Mercury 150 EFI Water pressure Mercury 150 EFI Water pressure
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    Mercury 150 EFI Water pressure
from Rick (  
1/8/1999 2:30:00 PM


 I have a 1998 Mercury 150 EFI hooked to the back of a Nitro 884 with a jack plate. I am concerned about the water pressure. I have around 10-12 psi at full speed which is about 65 mph. I have 5-6 at around 45 - 50 mph. My previous boat had a 115 Johnson faststrike and produced 2 times as much water pressure. I am not having any trouble with the engine over heating but it is just one of those things that make you wonder especially at the price that a new power head cost. I am looking for some input from other owners of the same engine on what there pressures are. The engine also doesnt seem to eject that much water from the engine at idle. Is this just the way the mercury is or should I be concerned. Thanks for any input.

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   150 efi from Triton Mike (  1/8/1999 3:06:00 PM
 Hey Rick, I have the same exact motor 1999. I wish I could tell you the exact psi but that sounds about right. I read the owners manual atleast 3 times. My psi gauge hasn't worked since I got my boat have taken it to the shop atleast 3 times trying to get it to work. Dangerous I know. The last time they messed with it there was a kink in the hose that goes to the gauge when they fixed that boy the water shot out of that motor like a steady stream. Use to be a spit stream before. damn gauge still doesn't work though. Boat is in the shop again. Hope this helps
   engine water pressure from ono (  1/8/1999 4:39:00 PM
 i have a 1999 175efi offshore. at idle maybe 3-5 psi. at 4000 rpm or so maybe 12-15. this subject has been discussed quite a bit and it seems that is about right for this motor. triton mike- i have the service manual also. there is nothing in there or in the owners manual that makes any mention of a water pressure specification. i also had a non-working gauge when i got my boat. the line from the engine to the gauge was pinched by a large washer on a bolt that held a grab rail. i moved the tube out from under the washer and it works fine (except the pressure seems low as you have noted). you might want to put a gauge back by the motor to see what reading you get. if your pressure is better there then inspect the tube for kinks, etc.
   Same Problem here from Norton (  1/8/1999 5:30:00 PM
 I had the same problem, it was reading low pressures at 6000 rpm's it read around 5 but now it reads 21. Here is what I found out from a Merc. service rep. at a tournament last year. At the engine where the water line went to the gauge was in the wrong place, it is suppose to be at the outlet at the engine head, mine was at the discharge line, my discharge water stream was weak also because it was also trying to send water to the gauge also. So I move the water line that goes to the gauge up the engine head and then everything was fine............Good Luck !
   150 efi from Don (  1/8/1999 6:54:00 PM
 Rick, I also have a '98 150 with a 6" plate running 15-18lbs. Motor was about 3/4" higher and I was running 10-12lbs so I dropped it til I got the pressure I'm running now.
   150efi from Jack (  1/8/1999 8:07:00 PM
 Rick, I have a 98 150EFI w/6" plate on my Triton TR18 and I get about the same readings as you
   Merc water press from Barry B (  1/8/1999 11:15:00 PM
 Rick, I experienced the same problem but it's not really a problem. I bought a new '96 Champion w/150 Merc on a 6" RapidJack, after owning OMC's all my life. The first time I took the boat out I made it a short trip because of my low water pressure(or what I thought was low). I called the factory and spoke with a tech and he said that a Merc/Mariner should run 2-5psi at idle, 8-12 at mid-throttle, and 15-20 when wide open. I found out from several other sources that comparing water pressure on OMC's and Merc's is like apples to oranges. For one the connecting point on these two outboards differ. Merc makes their connection at the tattle-tale line and OMC connects on the block in the water jacket. This itself accounts for a significant pressure drop. The Merc's do just fine at there designed pressure. But your pressure sounds a little low. You might try lowering your jackplate a notch or two because you might be sucking air when you are at full trim. I run my plate at a height where my propshaft is level w/ the pad. This gives my boat the best performance, yours may differ. Mercury also differs from OMC in the number of intake holes at the water pickup(Merc=10/OMC=4). I plugged the top three holes using silicone(another tip from performance enthusiasts). This allows you to raise your motor without getting the pickup above a good water supply thus maintaing good pressure. If you have about $300 to spare, Bob's Machine Shop in Florida will install a nice low water pickup on your foot. This will give you all the pressure you'll ever need. I would also suggest installing a temp gauge as a backup. Sometimes it's to late when those warning buzzers go off. Hope this helps. By all means talk to the factory or a trusted outboard mechanic for more info. Good Fishin' and God Bless



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