5.0 MPI Low Water Pressure Issue (solved - I think!) - Poppet Valve

[Jeff Copeland]

I wanted to share an issue I've spent a couple of weeks and a lot of money troubleshooting (and puling my hair out over!), in hopes that it helps someone else out there having a similar problem (and I actually found these forums extremely helpful when troubleshooting this and other issues, so wanted to give back):

• 2011 Sea Ray 220 SD, Mercruiser 5.0 MPI Bravo III, Raw Water Cooled with 310 hours
• Low water pressure alarm and occasional guardian mode at low rpms, alarm generally cleared by giving it some throttle, increasing RPMs a little.
• Verified water pressure read out with 0-15 PSI manual pressure gauge on a tee at the power steering cooler. It was in fact low (hovering between 0 & 1 PSI at idle), so the water pressure sensor (another common suspect) was not the culprit.
• Never got an alarm while running on a plane or even clutch ahead, only when coming back to idle, and boat never overheated. In fact, it ran at about 158 degrees max (which in retrospect I thought was a little weird since it has a 160 degree thermostat, but I always chalked that up to a couple of degrees sensor/gauge error).
• Thermostat and impeller less than one year old.
• It would generally be fine (around 1 PSI at idle, and 8-9 PSI at 3000 RPM - a little low, but not low enough to set off an alarm) at start up, and the problem was more likely to present after running for a while, stopping for a while with engine off, and then starting back up.
• Outdrive was off for rebuild 1 year ago, so I was confident there was no obstruction or "bravoitis"

All signs point to an impeller or raw water pump, right?

After doing some research, I replaced the raw water pump with an aftermarket stainless steel Hardin seawater pump (which I am very impressed with, by the way, and can confirm it works fine on the 5.0 MPI. Although the site states it's for a 350 MAG and other applications, many of the raw water cooling system components are the same). I especially liked not having to press the pulley off the old pump and back on the new one.

New raw water pump did not fix the problem or even help.

After further research, I found several instances of the water circulating pump brass impeller cracking or spinning on its hub in these engines, so I replaced the circ pump as well.

New water circulating pump did not fix the problem or even help.

(Side note: I was happy to replace these as preventive maintenance anyway, since they were original to the boat and over 10 years old, but I was truly stumped each time they didn't help because I was convinced there wasn't much else it could be!).

So at this point I was stumped. I had good water flow, no overheating, but a low water pressure alarm that was driving me crazy.

Then I stumbled upon a post in these very forums by @Seapig (Thank You Sir!) referencing the poppet valve in the thermostat housing. In many hours of researching this low water pressure problem, this was the only reference I found to the poppet valve on a 5.0 MPI.

It is located under the housing with the serpentine belt routing sticker and is held on by three bolts (credit to @Seapig for the photo, I added the circle):

It is essentially a pressure release valve, designed to bleed off some of the water pressure at high RPMs. It is very simple, and consists of a spring that is seated in the top housing (underneath the belt routing sticker) that presses a plastic "piston" (#15) down onto a seating surface:

In my case, the assembly wasn't exactly "stuck open", and the spring wasn't broken, but enough corrosion and scale had accumulated that it was allowing raw water to blow by freely.

What this meant was:

• At start up (thermostat closed, poppet open), the problem was barely evident.
• While running (thermostat open, poppet open), the boat was running just a little bit cool (158 on a 160 thermostat) because raw water was bypassing the thermostat via the poppet.
• After shutting down for a while and allowing the engine to "heat sink", and restarting with both the thermostat and the poppet open, the raw water pressure was too low at idle and would trip the alarm and guardian mode until the pressure came up with RPMs.

I disassembled the poppet valve housing this morning, and it was so simple that I just cleaned and reassembled it for troubleshooting purposes.

My raw water pressure immediately shot up to where it should be: 2-3 PSI at idle and 12-14 PSI at 3000-4000 RPMs, and I'm running at 161 degrees at cruising speeds (which makes much more sense on a 160 degree stat). So I'm reasonably confident the demon has been exorcised!

