overheating and loss of pressure

[dcfrank]

I have a 88 200 Black Max. Idling runs hot and has 1-2 lbs. of water pressure. Taking off, as my rpms increase the water pressure jumps from 0 to 10 and temp goes down to midrange, once rpms hit 4500, water pressure drops instantly to 0 and temp goes instantly to hot. Pulle lower unit and impeller doesnt seem to have been slipping on shaft and itself seemed fairly good shape but will replace it anyway. Just dont believe this is the problem, or only part of it. Any suggestions and possible fixes to do myself before leaving it at a repair shop. Called one and they seemed puzzled about the sequence above. Any suggestions?

 

[Laddies]

Re: overheating and loss of pressure

The impeller has a set in it or is failing, you should also ck the gasket and area beteen the w/pump and exhaust chamber for a exhaust leak, I think merc wants a RTV sealer applied in this area

 

[gss036]

Re: overheating and loss of pressure

It sounds like you are having poppet valve problems. My 89 Merc 200 runs a little on the warm side idleing out of the harbor, but not enough to set off the alarm, but I have new poppit parts waiting at the local dealer to replace. Might be wise to check it out.<br />The dropping of water pressure at high RPM and high temp rise would concern me also. How about the thermostats? Could the be sticking shut(closed)? Where does the water pressure take off from on your engine?

 

[dcfrank]

Re: overheating and loss of pressure

The pressure is right at 0 idling and doesnt pick up at all until taking off. Rising to 10 then falling to 0 at around 4500 rpms. I thought maybe the hub on the impeller or the keeper on the shaft had failed, but was not the case. There is noticeable amounts of water coming out of exhaust ports and as for the pee stream, fairly strong idling but weeker at higher rpms. With 0 pressure I dont understand having any strong peeing?

 

[dcfrank]

Re: overheating and loss of pressure

Is the poppet( is there just one) valve(s) and thermostats fairly easy the change myself? MY SELOC book doesnt seem to show this area well?

 

[Texasmark]

Re: overheating and loss of pressure

My engine is an '02 90 3 cyl. On the rear of the water jacket is a cover. Under the cover is a thermostat and a pressure relief valve. The thermostat controls cooling until the water pressure from the water inlet holes on the front of the "skeg" is high enough to unseat the pressure relief (bypass) valve. Then you get straight water thru your cooling system. <br /><br />I know this to be a fact as upon buying this rig, the thermostat was stuck closed. Any slow operation caused the overtemp horn to sound. But all I had to do was get on plane and after a couple of seconds the horn went off. Slow back down and the cycle repeats it'self.<br /><br />I pulled the thermostat out and put it in a pan of water on the stove. The water in the pot was boiling and the stat was still closed. On the pellet was stamped 120 (degrees F). Proof positive.<br /><br />So maybe you can apply this to your situation to help you figure out why your gauges are saying what they are saying.<br /><br />Will give you this: When that bypass valve opens, you are getting a big slug of water thru the water jacket so you should expect a rapid change of pressure and temp. It is about 4x the area that the thermostat controls so it will change things fast. <br /><br />Mark

 

[Texasmark]

Re: overheating and loss of pressure

Forgot something. The impeller is made of rubber. High pressure from these inlet holes and/or from high rpm's causes the blades to fold back and just go along for the ride; not needed. Unless you have a hole clogged from a piece of a blade, a bad impeller should have nothing to do with overheating at higher speeds.<br /><br />Mark

 

[dcfrank]

Re: overheating and loss of pressure

Thanks, does this sound more like the thermostats instead of poppet valve? The water pressure dropoff really concerns me. Since the impellor is sort of on the back burner to the high rpms increasing the pressure. What would cause such a dump in water pressure at 4500 rpms from 10 - 0?

 

[ronnieboy]

Re: overheating and loss of pressure

sounds to me like the motor is running out of water, make sure you dont have the motor jacked up too high on the transom or maybe you have installed something that blocks the flow of water along the hull. or mayby you have a blown water jacket gasket on exaust plate? notice any water comming out of back of cowl where it aint supposed to?

 

[Texasmark]

Re: overheating and loss of pressure

Seems (is for my engine) a little high in rpm's for the bypass valve to open (assuming you have one) but when it opens you would think that you would have an abrupt change in pressure. But I don't know. Never investigated that. If it were sticking it might take more pressure to open it....and remember, (at least on mine) the tell tale only says you have water to the powerhead, not thru it.<br /><br />Mark