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OMC 800 Water pump questions - answers

Repair help for OMC 800 outdrive
Question and Answers

  • OMC 800 / 400, 6 cylinder, V8 and 4 cylinder outdrive installation help
  • Impeller help
  • Lower unit gear set replacement

By author Trish Dougherty

Q: How can I tell if I need to change the impeller in my outdrive?

A: To test the water pump, start by disconnecting the line feeding into thermostat making sure that the idle speed on the engine is set between 550 – 650 RPM. Take a 2 gallon pale it should fill in approximately 2 minutes or less. If it is taking longer, replace your impeller.

Q: How do I change the OMC 800 water pump in my upper gear-case?

A: Disconnect the shift cable and the battery. Remove the 3 bolts on tilt lift area and the retainer clip under the ball gear. ¼ inch - 20 bolts on both sides of split bearing must be backed up 1 turn (not removed). Remove the rubber caps on split bearings, then four 3/8 inch bolts on the split bearing. The unit should pull away from the boat. Bolts on cap must be removed. Pull the shift cable through the upper gearcase.

*Note, this cable may be difficult to slide through the upper housing. Be patient and try different methods for removal. Do NOT cut the cable as this cable is upwards of $600 to replace.

The upper gearcase should lift straight up out of the exhaust housing. Turn unit upside down, and remove four ½ inch bolts with ¾ inch socket. Remove the four bolts on the ball gear retainer, pry ball gear out of upper gearcase.

Use a brass punch to push pinion gear down; this will allow access to the water pump area. You can now replace the assembly (#12190 kit).

Q: How do I test my lower unit for leaks?

A: You can purchase a common engine fuel pump, place a T-fitting on it and connect it to a bicycle pump line and fill to 12lbs. Clamp off the line so no air escapes. The other T-fitting should go to an oil fitting which will thread into the upper or lower gear case (where the drain screw is normally installed).

If pressure does not hold – spray unit with a 50 / 50 ratio of soap to hot water. Where the unit is leaking you’ll see bubbles appear from the soap. *Note – if unit has a mechanical shifter, the shift must be sealed off using a 5/8 inch inside diameter garden hose plugged at one end with clamping on shift cable flange where it goes into the boat. The unit can then be pressurized to 12-14lbs to find the leak.

Q: I've lost forward and reverse gear. What are some things I should look at to diagnose the cause?

A: Most commonly on OMC lower units units, if you have lost both forward and reverse – the spline on the drive shaft or water pump shaft is stripped in the upper gearcase. The upper gear case must be removed and these shafts inspected 1st then lower drive shaft in lower unit 2nd.

Q: What type of oil should I use in my outdrive?

A: You can use an 80 weight oil non synthetic or synthetic in both the upper and lower unit.

*Note – for the Stringer electric shift lower units, you must use a type-c oil which is designed specifically for the electric shift application. You should use an 80 weight in the upper gearcase.

Q: What type of sealant should I use when replacing the seals in my outdrive?

A: When installing seals, the area should be wire brushed or sand blasted first and then cleaned with lacquer thinner. Use a 680 or 609 Loctite compound when installing seals into a used or older outdrive. Gasket compound can be used in place of 680 / 609 Loctite only if the aluminum gearcase housing is new.

Q: What causes forward gear to wear out? How do I know if my OMC gears need to be replaced?

A: A common cause of forward gear wear is engaging into gear with an idle speed higher than 500 rpm. (Engaging into gear at an idle speed of 800 RPM will wear out the clutch dog and forward gear lugs). After about 100 shifts, the unit may not go into gear or may engage and then pop out of gear once throttle is applied.