Some other helpful nuggets I learned along the way:

• The seawater/raw water pump is a bear to replace in the boat. I seems impossible, but it's not. I found it necessary to remove the idler pulley bracket, alternator, and cool fuel module just to access the pump and hoses. Removing these isn't as hard as it sounds, and it's best just to bite the bullet and get everything out of the way (of course this will vary from boat to boat - if you have room to access it, consider yourself lucky).
• The water pressure sensor is at the back of the engine, on top, to the left as you face aft (stbd side), and is installed to the left as you face aft (stbd) side of the power steering cooler. If you put a life jacket on top of the spark arrestor and lay on top of the engine between the risers, it's reasonably easy to access, and my connector was labeled, making it easy to verify:
  
• The power steering cooler where the sensor screws in is just 1/8 NPT threads, so it's very easy to screw in a T-fitting and add a manual gauge to visually verify your water pressure and troubleshoot the sensor. I used this stainless T-fitting and this 0-15 PSI pressure gauge and the setup works well.
• I'm actually surprised this is not a more commonly known maintenance item (maybe it is and I just missed it?). Seems at the very least it should be disassembled and cleaned every couple of years when you do an impeller change!
• When you remove it, you are required to say "Hello, Poppet" in an English accent (Pirates of the Caribbean reference, lol).

 

[Fill Ess]

Thanks Jeff, that's great info!

For the Hardin pumps, I found the same thing. For some reason they don't advertise the 5.0s but they do fit, and I called them about it.

 

[Wanderer 290]

I wanted to share an issue I've spent a couple of weeks and a lot of money troubleshooting (and puling my hair out over!), in hopes that it helps someone else out there having a similar problem (and I actually found these forums extremely helpful when troubleshooting this and other issues, so wanted to give back):

• 2011 Sea Ray 220 SD, Mercruiser 5.0 MPI Bravo III, Raw Water Cooled with 310 hours
• Low water pressure alarm and occasional guardian mode at low rpms, alarm generally cleared by giving it some throttle, increasing RPMs a little.
• Verified water pressure read out with 0-15 PSI manual pressure gauge on a tee at the power steering cooler. It was in fact low (hovering between 0 & 1 PSI at idle), so the water pressure sensor (another common suspect) was not the culprit.
• Never got an alarm while running on a plane or even clutch ahead, only when coming back to idle, and boat never overheated. In fact, it ran at about 158 degrees max (which in retrospect I thought was a little weird since it has a 160 degree thermostat, but I always chalked that up to a couple of degrees sensor/gauge error).
• Thermostat and impeller less than one year old.
• It would generally be fine (around 1 PSI at idle, and 8-9 PSI at 3000 RPM - a little low, but not low enough to set off an alarm) at start up, and the problem was more likely to present after running for a while, stopping for a while with engine off, and then starting back up.
• Outdrive was off for rebuild 1 year ago, so I was confident there was no obstruction or "bravoitis"

All signs point to an impeller or raw water pump, right?

After doing some research, I replaced the raw water pump with an aftermarket stainless steel Hardin seawater pump (which I am very impressed with, by the way, and can confirm it works fine on the 5.0 MPI. Although the site states it's for a 350 MAG and other applications, many of the raw water cooling system components are the same). I especially liked not having to press the pulley off the old pump and back on the new one.

New raw water pump did not fix the problem or even help.

After further research, I found several instances of the water circulating pump brass impeller cracking or spinning on its hub in these engines, so I replaced the circ pump as well.

New water circulating pump did not fix the problem or even help.

(Side note: I was happy to replace these as preventive maintenance anyway, since they were original to the boat and over 10 years old, but I was truly stumped each time they didn't help because I was convinced there wasn't much else it could be!).

So at this point I was stumped. I had good water flow, no overheating, but a low water pressure alarm that was driving me crazy.

