I have a 1978 Mercury 115hp outboard. The tell tale is just a fast drip. The line is clear. I just replaced the water pump impeller (old one looked good too) The boat came factory with a water pressure gauge. Pressure in the water jacket runs approx 7psi at WOT... Does anyone know what pressure these are supposed to run. Is this gonna blow up on me.. Any ideas???
How did you verify the line was clear? 7 psi will produce a mighty strong telltalle so if it's just dripping it's plugged up still.
I'd recommend giving a strong puff of compressed air up the telltale and see what that does. If it doesn't help, pull the hose off the telltale and blow that out. Water should just gush out the open-ended hose, even at idle.
If it doesn't do that, you've got some sort of circulation problem in the powerhead and you'll need to start removing covers for inspection.
The best indicator of adequate cooling underway is to lay your hand on the exterior of the exhaust manifold (big retangular plate, port side, lotsa cast-in ridges). This motor has no thermostat so if it's very uncomfortable to keep your hand on the plate, that's around 140 deg. If you can touch it but for an instant before it's too hot to bear, that's 180+ and way too hot.
Normally the exhaust manifold should be lukewarm underway to a bit warmer after a full-power run. At idle in cold water it should be stone cold or just barely warm.
7 psi is a tad low but if it's keeping the manifold cool, might be OK. It's also possible your OE gage has become inaccurate and you actually have more pressure than you think.
One of those neat little infrared thermometers from Sears or Rad Shack work great for this but the old 'hand on the equipment' method will get you there, too.
Thanks for the temp info.... You can place your hand on it after running for about 15 minutes for about a second and is all you can stand it seems to be running a bit warm. Yes i have used compressed air(and have also just pulled the line and blew through it the old fashioned way) and you can hear the air come out down by the pump.. Also when you start it it has a stream coming out(although weak)...as it gets warmer(bout 4-5 minutes)...it dissappears...
I'm gonna grab another guage(and a temp guage) and check water for sure where the guage hooks up, check all the seals again coming from the pump, and just hope i dont have to pull the powerhead apart...this thing has me baffled...
Thanks!!!
When you have the lower unit off, hook up water source to the copper water supply tube.
Establish a baseline flow with the motor turned off. This should have a good stream out the telltale and of course will be gushing out the center section.
Crank 'er up and run for a few minutes. See if there's any unusual heating anywhere.
Don't throttle it up at all, just let it idle.
Be very careful 'cause there's no backpressure and no load from the pump or lower unit gears dragging, so this thing will rev like mad if you crack the throttle! Have someone at the control box ready to turn the key off in case of an emergency.
Note if the water flow changes with the motor running. I'm thinking that maybe you have some sort of exhaust gasket leak and it's pressurizing the powerhead cooling passages with exhaust gasses. This'll tend to blow all the water out of the block and make it heat up. And of course the faster you run it in the water, the higher the exhaust pressure will be.
Now, after you shut 'er down, pull all the spark plugs and check for droplets of water on them.
If you see water on any plug, it's a sure sign of leakage and I'd bet money on the exhaust manifold. Either gaskets leaking, bad baffle plate, or issues with the inner cover around the exhaust ports.
The reason I'm leaning towards this is how you describe the water flow getting weaker as the motor warms up. This is definitely not normal behavior for an Inline. They usually just pump water like crazy and run fairly cool.
If you do happen to run it in the water again before tearing into things, pull all the spark plugs, after running at a good clip then chopping the throttle to Neutral and shutting off the ign. If there's any water on them you've got issues.
Anyway, give that a shot, maybe it'll tell us something.......ed
1978 Mercury 1150 tell tale drips
Linear Mode
