Water pressure Merc 2.5L 280HP

[c starr]

My water pressure gauge reads 0 at any rpm, the temp ran 175-180 after 10 minutes (3000rpm), the tele tele was spiting, I blew air through the pressure line and the gauge seemed to be working fine. Any ideas why the gauge does not read pressure? Thanks Chris

[The Big Al]

If you over tightin the copression fitting, this will close the line.

Bad gauge?

[sosmerc]

On that engine your warning horn should activate at 180 degrees. You should have a minimum of 10 psi at WOT. You have a block pressure sensor that monitors pressure above 5000 rpm....and should put the engine into "guardian mode" if it does not see enough pressure for the rpm you are at...if you are above 5000 rpm.
At idle, these engines don't put out much pressure....4-.6 psi....but at 3000 rpm I think you should be seeing 3-5 psi and the temp should not be getting near as warm as you are getting.
Sounds like your intake may be restricted or you have a pump problem such as a spun hub, or blown gasket or improperly fitting water tube grommet.
Remove your lower unit and attach a garden hose to the water inlet tube and see if you can "adjust" the pressure to 3 psi. Lower the pressure to 1 psi and run engine at IDLE speed and see if it stays cool.
Inspect your water pump cover and gaskets and impeller. Inspect the water intake passages.
What is the serial number of your engine?

[c starr]

The serial # OG981568 what should the temp run on this ?

[sosmerc]

Sort of depends upon the air and water temps you are running in as well as the boat speed (which will affect max water pressure). The last 280 that I was test running in August ranged from 108 to 153 degrees as shown on my DDT...we were cruising around 5700 rpm and had about 7 psi...probably doing less than 70 mph at the time. (twin engine rig...very heavy and fighting some ignition issues which turned out to be bad block pressure sensors on both motors)

[Jay Smith]

I target 15-17 PSI at WOT on the 280's that I build ( Manual calls for 12 PSI min. @ 5700 RPM's ). When I do a JSRE drystack system on a motor ( Any 1 piece tuner adapter that come stock on Merc. Hi Per. engine, the 280's seem to respond the very well to water minipulation ) I can move the pressure up or down even as slightly as 1 lbs PSI increments at WOT...I also can furnish different quickly removable spent water discharge orfice plugs with the system for winter and summer to adjust for different ambient water temps to "hit" target temp numbers on the money also and make warm ups in winter quicker...

I find that pressure and temps on OB motors are determined by many issues several are : ambient cooling water temps, water pump health with a total seal from inlet to outlet ( no water leaks on discharge side of the pump anywhere or air on suction side leaks ), restrictor disc size, restriction somewhere in spent water discharge leaving block and or restriction on the suction side of inlet water entering suction side of the pump , also motor issues can cause temp spikes such as, timing, lean and or rich conditions with fuel mixture and loss or less that adiquate lubrication causing sleeve heat and gaulding this can cause water temp jumps up and down, but if a bore is so lean its causing water temp in block to sharply rise the water temp will be the least of the worry:cool: ...

Good luck,

Just food for thought.

[sosmerc]

That's a great summary of the situation, Jay. Thanks for posting that.

[c starr]

I took the lower unit off and inspected the impeller,housing, intake area and every thing looked fine. I tried the hose in the water tube and at any pressure the gauge reads zero,pulled pressure hose off of motor and very little water coming through. tale tale from lower side of motor lots of water, other side from cross over and top of motor no water, removed cross over hose and it is dry, blow through and seems like no blockage. is this the reason I have no pressure and if so what do I do next. Thanks Chris

[sosmerc]

Can you describe (or show us with a picture) where you are tapping off of for your pressure guage?
Has this engine been apart or modified?

[c starr]

The line for the pressure gauge is on the back of the motor, above and on the left where the 6 black boxs are, that go to the spark plugs. the motor is stock. Thanks Chris

[sosmerc]

Pull the thermostat covers off at the top of each head to confirm that there is a restrictor washer and gasket in place of thermostats. What is the size of the hole in the restrictor washers?

