75HP Chrysler overheating

[BigDaddy5517]

Hi. I have a 75hp Chrysler, model 757HB. The motor seems to run good however it overheats. It has a new impeller however it is a 6 blade instead of a 12 blade like it should. I don't know if this matters however if someone knows that would be helpful. I have checked the water pump, removed the thermostat, and dropped the lower drive unit and flushed the system. I removed the screw plug at the top of the head to purge the air and with the motor running I can feel pulses of air, however no water ever comes out. I removed the lower drive unit, hooked up a garden hose to it and succeeded in getting water from the screw plug hole. Any help is appreciated. Thanks

 

[eurolarva]

Re: 75HP Chrysler overheating

Most part sites I look at call out the 12 vane impeller which is the 3085 or the F438065. Do you know the impeller number you put in? Did you put an impeller in because it was overheating or cause it was due? A couple things to do is check to see if the impeller pin is in the drive shaft. This inserts in a small hole in the drive shaft and the impeller sits on it so it will spin. Next is use a flashlight and shine up in the motor leg with the lower unit off and you will see the grommet that the water tube connects to. It is possible to miss this hole during installation. Try running without thermostat and see if that improves. Only do this as troubleshooting and not as a fix. I suspect you have the wrong impeller however if you had an impeller dissinagrate you may have chunks still lodged in the motor somewhere.

 

[BigDaddy5517]

Re: 75HP Chrysler overheating

The impeller that is in there is the one it had when I bought the boat a few months ago so I don't know what number it is. The guy I bought it from told me it needed a new water pump but after I opened everything up the impeller and the pump were fine. I put in a new shaft key and I have ran the boat without the thermostat. But it still overheats. The 3085 impeller is the one I looked up and the one the marina told me it should have. I plan on ordering that one today but I was told by the marina that it should work with the six vane one. They also said that they don't work on Chrysler engines either so who knows. Thanks

 

[matt2002]

Re: 75HP Chrysler overheating

Don't run an outboard without a thermostat.Period.

 

[Frank Acampora]

Re: 75HP Chrysler overheating

While the 12 vane impellers are still available, the six vane are actually more efficient at low speed because the lower number of vanes allows more open area to suck in and "compress" the water. The extra vanes take up area that water could be in. So, the six vane should not be a problem.

Check the inlet screen on the lower unit. It may have some solidified gunk on it restricting water flow.

Also be absolutely certain that the brass delivery tube is sitting inside either the black rubber grommet on the water pump or the white plastic sleeve. (depending upon which water pump you have). If either is missing , most of the water will just blow out into the lower unit and not make it up into the block.

Since you reference a 12 vane impeller, I am assuming you have an older two piece lower unit with a 3/4 inch thick impeller. As far as I know, no one piece lowers with 1 inch thick impellers ever came with a 12 vane impeller. SO:

The impeller is located on the shaft with a rather small woodruff key. It is quite common for this key to slip out of position during installation. If this happens, the impeller will simply spin on the drive shaft and pump no water. Yet, with a hose, you will be able to get water to the top of the engine.

If you listen carefully, when you drop the lower unit, if you turn the shaft clockwise by hand before you remove the pump, you should be able to feel and hear the impeller turning.

The correct way to install the impeller is the locate the woodruff key in the shaft slot and slide the groove in the impeller over it. THEN, lower the water pump with a counter-clockwise twisting motion over the impeller. Lubricate the impeller beforehand with dish detergent.

Oh, yes! if you take apart the pump, be sure that the bottom stainless plate and the gasket are present and are in good condition

 

[BigDaddy5517]

Re: 75HP Chrysler overheating

OK so I have checked the screen and its not clogged. Also I double checked the key and water pump and both are good. The pipe is seated into the rubber grommet. Any other ideas? Thanks

 

[eurolarva]

Re: 75HP Chrysler overheating

Couple of things. The previous owner might have had his impeller grenade on him. When this happens parts of the impeller can get lodged in the exhaust port water chamber on the port side. Problem with this is the age of the motor chances are if you try to remove that area you will break bolts that are probably corroded. With lower unit off and water tube attached hanging in the motor leg try using a garden hose attached into where the thermostat goes and see if water will back flow through the water tube. Could also be the wrong impeller was installed. Merc site follows frank with a 6 vane impeller which is F438065-1 however all other sites state only a 12 vane for the 75 hp. Most sites give an option of 6 and 10 vane for the 35-55 and a 6 vane or 12 vane over 100 HP. It is possible that even though you have a 6 vane it might be the wrong six vane impeller installed. Pricing for a 6 vane is half the price of the 12 vane from 17 bucks to 35 bucks. You mentioned you have the other impeller on order. When you install this make sure the inside of the water pump is clean and not pitted and that the exit grommets are not broken or cracked.

This may seem like a dumb idea but if you have a decent size drill and you can chuck it up to the drive shaft with the lower unit submerged in water with the water tube attached you should be able to achieve at least idle speed and see if water comes out the tube and at what volume. This would require two people to accomplish. I would start with the new impeller and if that works get a thermostat installed. Good Luck.

 

[BigDaddy5517]

Re: 75HP Chrysler overheating

OK so here is what I found today. I dropped the drive unit yet again and ran water up through the water tube and ran the engine and it did not over heat. After a new test of the water pump with the lower unit off the boat I found that the water pump had a hairline crack by one of the mounting screws and was loosing alot of water and that was with a low pressure water hose. So I believe I have found the problem. On a different note does anyone know where I might be able to get a new water pump with the correct 12 vain impeller. Also thanks to everyone who offered advice on this problem. This is the first outboard I have ever owned and needless to say I am learning a heck of alot on this great forum. So thanks all......

 

[eurolarva]

Re: 75HP Chrysler overheating

The following are mercury or Quick silver parts and can be ordered from a mercury outboard parts dealer. The water pump might be found cheaper as an after market or ebay part.

FK500060 Water pump body $87.92
25-F84504 Thermostat grommet $5.24

The thermostat for that motor has been discontinued however an after market one exists Sierra 18-3559. That goes for around $35. The gasket for the thermostat I cant find in the parts diagram.

 

[BigDaddy5517]

Re: 75HP Chrysler overheating

Well this might be a stupid question but there is a little hole on the water pump housing at the base of where the water tube goes in. Is water supposed to come out of this hole all the time. I noticed that the pictures of the new ones have this hole as well.