Then I stumbled upon a post in these very forums by @Seapig (Thank You Sir!) referencing the poppet valve in the thermostat housing. In many hours of researching this low water pressure problem, this was the only reference I found to the poppet valve on a 5.0 MPI.

It is located under the housing with the serpentine belt routing sticker and is held on by three bolts (credit to @Seapig for the photo, I added the circle):

It is essentially a pressure release valve, designed to bleed off some of the water pressure at high RPMs. It is very simple, and consists of a spring that is seated in the top housing (underneath the belt routing sticker) that presses a plastic "piston" (#15) down onto a seating surface:

In my case, the assembly wasn't exactly "stuck open", and the spring wasn't broken, but enough corrosion and scale had accumulated that it was allowing raw water to blow by freely.

What this meant was:

• At start up (thermostat closed, poppet open), the problem was barely evident.
• While running (thermostat open, poppet open), the boat was running just a little bit cool (158 on a 160 thermostat) because raw water was bypassing the thermostat via the poppet.
• After shutting down for a while and allowing the engine to "heat sink", and restarting with both the thermostat and the poppet open, the raw water pressure was too low at idle and would trip the alarm and guardian mode until the pressure came up with RPMs.

I disassembled the poppet valve housing this morning, and it was so simple that I just cleaned and reassembled it for troubleshooting purposes.

My raw water pressure immediately shot up to where it should be: 2-3 PSI at idle and 12-14 PSI at 3000-4000 RPMs, and I'm running at 161 degrees at cruising speeds (which makes much more sense on a 160 degree stat). So I'm reasonably confident the demon has been exorcised!

Some other helpful nuggets I learned along the way:

• The seawater/raw water pump is a bear to replace in the boat. I seems impossible, but it's not. I found it necessary to remove the idler pulley bracket, alternator, and cool fuel module just to access the pump and hoses. Removing these isn't as hard as it sounds, and it's best just to bite the bullet and get everything out of the way (of course this will vary from boat to boat - if you have room to access it, consider yourself lucky).
• The water pressure sensor is at the back of the engine, on top, to the left as you face aft (stbd side), and is installed to the left as you face aft (stbd) side of the power steering cooler. If you put a life jacket on top of the spark arrestor and lay on top of the engine between the risers, it's reasonably easy to access, and my connector was labeled, making it easy to verify:
  
• The power steering cooler where the sensor screws in is just 1/8 NPT threads, so it's very easy to screw in a T-fitting and add a manual gauge to visually verify your water pressure and troubleshoot the sensor. I used this stainless T-fitting and this 0-15 PSI pressure gauge and the setup works well.
• I'm actually surprised this is not a more commonly known maintenance item (maybe it is and I just missed it?). Seems at the very least it should be disassembled and cleaned every couple of years when you do an impeller change!
• When you remove it, you are required to say "Hello, Poppet" in an English accent (Pirates of the Caribbean reference, lol).

That is fantastic work by a tenacious person. Tip of the hat to your efforts!

 

[Espos4]

I wanted to share an issue I've spent a couple of weeks and a lot of money troubleshooting (and puling my hair out over!), in hopes that it helps someone else out there having a similar problem (and I actually found these forums extremely helpful when troubleshooting this and other issues, so wanted to give back):

• 2011 Sea Ray 220 SD, Mercruiser 5.0 MPI Bravo III, Raw Water Cooled with 310 hours
• Low water pressure alarm and occasional guardian mode at low rpms, alarm generally cleared by giving it some throttle, increasing RPMs a little.
• Verified water pressure read out with 0-15 PSI manual pressure gauge on a tee at the power steering cooler. It was in fact low (hovering between 0 & 1 PSI at idle), so the water pressure sensor (another common suspect) was not the culprit.
• Never got an alarm while running on a plane or even clutch ahead, only when coming back to idle, and boat never overheated. In fact, it ran at about 158 degrees max (which in retrospect I thought was a little weird since it has a 160 degree thermostat, but I always chalked that up to a couple of degrees sensor/gauge error).
• Thermostat and impeller less than one year old.
• It would generally be fine (around 1 PSI at idle, and 8-9 PSI at 3000 RPM - a little low, but not low enough to set off an alarm) at start up, and the problem was more likely to present after running for a while, stopping for a while with engine off, and then starting back up.
• Outdrive was off for rebuild 1 year ago, so I was confident there was no obstruction or "bravoitis"

All signs point to an impeller or raw water pump, right?

