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BrianS
09-24-2004, 10:54 AM
I had something wierd happen last night on the boat and I thought I would run it by you guys.

I have a '93 Prostar 190 with the 351 Ford. This is the first season I've owned it and I have put about 70 hours on it. The temp guage has always read fine, around 150-160 degrees. Typically when I shut it off the guage will creep up around 170, never above 180. All of this seems normal to me.

Last night after pulling skiers for a few hours, the last guy was getting in the water and the motor was off. When I sat down to start it the temp guage was pegged at 220+ degrees!! Obviously, I was concerned and lifted the motor cover but everything seemed fine, it did not feel excessively hot. I started the motor and the guage immediately dropped back to about 180 and then fell back to normal about 160. So I shut it down again, and the guage went right back to 220+. I cycled power a few times and the same thing happened. What could possibly cause this to happen? I'm thinking either the guage is bad or it may be related to my alternator problem (http://www.tmcowners.com/teamtalk/showthread.php?t=789)??

Does anyone have any thoughts?

G-man
09-24-2004, 11:00 AM
I would check out the impeller first and replace it. Pull it out of the housing and coat it in silicone grease in a plastic bag when you winterize. Most people on the board change their impeller every spring. It's cheap insurance. I would also change the thermostat. It should have a running temp of 143 degrees.

BrianS
09-24-2004, 11:15 AM
I need to pull the impeller and check it again. The previous owner changed it every other year, the current one is on it's second season, I already have a replacement. I was just hoping to make it through this season.

Does the Ford 351 have only 1 thermostat? I've read about some motors having 2 but I think that is the LT-1. Skidim has a 143 and a 160 degree, which is correct?

Also, how do I check the screen near the transmission cooler for debris?

Leroy
09-24-2004, 11:41 AM
It likely points to impeller, but I think that as they wear out it allows water to drain back out and with less water in the block it heats up like you are talking about.

G-man
09-24-2004, 11:47 AM
It's a 143 degree on a 285ho engine. Alsways best to get the gasket. skidim.com is great for this stuff if your dealer isn't convenient.

east tx skier
09-24-2004, 11:55 AM
I'm a huge advocate of new impellers each season. Such cheap insurance. Plus, if you have one shred on you, it's a pain to try to flush all the little pieces out, not to mention the potential problems that could erupt following an overheat.

BrianS
09-24-2004, 12:37 PM
Leroy's reply seems most logical.

Can anyone verify that if the impeller is worn or bad, does the water drain back out of the pump on shutdown. If the water drains out and the temp sender is no longer in the coolant water, that could explain the abnormally high reading.

Does that sound like a reasonable explanation? Either way, I'll definitely check the impeller before heading out next time.

Leec
09-24-2004, 02:37 PM
Leroy's reply seems most logical.

Can anyone verify that if the impeller is worn or bad, does the water drain back out of the pump on shutdown. If the water drains out and the temp sender is no longer in the coolant water, that could explain the abnormally high reading.
Does that sound like a reasonable explanation? Either way, I'll definitely check the impeller before heading out next time.
Mine has always done that. I attributed it to the water sitting still in the engine and as soon as you start it up it cools down. When it doesn't cool down that is when you have a problem. I have see my cars do it to. If you have a guage in your car once it is hot let it sit a few minutes and then look at the guage once you turn on the key but not start the motor.
:smile:

BrianS
09-24-2004, 02:54 PM
Yeah, wasn't really worried that the engine was overheating since the temp came right back to just above normal when you started it.

I was just worried because something changed, it has never done this before. It does not seem like normal heat soak because the rise in temp isn't gradual. It jumps 80+ degrees as soon as you click the key off.

Does anyone know what the temp sender will do if it is not immersed in water? If it gives an incorrect high reading if not in water, that will back up Leroy's theory.

ski_king
09-24-2004, 03:08 PM
Does anyone know what the temp sender will do if it is not immersed in water? If it gives an incorrect high reading if not in water, that will back up Leroy's theory.

I saw someone who just reinstalled his water pump. Only problem was he put it on backwards.
The temp gauge never read a temperature. The senser was in air, not water. He never knew he had a problem until he the exaust hoses began to melt.

I believe there must be water in the system for the gauge to have a reading.

