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How do you diagnose a failed water pump vs thermostat?


kfennell
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My boat (Carbon pro with PCM 343)is throwing a CGM light, and the temp gauge was over 180 yesterday after idling down the lake, so I guess that the water pump or thermostat have failed on me. (Oil looks good still and was just changed). How do I diagnose the water pump with the boat on the lift, if I disconnect the suction line and place it in a bucket of water will it pull water through the system if it is working right or is it not self priming?

 

Or is it just the kinda thing that it's easier and cheaper to just change both if there is a question?

 

 

 

 

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@kfennell

Start with checking the screen filter in the water strainer ( should be checked and cleaned out regulary anyway). If that's all good check the impeller (always good to know it is in good condition) pay attention at the direction the wings are bent so you get it right putting it back. If all the wings are present it' s not at fault however if you see small cracks or it 's badly worn replace it with a new.

Assuming you still have not found anything wrong you need to take off the water hoses from the transmission cooler and the engines heat exchanger ( if you have closed cooling system) to make sure nothing is clogging them. Make sure you check both ends, I have encountered algae growht clogging our transmission cooler more than once.

By now you have made sure nothing is wrong with the lakewater feed and if the impeller was intact and nothing is clogged, change the thermostat.

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OK! their might be a service notice on that motor that requires the circulation pump to be replaced. their were a number of circulation pumps that the inner hub cracked and broke causing a overheat situation. I would get the serial number off the engine and contact your nearest PCM dealer and check if it falls in the service bulletin serial number sequence.
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I could be wrong, but when a thermostat fails, it is designed to stay open and it is hard to raise the temp to operating temps.

Assumimg the water filter or strainer box on the bottom of the boat isn't plugged, Chances are its the impellor. That is a $35 fix and an hour or so of work. Easy enough to rule out, and not bad to replace anyway.

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Your right, no serial number series.

From PCM:

 

In the event you are diagnosing an overheat or running hot condition, the engine circulation pump should be tested and inspected prior to extensive diagnostics.

 

PCM is issuing Service Update SUP2012-03 to address a problem with 5.7 engines.

General Motors vendor for the circulation pump has supplied some pumps with impellers that do not meet the "hardness" spec on the metals. The impeller metals in some cases are too hard. This condition results in the impeller cracking and is no longer able to circulate water through the engine and exhaust system. The crack can be anything from a hair-line crack to completely broken as shown below.

 

 

 

 

933450b1016733a17cb328449fa739.jpg

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@wilecoyote, I did some researching on the web, and thermostats are indeed engineered to fail in the open position. I can't comment on what would cause them to fail in closed or half-closed mode. Every thermostat that has ever failed me stayed open, and typically, was in the winter when I needed the heater to work!
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When the boat is running in water put your hand on the face of the impeller case and it should be cool to the touch... If its warm or hot, the impeller has failed. I'm with Alberto Soares and am betting it's the impeller. Easy to replace on most boats.
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The thing about impeller failure is that not only does it stop pumping water but sometimes the pieces of rubber get pushed into your system further blocking the water flow. The impeller change is easy, it's the pieces that are a PITA to get out.....

@kfennell How many hours on the boat and has the impeller ever been done?

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Never heard about thermostats being designed to fail open, in fact if you look at how they are designed it is just as likely they would fail to open enough.

When it comes to troubleshooting guessing really doesn't do any good, you need a step by step approach where some maintenance might be taken care of along the way.

As @Jody_Seal pointed out if blocking/impeller and thermostat are ruled out circulation pump comes next.

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It has been a number of years since I have seen PCM impellers spin on the hub, however Indmar had a rash of them last summer. I posted the PCM Service bulletin because it is something that could be the problem otherwise PCM would not have sent out that service bulletin. I replaced two Circulation pumps last summer.
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I think that many of you are talking past each other - There are two water pumps on a typical Ski boat - 5.7. 350 GM 0r 351 Ford Block engines.

 

The Raw water pick-up is straight off the crank shaft (except on some older Mercruisers & Correct Crafts - then it is belt driven & off to the side) and it is bolted directly to the harmonic balancer. That is about a $40.00 part - I keep a spare one in my glove box - quick and easy fix.

 

The circulating pump (the one in Jody's picture) is talking about is directly above the Harmonic balancer's pulley (end of crankshaft). When it is begining to fail - it generally drips small amounts of water as the bearings begin to go .... or there is a failure in the casting as Jody's picture shows (probably because the bearing has seized).

 

The Thermostat sits directly above that. It is sort of diamond shaped with bolts at either end is covered. They are fairly simple and cheap sometimes they get stuck because scale keeps the disc from closing properly. Be sure to replace the gasket if you take this apart - even if you only have to clean it.

This is where my winterizing gizmo is helpful - if the raw water pump won't pull the anti-freeze over the gunnel replace the impeller. Inspect the back of it of it is driping water - ther bearings are about to fail. If the engine doesn't reach operating temp for a long time it is probably the thermostat stuck open. Check and see if the circulating pump is driping around the pulley.......

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The circulating pump failure Jody is talking about is not due to bearing seizing. It is the result of manufacturing defects. The metal that was use for the impeller was out of spec - to Hard. In general the harder a metal is the more brittle it is. Think of it as like the difference between glass and plexiglass. The glass is much harder and will shatter. The plexiglass will bent and take impacts etc., much like a softer metal.
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The only way to diagnose a failed thermostat is to remove it and place it on a pan of water then heat it to 170˚. If it opens, then it is good. Since it is only a $10 part, I would just replace it as a preventative maintanence item. Impellers are supposed to be replaced every 200 hours or 2 years, whichever occurs first. If you are unsure as to the age of the impeller, then change it now. These are normal wear items and they are cheap enough that you should replace them if in doubt.
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