Temp question

[ericcarr2000]

OK quick question here, I have a 1990 7.4 merc motor. While taking her out this past weekend I had noticed that the temp gauge was fine while cruising around 1800 rpms, then I went to wot. I noticed that the gauge slowly started to rise while wot. After a few minutes of running wot, I backed off the throttle and noticed tha gauge start to rise close to the 200 range. I gues my question is if this is normal behavior for a motor?

 

[DeepBlue2010]

Using the serial number for you engine will be able to lookup the thermostat which will tell you what is the normal operating temp for your engine. Mine runs steady at 160-170 (7.4 as well). At WOT, you might not have enough seawater flow to cool down the engine. When was the last time you changed the impeller?

 

[ericcarr2000]

I changed it this year before I put her in the slip. In pretty sure that's what mine is as well 160-170. Does yours get up to around 200 when you back off of wot?

 

[DeepBlue2010]

No. As the RPM increases so does the water flow of the seawater pump, it is belt driven. The engine should run at a steady temp. Does your alarm go off?

 

[ericcarr2000]

No not at all.

 

[DeepBlue2010]

Did you overheat before? was the impeller intact when you changed it last time. Was the impeller installed correctly and lubricated before first run?

You either not getting enough water to cool the engine or your system is restricted somewhere. You are not at the overheat zone yet but better find out what is going on before you start overheating. Try to put your hands on your risers while the engine is running, they should be cool to touch and keep your hands on. If you can keep your hands on them, something is not right.

 

[ericcarr2000]

Well as far as I can remember the impeller I took out really didn't look worn at all, I just changed it anyway. I'm thinking that since its slipped that my intake holes on the side of the drive might have some of that damb mascke buildup on it. Next time I go up I'll start her up and let her run to temp and feel the risers to see how they feel.

 

[DeepBlue2010]

My alarm went off last time I was on my boat. Tried everything possible but my gut feel was telling me it is a restriction in the inlet on the drive itself (hint, I was fishing and there was a lot of seaweed on my flashers) Called tow company, took forever. Finally gave up and jumped in the nice freezing cold water of Seattle and cleaned the inlet, was full of seaweed and I was able to continue my day. You never know, take a look over there and see what you find

 

[stonyloam]

If you see a temperature rise with increased RPM as deep blue says you have a cooling water flow problem. If you are approaching 200 you are flirting with disaster! Additionally you are not getting enough cooling water dumping into your exhaust and could seriously damage your rubber exhaust shutters and bellows. I would recheck the impeller, test the thermostat, and look for restrictions in the water flow.

 

[ericcarr2000]

Ok, I've never changed one while it was in the water. Can it be changed while the boat is in the water? You'd think that when you take off the hoses that water would flow into the bilge.

 

[DeepBlue2010]

Put the water in an up position (vertical) and you should be fine. If you are worried, get a PVC plug from Lowes or HomeDepot and plug the hose.

 

[tpenfield]

I had a similar problem a few years ago. So, here are a few comments in addition to the input already provided.

at WOT you may see about a 5 degree temp rise versus what you see at regular cruising speeds, but it should reach a steady state. When you back off the throttle and slow down you will see a momentary 'spike' in the temp by about 15-20 degrees as the water flow is reduced, but there is still quite a bit of heat needed to come out of the engine. This should last for only about 30 seconds or so.

If you are seeing a constant temp rise at WOT and a big spike after slowing down that lasts for a while (few minutes), then it is a pretty good indication that you have restricted flow in the cooling system. With that it is a matter of hunting it down, starting from the outdrive and working your way through the cooling system all the way to the exhaust elbows.

In my case I found a barnacle had lodged in the water inlet hose at the transom assembly. sort of the last place I looked as I had to remove the outdrive to find it . . . and there it was.

 

[ericcarr2000]

That's exactly what happened, it spiked about 20 degrees after o backed off of wot. I did notice it only took about 15-20 seconds to get back to where it was before.

I had a similar problem a few years ago. So, here are a few comments in addition to the input already provided.

at WOT you may see about a 5 degree temp rise versus what you see at regular cruising speeds, but it should reach a steady state. When you back off the throttle and slow down you will see a momentary 'spike' in the temp by about 15-20 degrees as the water flow is reduced, but there is still quite a bit of heat needed to come out of the engine. This should last for only about 30 seconds or so.

If you are seeing a constant temp rise at WOT and a big spike after slowing down that lasts for a while (few minutes), then it is a pretty good indication that you have restricted flow in the cooling system. With that it is a matter of hunting it down, starting from the outdrive and working your way through the cooling system all the way to the exhaust elbows.

In my case I found a barnacle had lodged in the water inlet hose at the transom assembly. sort of the last place I looked as I had to remove the outdrive to find it . . . and there it was.

 

[tpenfield]

15 second recovery seems fine . . . The questions would be if it continually rises at high speeds/WOT and the 20 degree spike seems a bit high. If it is different than before, it would be something to check out.

 

[ericcarr2000]

Next time I take her out I'll let you know what I find out.