1984 Mercury V150 - High water pressure and blowing oily carbon out exhaust

**04300exc** · 02-17-2020, 04:18 PM

I rebuilt the power head on my 1984 merc v150 a couple years back - everything has been working great. I took the boat out of hibernation this year (last ran in August 2019) but could not get motor to start - discovered a fuel line plugged due to delaminating liner in the line....good ol CA gasoline. I did get boat to run sporadically while at the dock, and noticed I had little to no water pressure. I took boat home and replaced my fuel lines and put in a new water pump. Old water pump impeller blades had taken a set, but all still in tact. I took boat back to lake...fired up fine.. had good water pressure... so I ran it a little. Noticed water pressure would fluctuate when accelerating...but it has always done that. I ran rpms up once motor was warm - at 5000 RPM I had 25 PSI water pressure - this seemed high to me as I do not ever remember seeing over about 20 PSI at 5500+ RPM. I took back home an put motor in a barrel of water. I found water leaking from my poppet valve weep hole. I found a hole in my poppet valve diaphragm...replaced diaphragm. Thought I had found my problem for high water pressure. Fired up motor in the barrel again and I have 14 PSI at idle. This seems way too high. While motor was running in barrel, I also noticed a lot of oil residue on the water in addition to oily carbon deposits floating on the water...??????? I don't ever remember seeing carbon deposits in my water barrel??? Picture attached. I ran a compression check on the cylinders thinking maybe I have a bad head gasket... all cylinders are 90 PSI. I have read that my water pressure should be 4-7 PSI at idle and 18-21 PSI at 5000 RPM. Any thoughts as to what is going on with my motor????? I am thinking I have a plug somewhere in my cooling system and am preparing to pull the powerhead to check the midsection plate housing my upper motor mounts..... I really don't want to do this...but I'd rather be safe than sorry. Your help is appreciated.

**Mrzip** · 02-17-2020, 04:46 PM

Nothing wrong it. Needs the piss run out of it

**Merc 2.5** · 02-17-2020, 05:28 PM

14 at idle seems little high to me , carbon, ya maybe seafoam or sumthin to break it up and run chit out of it , make sure u got all ur waterpump gaskets and plates correct. Back flush from copper tube with stats and poppet out , and make sure those r working rite , if not prob n plate, but with all that said. I've seen motors with high water pressure run for yrs , warm water tho

**Merc 2.5** · 02-17-2020, 05:29 PM

Mine barely reads on guage at idle 3-5 in neutral lower n gear
Idles round 140. O and welcome to s&f

**mrichartz** · 02-17-2020, 06:38 PM

Did you fog it when you put it away and have you been able to really run it since then or no? If not need to run it and like mentioned throw some seafoam etc in it...
What kind of oil do you use? I know if going from regular to synthetic you will have oil running out the exhaust etc..
Yea 14 at idle is high..
And your right...as much as not wanting to pull powerhead better safe than sorry and not having it nagging at you

Michael

**04300exc** · 02-17-2020, 09:18 PM

Well I pulled the powerhead and the two sections from the lower. Didn't find any smokin gun, but I did find water where it should not have been and some leakage of the exhaust past a gasket. will clean it up, put in new gaskets and try back flush. Anything else I should do while its apart? thanks guys - appreciate the advice.