77' merc 1500 no cooling water at high speeds

[bqsburb]

Hello I have a 77' merc 1500(150hp) in-line 6 that has good water flow "peeing out" at low speeds and idle, but looses flow with increasing speed. The exaust gasket from tube to lower end needs to be replaced. Could it be the problem? Any ideas will be appreciated. thanks bq

 

[fgbutler]

Re: 77' merc 1500 no cooling water at high speeds

If you know the gasket is bad connecting your powerhead to your shafthouse, that could definitely be the problem. Do you have water spraying form the lower end of the powerhead? Have you run the engine on a hose to see if you have water coming out your exhaust "pipe"?

 

[bqsburb]

Re: 77' merc 1500 no cooling water at high speeds

When on a hose I did notice some water spraying from the two square exhaust holes, but most of the exhaust comes out of the prop. I never really reved the engine though. Could the resistance of the lake water to the exhaust cause the cooling water supply to blow out with high exhaust flow?

 

[mellowyellow]

Re: 77' merc 1500 no cooling water at high speeds

drop the L/U and install a new impeller to start.<br />good luck,<br />M.Y.

 

[bqsburb]

Re: 77' merc 1500 no cooling water at high speeds

This was with a new impeller and housing. The last one did the same thing, but didn't look too good so new installed.

 

[bqsburb]

Re: 77' merc 1500 no cooling water at high speeds

I am now trying new gaskets on exhaust side. Maybe exhaust pressure is overtaking the water pump at the high speeds (if gaskets failed)? I am running out of ideas.

 

[mellowyellow]

Re: 77' merc 1500 no cooling water at high speeds

a decarb can help unclog partially closed exhaust,<br />but the gasket is gonne need to be replaced.<br />good luck,<br />M.Y.

 

[bqsburb]

Re: 77' merc 1500 no cooling water at high speeds

ok I replaced the gaskets on the powerhead. They were leaking water due to loose bolts before. Now no water leaks, but still no water at higher rpm. Do I have to remove the powerhead to drop the driveshaft housing? I have loosened the 8 bolts that are visible and the clip on the lower mount splined shaft, but the housing doesn't want to budge. The powerhead is loose though. What do I need to do to spit that sucker? Again Thanks for your help.

 

[dvchristoff]

Re: 77' merc 1500 no cooling water at high speeds

I bought a used 1980 in-line powerhead form a engine rebuilder who has a lot of experience with in-lines. We hooked are water pressure gauge hose to the #3 cylinder water jacket and the pee tube to the outlet down by # 6. We experience the same deal, good pee at low RPM and very little pee at high rpm with more water exiting the right square exhaust housing at high rpm. The pressure gauge reads low at idle and 10-12 psi at high rpm. Anyway he says this is normal and not to worry. Have ran powerhead like this for 50 hours with no problems.

 

[corm]

Re: 77' merc 1500 no cooling water at high speeds

Hi,<br />I almost hate to mention this but. Do you have anything mounted on the transom of the boat in front of the motor like a spedo pick-up or transducer that would push the water away from the motor water intakes at speed. Is the compression good on all the cyl?<br /><br />Yes you have to remove the powerhead to take off the mid section. I don't know why you would need to remove the mid section. All that is in there is a copper water tube from the water pump to the powerhead. With the powerhead & lower unit off you can see everthing you need to.

 

[bqsburb]

Re: 77' merc 1500 no cooling water at high speeds

Thanks for responding. I guess I will pull the powerhead and hope for a gasket problem. It looks like there are a few others experiencing similar problems. I am hoping to be on the lake this weekend.

 

[bqsburb]

Re: 77' merc 1500 no cooling water at high speeds

Found it! FYI There was a poor seal under the pump base (inlet) and an exhaust leak right next to it. I sealed the inlet and sealed the exhaust and no more problems. bq

 

[mellowyellow]

Re: 77' merc 1500 no cooling water at high speeds

way to keep at it BQ! <br />I'm sure this thread will help lots of folks here.<br />safe boating,<br />M.Y.