Leroy
09-24-2004, 03:30 PM
If its jumping 80 degrees at the turn of a switch, it's not real, there is too much mass where the sensor is for that to happen. Some engines seem to do this upward drift of temp and others don't. Could be position of sensor or internal design of water flow also.

mgurley
09-24-2004, 03:33 PM
I am no expert by any means. Others on this board could probably either verify or discount this, but in my experiences the temp sensor will not read any temp when not sumbmersed in water. :twocents:

mgurley
09-24-2004, 03:34 PM
In other words "no water no temp". :(

mgurley
09-24-2004, 03:41 PM
Sorry Powerslot just saw where you said the same thing. :D

I agree. :worthy:

Leroy
09-25-2004, 11:18 PM
Just from engineering knowledge, not engine knowledge, I think it's possible for temp to not read anything if you just start it up and it's cool and there is no water running. I would be surprised if the sensors are anything other than two metal thermocouple but would like to hear from JimN on this.

Bert
09-26-2004, 12:06 AM
I saw someone who just reinstalled his water pump. Only problem was he put it on backwards.
The temp gauge never read a temperature. The senser was in air, not water. He never knew he had a problem until he the exaust hoses began to melt.

I believe there must be water in the system for the gauge to have a reading.
I can verify that. A friend of mine purchased an incorrect craft and was chomping at the bit to ski behind it and asked me to drive for him. While waiting for it to warm up at idle he was impatient and wanted to ski now. Away we went and burned up the engine. The guage was still on cold but the smell of the exhaust melting got my attention :eek: You can imagine how upset he was, it was the first time in the water with it.
I drive a friends 94 pro star from time to time and it behaves as described whe restarting after sitting between skiers.

Leroy
09-27-2004, 12:07 PM
Would like JimN's comments!

I just like understanding how things work! Not much on the internet explaining this. I believe the sensors are variable resistance sensors and do not need water, other than if there is no water and only air flowing around the sensor probably doesn't read right until the air soaks it for a long time as air doesn't conduct heat as well as water or conducts through the metal where the sensor is screwed in.

JimN
09-29-2004, 11:15 AM
The temperature sensor is actually a thermistor, where the resistance goes down as the temperature goes up. The body of the sensor is insulated from the thermistor so it doesn't influence the reading of the coolant. If the sensor was located in a hot spot and the thermistor wasn't insulated, the ECM would deliver less fuel and running too lean is not a good thing, unless you like melting valves and holes in pistons. Water is definitely a more efficient conductor of heat than air.

NeilM
09-29-2004, 12:17 PM
Right. To clarify, the temp sensor doesn't need water to work, per se. It will read what it 'thinks' the ambient temperature is, but air (ie the absence of water flowing past and in contact with the sensor) is such a relatively poor heat conductor, the engine is fried before the gauge displays there is a temperature problem. Eventually, the temp sensor will display the temperature of the environment surrounding it.

BrianS
09-30-2004, 09:21 AM
As usual you guys were right on the money. I pulled the impeller and it had a chunk missing from one vane about the size of the nail on your pinky finger. The other vanes had noticable wear on each of the other vanes from passing over the holes in the housing.

I destroyed the gasket to the thermostat housing when I pulled it checking for the missing piece of impeller. After replacing the impeller, thermostat housing gasket, and thermostat (might as well while in there) it runs just like the day I got it again.

I guess I still don't understand why the temp guage was immmediately spiking when the key was turned off. You guys have thoroughly disproved the theory about water draining out of the block.

Edit: Just to add, the thermostat was a 160 degree. The one I replaced was 160* as well as a spare the previous owner had given me.

JimN
09-30-2004, 11:18 AM
The block won't drain through the knock sensor and petcock holes and the only other place it could go out is by flushing back out of the circulating pump.The only water than could drain out would be from the circulating pump down to the level of the water pressure under the hull and if you have ever removed the hose from the raw water pump when the boat is in the water, you know how fast the water comes out.

Basically, if you have a temperature issue, suspect the impeller and/or the oil cooler, especially if the thermostat is fairly new. When you buy new impellers, get the kit that includes the gasket.

east tx skier
09-30-2004, 11:30 AM
Just a little aside to Jim's last statement. Sort of an FYI. I bought my impeller from Overtons this year because their website said "impeller and gasket" for the Johnson 812. Skidim's site didn't mention the gasket. My father-in-law bought one from skidim, and it did, in fact, come with the gasket.

lakes Rick
09-30-2004, 01:03 PM
There is a different Part # for the impeller w/ gasket vs. the impeller only.

JimN
09-30-2004, 01:28 PM
Yes, the part # is different, but the impeller probably shouldn't be replaced without the gasket, anyway. It may not leak water, but if it sucks air, it's not gonna cool the way it should. Whether the dealers and parts distributors stock the impeller separately or w/ the gasket is up to them. I would replace both, since replacing the gasket at the same time is really a cheap way to not have problems.

MarkP
09-30-2004, 01:46 PM
Doug

I got one from dim about two weeks ago (812). It too came with the gasket