OMC Impeller.

[Senior B]

Another friend of mine just took permanent posession of his dads 1985 Renken with a 2.5L OMC. Went to go for a ride on this boat a few months ago and didn't get 5 min from the ramp before the temp gauge was peged. He's pulled the thermostat to replace it and I have told him to replace the impeller. Is it in the lower unit like on a merc or does it have a raw water pump on the motor like a Volvo? Will look at the engine water pump and the manifold/riser as well if the impeller and the thermostat don't fix it. He like my other friend with the trashed 3.0L doesn't want to spend alot of $$. Thankyou..

 

[Boatin Bob]

Re: OMC Impeller.

The impellar is in the drive, recommend your friend pick up a manual even an aftermarket Seloc would be better than nothing for the impellar change.

 

[Gary H NC]

Re: OMC Impeller.

Is it a Cobra or Stringer drive? If it pegged the temp guage he could have some other damage..

 

[Boatist]

Re: OMC Impeller.

First step is to tell us if it a Stringer or Cobra. 1985 is when the change was made. Cobra easy to change impeller and the stringer is not so easy.

How does it lift the outdrive, a big 1/4 round gear (Stringer) or two hydraulic cylinders (Cobra).
Either way you need the correct OMC manual.

Stringer can have a flushing port to hook a hose to. If the cap is left off motor will overheat fast at low RPMs. At higher RPMs with a good pump will still cool ok but when slow down all the water pumped from the impeller will run out the hose conection and motor will overheat.

 

[THE BEEF]

Re: OMC Impeller.

If it's a stringer go down half way on the drive on the transom side and look for a grease zerk. This is for the swivel bearing If the seals are leaking thay will suck exhaust gases in the lower pump causing overheating right now.
Give that zerx about 12 shots of triple guard omc grease
and try it again. This happend to me .I replaced the impeller too. No help . A OMC mechanic told me about this. He said the seals were tp keep grease in not exhaust out. The grease will seal this area. After I did it
no problem 160 * at WOT. I'll change the seals this fall.
BEEF d

 

[Senior B]

Re: OMC Impeller.

First step is to tell us if it a Stringer or Cobra. 1985 is when the change was made. Cobra easy to change impeller and the stringer is not so easy.

How does it lift the outdrive, a big 1/4 round gear (Stringer) or two hydraulic cylinders (Cobra).
Either way you need the correct OMC manual.

Stringer can have a flushing port to hook a hose to. If the cap is left off motor will overheat fast at low RPMs. At higher RPMs with a good pump will still cool ok but when slow down all the water pumped from the impeller will run out the hose conection and motor will overheat.

His boat has the Stringer style outdrive.

 

[Boatist]

Re: OMC Impeller.

If it is a stringer I would not just pull the outdrive and change water pump without doing some Diagnostics. If you have no idea how old pump is might be a good Idea to change but locate the real problem first.

Check the water pump input holes on the bottom of the outdrive below the cavition plate. Make sure the screen is not plugged with sand, mud or debree.

First look over the transom with the motor running and you should see a stream of water of water form the starboard tilt pin spraying on the ball gears. If your boats stern heavy this could be under water. My 21 foot it is well above the water at idle. On plane for sure it will be above the water.

Also as I said before check if the unit has a flush adapter installed on the starboard tilt pin cap and if it does make sure the cap is on the garden hose connection. if not caped the water from the water pump will just run out there with very little getting to the engine.

Many will say change impeller every 2 years reguardless.
I changed mine twice since 1980 when it was new and both times when had the outdrive off doing other work. One time for a upper gear case reseal and the when the water pump short shaft splines failed. Both times I check the old impeller and they looked perfect. No wear no cracks and flex just like the new one.

To test it on the water or with the drive in a big bucket. Pull the hose from outdrive going into the thermostat Houseing and direct it into a 5 gallon bucket. At idle it should put out a steady stream and shoot out at least 1 foot like a garden hose. Mine was almost 2 foot.

On muff the hose pressure can give you false readings. Also this should be a short test as no water go to the exhaust manifold while you have it off.

If water presure is good there then I would reinstall. Next let the motor warm to above normal and shut off. Be ready before hand and pull the hose off that goes from the thermostat out let to the exhaust manifold. Be Carefull this water will be engine temperature. Install a longer hose over the side and start the engine if the temperature dropps fast then you have a plugged exhaust manifold or Riser. Again this should be a short test because no water to exhaust manifold.

If had good water input and good water output and the motor is still running hot then check and make sure the input hole thru the thermostat gasket is not plugged with rust or someone installing the wrong gasket. Make sure the thermostat is open.

Check the water circulation pump on the front of the engine and make sure belt is turning pump and it has a impeller in tact inside.

Now you can Fix what is bad.

 

[THE BEEF]

Re: OMC Impeller.

If you want to find out if its exhaust gases get a clear tube hosing
and install it where your water feed goes to the therm housing.
If you see a lot of bubbles its sucking gases.
Its a quick diagnostic tool.