Q: Why does temp go up when motor is shut off?

This is regarding an '86 Nautique with the PCM 351. Situation:

1. Engine is running at normal temp.

2. Shut off engine for a minute or two.

3. Turn on engine. Now the temperature on the gauge reads very high (up into the 200's) and the alarm is going off. This lasts for a short period of time until the indicated temperature drops back to normal.

Questions: What causes this? Is there anything you can do about it (the alarm is annoying).

because the waterpump isnt flowing cool water into the engine when its shut off so the water temp will go up a little. i wouldnt be to worried about the temp only going into the 200's. new cars run at something like 220, but i dont know what you can do about your buzzer.... sorry

It's also not producting any extra heat though. Where is the temp sender on a marine engine anyway? I'd assume it's not in the cooling passages, i.e. in the water, since that will probably be considerably cooler than the engine, since it's not recycled. Or...maybe it IS in the cooling passages so when you shut off the engine, the water is not cooling the sender and it absobs extra heat from the block?

Anyway, as long as it's normal, I was thinking it would be pretty cool to have some kind of timed alarm override so that the alarm would be silent for the first 10 or 15 seconds of operation. As long as you look at your gauges, that would be fine since the alarm would still alert you to any trouble that happened while you were running. The simpler option would be to install a push button to cut power to the alarm while you are pressing it. Then you'd have to hold that in after you started the boat though.

When the engine is running there is a constant flow of water through the engine and past the temperature sensor. The engine itself is a lot hotter than the water, which of course is a requirement to get a transfer of heat. How much hotter the engine block has to get in order to transfer the necessary heat out is a function of the "thermal resistance".

So, as you run along, the engine reaches a stable operating point, with heat flowing out of the engine block into the constant flow of water going by.

Then you shut the engine off, and the flow of water stops immediatly. The flow of heat out of the engine block continues, however, and the water gets hotter than usual.

If you leave it for a while the engine cools off and everything cools down. But for the first minute or two the water gets a lot hotter then when it was running.

Since the alarm going off is not "normal" it is an indication that something has changed. The changes could include:

1) The temperature sensor has become more sensitive, and is giving the alarm at a lower temperature then before.

2) The thermostat is stuck and is keeping the "normal" temperature a little bit higher then before.

3) The "thermal resistance" has increased, which would indicate that the water passages have been coated with rust, water deposits, dirt, etc. Do you operate in salt water? I have heard of salt deposits causing problems like this.

4) The water pump has gotten weak and the volume of water is less, making the "normal" operating temperature higher.

Since your guage agrees with the alarm sensor (they are usually separate sending units) I wouldn't suspect the sending unit. If you haven't changed your raw water pump impeller in a while I would start there. Next I would replace the thermostat.

If you suspect a buildup of deposits on the cooling passages then the best thing you could do is to rig up a closed loop system and run a radiator flush through it.

Rod

you have to leave your boat on iddle for a few minutes after use before you shut it off, so you'll get the engine block cooled and you wont get those temp readings.

Lunchog,
You shouldn't have to do that. Besides, the engine will stay close to normal op temp for close to a half hour or more on a warm day. A few minutes isn't going to cool the engine more than a few degrees as long as it's running.

Either way, I wouldn't think that is normal.

It isn't a huge problem, but as sad by Rod; something shows his age. I think if you change the impeller everything should be fine. If not, test you thermostat.

My 81 has always read higher after being off for a few minutes, but it comes down within a matter of seconds after starting. I simply attributed it to the fact that the closed engine compartment is not getting any cooling (water or air).

I had the exact same problem with my PS 190. In addition to the "heat soak" effect yo were having it was cooling properly. It ended up being a bad thermostat. It was a little sticky. I replaced the thermostat and no more heat soak. Mine would go from normal operating temp to like 200 ish in seconds. but once you start the motor it would drop right back down.

Suggest the following:

Inspect/clean strainer at the transmission cooler.

Check/replace raw water pump impeller. Even if the fin tips look good, cracking of the fins at the base, or a permanent "set" on the fins could be causing the engine to operate above normal temps leading to higher heat soak.

Check/replace t'stat.

Hope this helps out some...

To everyone suggesting that I check the impeller, strainer, thermostat, etc:

Wouldn't those issues show up while the engine is running as well? I'm unclear on how those issues would cause the indicated temperature to rise only after the motor is shut off. I understand why it is normal for the temperature to rise (the engine and cooling water never reaching equilibrium while running and the remaining water--and thus the temp sensor--absorbing extra heat once the water stops flowing).

Also, does anyone know at what temperature the alarm is supposed to sound? I couldn't find that info anywhere.

Thanks.

It is simple physics. Rod is totally right. The water in the lake removes extraordinary amounts of heat. It is an infinite heat sink. Boat engine temps are lower than car engine temps because ot the availability of an infinite heat sink instead of a closed loop cooling system.

When you stop your boat, the flow of water across the sensor stops, and the water temp will quickly rise 15 deg or higher from heat radiating out of the block. Combustion temps are 600deg+ so without a heat sink so the engine gets warm quick. . This is why cars spit coolant back into the reservoir, and why old cars used to spit it on the ground. You don't notice it in a car because unless you are a teenager you don't sit around in your car looking at the gauges when the engine is not running and the ignition is on. At least I hope you don't In a boat, that is normal. Don't worry so much!

Psyclone,

You just explained the part that I already understand, i.e. why the temperature rise is normal. I was prompted to ask the question because I didn't think the alarm going off for 10 seconds or whatever after restarting should be normal.

I didn't know what the temp sensors were measuring (water, block, etc.) but it became perfectly clear after several posts and a look through the engine manual.

What I still don't understand (and maybe I'm just being lazy in thinking about it here) is how a bad impeller, thermostat, or strainer would exaggerate this post-shutoff temperature rise phenomenon, i.e. why people are saying to check them. I would think that a problem with any of these would affect the temperature while running.

My thought is that it is either increased thermal resistance, a problem with the sensor or alarm, or some other phenomenon that I'm unaware of (something to cause localized heat build-up or something).

I just got that boat so I guess I'll just see if anything changes after I run a couple of times.

The only way I can think of that a marginal cooling system part would cause MORE of a rise would be if you had a weak impeller and that the boat was running hotter than usual to start with. Or a stuck thermostat.

I recommend that you test replacement thermostats on the stove in a pan of water, with a meat thermometer to verify that it will open all the way and at the proper temp. I have put bad ones in before I started doing this.