1989 Ford 5.8 omc, after changing log type exhaust to center rise, overheats.

Ideas? I recently changed from log to center rise type exhaust manifolds, and the water supply does not fill the exhaust riser to the top so both exhaust hoses overheat. Had the stern drive pump and back plate assy reworked by omc mechanic. Still not filling the riser. I was wondering how much water should flow from stern drive raw pump. When I disconnect the raw pump hose to thermostat housing and run engine, the water flows about 1 inch upward from tip of hose. I've checked the power steering cooler and it is clear. I am in process of rechecking risers to make sure there are no restrictions in top of riser. Does anyone know how much raw water flow should be? Has anyone made modifications to flow pattern to support changing to center rise manifolds? Thanks for any support!

Re: 1989 Ford 5.8 omc, after changing log type exhaust to center rise, overheats.

dalawson said:

Ideas? I recently changed from log to center rise type exhaust manifolds, and the water supply does not fill the exhaust riser to the top so both exhaust hoses overheat. Had the stern drive pump and back plate assy reworked by omc mechanic. Still not filling the riser. I was wondering how much water should flow from stern drive raw pump. When I disconnect the raw pump hose to thermostat housing and run engine, the water flows about 1 inch upward from tip of hose. I've checked the power steering cooler and it is clear. I am in process of rechecking risers to make sure there are no restrictions in top of riser. Does anyone know how much raw water flow should be? Has anyone made modifications to flow pattern to support changing to center rise manifolds? Thanks for any support!

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Changing the style of manifold should not make a difference you have a water flow problem, either a blockage or a stern drive water pump issue.
If you are testing it in the driveway on muffs you need to ensure you have plenty of water pressure up to the drive.

Re: 1989 Ford 5.8 omc, after changing log type exhaust to center rise, overheats.

dalawson said:

Ideas? I recently changed from log to center rise type exhaust manifolds, and the water supply does not fill the exhaust riser to the top so both exhaust hoses overheat. Had the stern drive pump and back plate assy reworked by omc mechanic. Still not filling the riser. I was wondering how much water should flow from stern drive raw pump. When I disconnect the raw pump hose to thermostat housing and run engine, the water flows about 1 inch upward from tip of hose. I've checked the power steering cooler and it is clear. I am in process of rechecking risers to make sure there are no restrictions in top of riser. Does anyone know how much raw water flow should be? Has anyone made modifications to flow pattern to support changing to center rise manifolds? Thanks for any support!

Click to expand...

Ayuh,.... You need the T-stat housing for center riser manifolds..... least, that's My guess anyways...

The water flows change in that swap on ole Mercs....

Yer water flow test sounds kinda weak too.... Run a clear hose on it just before the T-stat housin', 'n look for Bubbles....
Ya might have a leak under the pump...

Re: 1989 Ford 5.8 omc, after changing log type exhaust to center rise, overheats.

Black as said:

Changing the style of manifold should not make a difference you have a water flow problem, either a blockage or a stern drive water pump issue.
If you are testing it in the driveway on muffs you need to ensure you have plenty of water pressure up to the drive.

Click to expand...

Thanks for the feedback! After changing out manifolds and risers a while back, I took it out on the water and it burned up my exhaust hoses. After that, I back flushed all hoses, ports, engine block, stern drive, etc., and then had stern drive rebuilt. I have not retested in the water, but just by hose adapter on trailer. The only other change from stock was years ago when I installed a different power steering cooler that required a bit larger diameter cooler inlet and discharge hoses...don't think that would cause a problem, as it didn't in the past. I intend to run flex brush through the risers to ensure that there was not a foundry problem, then reassemble, recheck hoses, and retest. I read that someone else had a pin hole leak in the raw pump suction pipe. Hopefully that is not the case.

Re: 1989 Ford 5.8 omc, after changing log type exhaust to center rise, overheats.

Bond-o said:

Ayuh,.... You need the T-stat housing for center riser manifolds..... least, that's My guess anyways...

The water flows change in that swap on ole Mercs....

Yer water flow test sounds kinda weak too.... Run a clear hose on it just before the T-stat housin', 'n look for Bubbles....
Ya might have a leak under the pump...

Click to expand...

Thanks for the feedback. I did install a clear hose and had bubbles prior to having the stern rebuilt, but not afterwards. It will probably be a week or two before I can put some more time into it.

Re: 1989 Ford 5.8 omc, after changing log type exhaust to center rise, overheats.

Put the drive in a tub of water with the water level well above the water intake holes on the drive. (better yet, back the boat down into the water) Then start the engine and see how much water is pumped. You should see a lot more than 1" of water above the hose, that is just a trickle.
Along with low water pressure on muffs, a leaky raw water pump housing could cause the problem too. It takes more than just a good impeller to make the pump work.

PS, thermostat housing is the same on those engines for both the log and the center rise manifolds.

Re: 1989 Ford 5.8 omc, after changing log type exhaust to center rise, overheats.

Don S said:

Put the drive in a tub of water with the water level well above the water intake holes on the drive. (better yet, back the boat down into the water) Then start the engine and see how much water is pumped. You should see a lot more than 1" of water above the hose, that is just a trickle.
Along with low water pressure on muffs, a leaky raw water pump housing could cause the problem too. It takes more than just a good impeller to make the pump work.

PS, thermostat housing is the same on those engines for both the log and the center rise manifolds.

Click to expand...

Thanks, Don! I intend to test in the water while on the trailer, hopefully next week. I'm gonna recheck the pump as well.