attach a pic of what I mean.

 

[MikeDobbs]

OK- final update for today. I think everything is fine regarding temp, but I have a wiring issue to the gauge. Here's what I did.

Started the engine up and ran it under a watchful eye for almost 30 minutes. Temperature hovered high around 140 on the gauge the entire time. Used a non-contact thermometer to confirm, and no matter where I shot on the engine, I couldn't get a reading above about 142 degrees.

Here's where it gets interesting. If you've been following my posts, you know I had a problem with the purple wire that brings power from the ignition key switch to the engine for the coil, shift interruptor, alternator, etc. I had recently ran a new purple wire from the front of the boat, but had not yet hooked it up. For the almost 30 minutes that the temp was fine, I was running the engine with a jump wire directly from the battery, to the purple wire on the boat side of the harness. However, when switched that to the new purple wire I ran from the front of the boat- the engine kept running fine, but the temp gauge almost immediately started to shoot up. I repeated this on two separate occasions, spaced about 2 hours apart, so I'm fairly convinced it's not a coincidence. Now all I need to do is figure out what that wire has to do with the temp gauge, and why hooking it up seems to interfere with the operation of the gauge.