After doing some research, I replaced the raw water pump with an aftermarket stainless steel Hardin seawater pump (which I am very impressed with, by the way, and can confirm it works fine on the 5.0 MPI. Although the site states it's for a 350 MAG and other applications, many of the raw water cooling system components are the same). I especially liked not having to press the pulley off the old pump and back on the new one.

New raw water pump did not fix the problem or even help.

After further research, I found several instances of the water circulating pump brass impeller cracking or spinning on its hub in these engines, so I replaced the circ pump as well.

New water circulating pump did not fix the problem or even help.

(Side note: I was happy to replace these as preventive maintenance anyway, since they were original to the boat and over 10 years old, but I was truly stumped each time they didn't help because I was convinced there wasn't much else it could be!).

So at this point I was stumped. I had good water flow, no overheating, but a low water pressure alarm that was driving me crazy.

Then I stumbled upon a post in these very forums by @Seapig (Thank You Sir!) referencing the poppet valve in the thermostat housing. In many hours of researching this low water pressure problem, this was the only reference I found to the poppet valve on a 5.0 MPI.

It is located under the housing with the serpentine belt routing sticker and is held on by three bolts (credit to @Seapig for the photo, I added the circle):

It is essentially a pressure release valve, designed to bleed off some of the water pressure at high RPMs. It is very simple, and consists of a spring that is seated in the top housing (underneath the belt routing sticker) that presses a plastic "piston" (#15) down onto a seating surface:

In my case, the assembly wasn't exactly "stuck open", and the spring wasn't broken, but enough corrosion and scale had accumulated that it was allowing raw water to blow by freely.

What this meant was:

• At start up (thermostat closed, poppet open), the problem was barely evident.
• While running (thermostat open, poppet open), the boat was running just a little bit cool (158 on a 160 thermostat) because raw water was bypassing the thermostat via the poppet.
• After shutting down for a while and allowing the engine to "heat sink", and restarting with both the thermostat and the poppet open, the raw water pressure was too low at idle and would trip the alarm and guardian mode until the pressure came up with RPMs.

I disassembled the poppet valve housing this morning, and it was so simple that I just cleaned and reassembled it for troubleshooting purposes.

My raw water pressure immediately shot up to where it should be: 2-3 PSI at idle and 12-14 PSI at 3000-4000 RPMs, and I'm running at 161 degrees at cruising speeds (which makes much more sense on a 160 degree stat). So I'm reasonably confident the demon has been exorcised!

Some other helpful nuggets I learned along the way:

• The seawater/raw water pump is a bear to replace in the boat. I seems impossible, but it's not. I found it necessary to remove the idler pulley bracket, alternator, and cool fuel module just to access the pump and hoses. Removing these isn't as hard as it sounds, and it's best just to bite the bullet and get everything out of the way (of course this will vary from boat to boat - if you have room to access it, consider yourself lucky).
• The water pressure sensor is at the back of the engine, on top, to the left as you face aft (stbd side), and is installed to the left as you face aft (stbd) side of the power steering cooler. If you put a life jacket on top of the spark arrestor and lay on top of the engine between the risers, it's reasonably easy to access, and my connector was labeled, making it easy to verify:
  
• The power steering cooler where the sensor screws in is just 1/8 NPT threads, so it's very easy to screw in a T-fitting and add a manual gauge to visually verify your water pressure and troubleshoot the sensor. I used this stainless T-fitting and this 0-15 PSI pressure gauge and the setup works well.
• I'm actually surprised this is not a more commonly known maintenance item (maybe it is and I just missed it?). Seems at the very least it should be disassembled and cleaned every couple of years when you do an impeller change!
• When you remove it, you are required to say "Hello, Poppet" in an English accent (Pirates of the Caribbean reference, lol).