[sosmerc]

You may also want to pull the heads to inspect for proper location of the 4 "water dams" that direct water through the heads.
But if you have washers with small holes and gaskets in place, and are still not getting a good stream of water out of the location you are tapped into...I think you may have a blockage under the powerhead.

[c starr]

If there is a blockage under the powerhead what would it be from ? where my pressure line is should this be the most water pressure coming into the motor ?

[sosmerc]

From looking at the diagrams in my manual, the spot where you are reading water pressure is coming from the main galley in the center of the block, however, it may have to fill the exhaust cover first before it gets to your fitting...I am not sure. Do you have a large pipe plug on the very top of your block just behind the flywheel between the cylinder banks? (I think most 2.5 litre blocks do) If so, you could attach your water pressure fitting there and retest. The water in the chamber under that fitting is coming directly from the water tube. That chamber should completely fill unless there is insufficient pressure or volume. (or exhaust/air getting into the water pump housing from an improperly sized pump housing guide tube/grommet or broken gaskets on either side of the wear plate. Have you removed the inlet covers on the side of the gearcase to inspect for debris?
Could the powerhead base gasket be bad?
Has a DDT been plugged into the engine to read stored fault codes and to download the run history?
I am thinking some more about that fitting that you currently have in the back of the exhaust cover. I am pretty sure that you will not get much of a pressure reading UNTIL the cover fills up. So why isn't the cover filling up?
It could be plugged inside and since water has to get through the cover FIRST before it lets water in around the cylinders this could explain why you are running so warm.
I think you need to remove that cover and inspect it and the gasket.
(did you pull the thermostat covers and find the restrictor washers ok?

[sosmerc]

One other thought...the electronic block pressure sensor taps from a location that I believe gets water BEFORE your pressure guage tap. Why not try hooking up your mechanical pressure guage temporarity to this fitting. If your pressure reading is much higher this would indicate that the problem is a blockage in the exhaust cover itself.
Please keep in mind that these pressure checks should not be taken on a hose...the lower unit and exhaust housing need to be submerged to get proper readings. I use 300 gallon cattle feed tanks to test in...if I can't get to the boat ramp easily.

[c starr]

The holes in the restictive washers are 1/8" there is no build up at all in them. What next?

[sosmerc]

If you still have the lower unit off, I would put the garden hose back on the water tube and turn it on and see if there is any difference in the pressure as measured on the water jacket compared to measuring the pressure where the factory pressure sensor taps off of.
Did you find the large pipe plug I was talking about on the very top of the block? You could check pressure here as well.
Once the block is "filled" the pressure should really be the same at all these locations. If the pressure is the same, but very low, I think the problem is under the powerhead where the water tube enters the exhaust plate. If you have good pressure everywhere except on the back of the exhaust manifold I would think the problem is the exhaust manifold itself..or the gasket.
What does the run history from the DDT say?

[ROLF]

How or was this resolved?
Rolf

[sosmerc]

Yes, I am curious about this too. Was the problem fixed, and how?

[c starr]

The problem was the impeller, looking at it in the housing it looked fine, once I pulled it out and looked it over it was obviously the problem. Also big Al was right the compresion fitting was over tighten and not letting presure through to the gauge.
Now the tell tales both push a hard and steady steam at idle with 3-4 pounds presure, motor runs nice and cool.

[j_martin]

I just ran into the same problem. One year old, nylon hub impeller. Took the pump off and it looked fine. Pulled the impeller out, and it didn't unfold at all, stiff as a board. Bein' that the pump action involves the flexing of the impeller, I could see how this would just slip by the water and develop no pressure.

John

[c starr]

John, Sounds like the same problem, when I dropped the lower it looked ok and I kept looking for some thing else.I should of just did the simple impeller first and saved a bunch of time. Chris

[j_martin]

My mechanic told me that the brass hub impellers are better, and the nylon hub impellers will get stiff. He thinks it has something with what they have to do to vulcanize it to the hub. I think it's just cheaper rubber, along with the cheaper hub.

John

[c starr]

The new impeller I put in was brass, the old one was nylon and stiff as a board, I think the older ones only came in nylon and maybe Merc realized the problem and went to brass. Chris