Glad you got it figured out. It’s a bit of an oddity as I have a 2007 350 MAG raw water cooled. It doesn’t have that poppet valve. I run 158° all day long. Idle psi is right around 1.2 psi. At 3500 rpm I get 8.5-9.5 psi. It’s been like this for the 6 years that I’ve owned it and (knock on wood) no low pressure alarms.

 

[Jeff Copeland]

I am the original poster and wanted to provide an update for anyone following along or reading this in the future.

While cleaning out the poppet valve did seem to help (and may have been contributing to my problem) as noted above, the low water pressure alarm returned.

As noted above, I had convinced myself it wasn't the water pressure sensor because I installed a 0-15 PSI pressure gauge on a tee. As it turns out:

1. The 0-15 PSI gauge I used was very inaccurate (reading zero when the actual pressure was closer to 2) at the low end 0-3 PSI (exactly the range I was troubleshooting), and
2. While my water pressure sensor hadn't stopped working altogether, it was reading about 2 PSI low and was the likely source all along.

I finally bit the bullet bought a new $200 water pressure sensor, and viola, problem solved! (if you are lucky enough to have a twin engine, this is an easy 2 minute swap to check your sensor).

A few more nuggets that might help others out:

• Because I couldn't leave well enough alone, I bought a 0-3 PSI pressure gauge to verify the readings at the low end of the scale and double check the new sensor (and I can confirm an average human can blow about 2.5 PSI with their mouth, so it's very easy to check the low end of a manual pressure gauge!)
• With the new sensor (and verified with the 0-3 PSI gauge), I consistently get 4 PSI at idle with the thermostat closed (which admittedly pegs the gauge beyond 3), and about 2 PSI at idle with the thermostat open (which was reading ZERO on both my bad pressure sensor and my 0-15 analog gauge - the source of all my frustration!), and water pressure gets up in the teens (10-14 PSI) on a plane. I've read elsewhere that the Merc service manual states 7-17 PSI is the normal operating range.
• My parts diagram called for sensor P/N 8M6000623, which is still widely available, but this has been superseded by a new P/N 8M0157742. The old one ending in 623 was brass, the new one ending in 742 (which is the one I got) is stainless. Hopefully the new one is better, more accurate, and/or more reliable.
• I've learned a lot about the raw water cooling system on these boats during this process, and one thing that stands out in my mind is the need to let them engine run long enough to open the thermostat when flushing with freshwater after saltwater use (otherwise the saltwater just stays in your heads/manifolds). This can take several minutes if the engine is cold (such as after a long trailer ride home), and you can see this pretty easily by watching your water pressure reading - it dips by about 2 PSI when the thermostat opens. This may seem obvious, but I'm sure I've made the mistake in the past of firing it up for 5 minutes to rinse the engine, but not waiting for the thermostat to open.

Hope that helps someone! Happy boating!

 

[Jimmy Buoy]

I am the original poster and wanted to provide an update for anyone following along or reading this in the future.

While cleaning out the poppet valve did seem to help (and may have been contributing to my problem) as noted above, the low water pressure alarm returned.

As noted above, I had convinced myself it wasn't the water pressure sensor because I installed a 0-15 PSI pressure gauge on a tee. As it turns out:

1. The 0-15 PSI gauge I used was very inaccurate (reading zero when the actual pressure was closer to 2) at the low end 0-3 PSI (exactly the range I was troubleshooting), and
2. While my water pressure sensor hadn't stopped working altogether, it was reading about 2 PSI low and was the likely source all along.

I finally bit the bullet bought a new $200 water pressure sensor, and viola, problem solved! (if you are lucky enough to have a twin engine, this is an easy 2 minute swap to check your sensor).

A few more nuggets that might help others out:

• Because I couldn't leave well enough alone, I bought a 0-3 PSI pressure gauge to verify the readings at the low end of the scale and double check the new sensor (and I can confirm an average human can blow about 2.5 PSI with their mouth, so it's very easy to check the low end of a manual pressure gauge!)
• With the new sensor (and verified with the 0-3 PSI gauge), I consistently get 4 PSI at idle with the thermostat closed (which admittedly pegs the gauge beyond 3), and about 2 PSI at idle with the thermostat open (which was reading ZERO on both my bad pressure sensor and my 0-15 analog gauge - the source of all my frustration!), and water pressure gets up in the teens (10-14 PSI) on a plane. I've read elsewhere that the Merc service manual states 7-17 PSI is the normal operating range.
• My parts diagram called for sensor P/N 8M6000623, which is still widely available, but this has been superseded by a new P/N 8M0157742. The old one ending in 623 was brass, the new one ending in 742 (which is the one I got) is stainless. Hopefully the new one is better, more accurate, and/or more reliable.
• I've learned a lot about the raw water cooling system on these boats during this process, and one thing that stands out in my mind is the need to let them engine run long enough to open the thermostat when flushing with freshwater after saltwater use (otherwise the saltwater just stays in your heads/manifolds). This can take several minutes if the engine is cold (such as after a long trailer ride home), and you can see this pretty easily by watching your water pressure reading - it dips by about 2 PSI when the thermostat opens. This may seem obvious, but I'm sure I've made the mistake in the past of firing it up for 5 minutes to rinse the engine, but not waiting for the thermostat to open.

Hope that helps someone! Happy boating!

Excellent work and a great write up! I've been dealing with a similar issue with mine, but never looked at it from a water pressure aspect, just having problems getting the engine past 85 degrees. With my idle pressure around 1.1 @ 720rpm and the t-stat closed (engine temp 82F) it helps confirm that the problem stems from too much raw water being let out of the system (down the tubes to the exhaust manifolds and out into the lake). From a temp point of view it explained my cold temps, but with your post it adds another view from the pressure side of things. My low water pressure is probably a result of the same flushing of water out of the system. I know the pump is using more water at idle than my home garden hose can provide so it seems to be operating well, but pressure won't build if there's a leak in the system. I didn't put much stock in the water pressure figure being too accurate before.

Screen showing water pressure at that rpm

 

[Jeff Delbuono]

Great info.

 

[Fill Ess]

Thanks for following up!

I too had a low pressure alarm on one engine and it turned out to be just a bad sensor (in the back on the PS cooler). I did the OEM sensor from Merc which is criminally expensive at $150 - $200. But it's really just a generic GM pressure sensor/sender that you can get for $15 .... or $30 if you go with the original AC Delco. I know everyone says you must use Merc official parts for anything electronic, and I do in most cases, but it would be great to see some evidence that these Merc parts -- which are identical to the GM equivalent -- are any different and worth the 500% markup

 

[Jeff Copeland]

Thanks for following up!

I too had a low pressure alarm on one engine and it turned out to be just a bad sensor (in the back on the PS cooler). I did the OEM sensor from Merc which is criminally expensive at $150 - $200. But it's really just a generic GM pressure sensor/sender that you can get for $15 .... or $30 if you go with the original AC Delco. I know everyone says you must use Merc official parts for anything electronic, and I do in most cases, but it would be great to see some evidence that these Merc parts -- which are identical to the GM equivalent -- are any different and worth the 500% markup

I actually tried a sensor from the auto parts store that cross referenced and looked identical, but it read 0 psi when I tried running it. So I just bit the bullet and bought the Merc part!

 

[Jon T]

I just started having the same problem last week. This is a great thread and gives me a place to start. I had overheating problems early this year and after replacing the impeller and thermostat, I had to have the risers replaced since they turned out to be the culprit. Now I just got the low water pressure alarm. I ordered a new pressure sensor to see if that helps. I have heard that they are a pretty common failure. Fingers crossed. Thanks for the great post.

 

[jfed]

Anyone figure out a GM one that works?