Frustrated - Stuck on the hard on a beautiful weekend.

skibum

skibum

Well-Known Member
Jul 30, 2007
2,769
Perry Hall, MD
Boat Info
2005 Sundancer 260
Engines
496 Magnum HO
I have a seemingly simple to resolve issue that I can't seem to fix. I've had a mechanic in my bilge for close to 3 hours, and I've been down there for about 10 over the last week. I have a low sea water pressure problem engaging Guardian mode preventing me from getting on plane. My water pressure gauge and the mechanic's scanner are showing 3.6 psi max.. Things I've done so far:
  1. Checked 1yr old impeller - it was fine.
  2. Pump had scoring on both halves of housing, so I replaced sea water pump with Hardin Marine SS pump. New pump, same wrong pressure.
  3. Bought new sending unit (8M6000523). Got 36 psi on the gauge while running on the muffs. Discovered that new sender was showing 29.5 psi with key on / engine off. Original sender - 0 psi
  4. Returned sender as defective. Installed new one. It also 30 psi with key on / engine off. Original sender - Still 0 psi
  5. Unplugged harness from sender and PCM 555. .1 ohm (1/10) resistance on all 3 wires from plug to PCM harness connector.
  6. I knew I had 1 bad battery, so I replaced both just to make sure that it was not a power problem.
  7. Low psi indicates an inlet blockage, so I pulled the drive and back flushed from the pump inlet through the transom pickup tube with a hose. No blockage found.
  8. Used muffs on removed drive to verify full water flow up to transom pickup tube area. No blockages.
Notes: Original WP sender shows 0 psi key on / engine off. Both replacement senders showed ~30 psi. The new (2nd) sender will slowly settle to 0 psi after about 2-3 minutes with key on / engine off. Running on the muffs with the new sensor shows about 5 psi. I am going to the boat with a mechanical pressure gauge today to find out what my WP really is. I suspect that it is fine, which leaves me with either a bad sender, harness, or PCM. Can anyone think of anything I might have overlooked???
 
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The boat should run now, no? I don't get too concerned with the psi #'s, it's all BS anyway. The true indicator for flow is your temp gauge. Not so smart idea from merc. Problematic device that can leave you in a bad situation.
 
No... The PCM shuts down the engine at 3100 RPM due to no pressure.
 
5 psi on muffs at idle sounds like plenty, mines at 1 or 2. Then what happens?
 
Check the hose on the out drive that goes to the pipe that goes through the transom. Take it off and inspect it and the pipe. The hoses are known to collaspe and the pipe is known to corrode and clog up. Both of which could cause restricted flow and in paticular low volume at high rpms in turn causing your pump to cavitate resulting in low pressure.
 
Thanks, Mitch. I had someone reach up under the bell housing while I reved the engine on the muffs. The hose did not collapse. I pulled the drive, and that end of the tube was in perfect shape. I also backflushed it from the sea pump side with the drive removed. Full flow, no apparent restrictions...

Edit: Re-read your post. Were you saying to remove the hose that is on the inside of the boat at the transom pipe?
 
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A couple of observations.

You mentioned the following:


  1. Bought new sending unit (8M6000523). Got 36 psi on the gauge while running on the muffs. Discovered that new sender was showing 29.5 psi with key on / engine off. Original sender - 0 psi
  2. Returned sender as defective. Installed new one. It also 30 psi with key on / engine off. Original sender - Still 0 psi


Did you get a third sender because this could be your problem. At WOT throttle my pressure is typically around 10 to 10.5 max PSI. Granted I have a 350 MAG but I would think the big block would be similar. Maybe the ECM is also programmed to go into Guardian Mode with high PSI readings as well.

You mention it shuts down at 3100 RPM. Is the engine temperature OK under 3100 RPM? If so then the sender still sounds like the culprit to me.

Dave
 
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techmitch said:
Check the hose on the out drive that goes to the pipe that goes through the transom. Take it off and inspect it and the pipe. The hoses are known to collaspe and the pipe is known to corrode and clog up. Both of which could cause restricted flow and in paticular low volume at high rpms in turn causing your pump to cavitate resulting in low pressure.
Click to expand...

Here is a pic of what he is talking about. This is on the inside with inlet hose to pump removed. This is a very common problem. Does the temp start to climb when the ecm goes to protection mode?
 
Dave, no... I haven't tried a 3rd sender. I just put a mechanical gauge on and still have very low pressure.

Kevin, The opposite end of that connection looks pristine. I backflushed it fromthe pump inlet, and the water flow is perfect... W
e keep our boat on a rack, so that shouldn't really causes a problem.
 
The dreaded Bravo disease happens to rack stored boats all the time. the water still flows you just can't get enough volume. You can either have the mechanic take it all apart and repair it or just have him install a thru hull water pickup and it will be problem solved. This problem is fully documented and has been the subject of conversation on the forum and other forums as well. Stop scratching your head and install the thru hulls you will be satisfied with the results. When I test for this I stick a piece of 1.25 hose on the back of the seawater pump inlet side and stick it over the side and so far every time it proves the pick up tube is clogged.
Rob
 
The boats kind of new to be corroded to that point, and the OP hasn't said anything about an overheat condition.
 
Okay the boat is a 2005 and a big block so I say it's old enough to have a corroded water pick up tube! My friend has a 2003 that had this problem by the third year, it was repaired under warranty at that time and last year he had to put thru hull pick ups in because it happened again. The easiest way to rule it out is stick a piece of 1.25 hose on the seawater inlet side of the pump and stick the other end over the side of the boat or in a pail of water. If the pressure goes up that IS your problem.
Rob
 
Mystery solved - sorta...

I checked ALL of the plumbing. Transom inlet looks as new as the day she was made. No clogs, obstructions, restrictions, etc.. All hoses removed one at a time and water flow visually verified. I removed, inspected, and reinstalled the new water pump. No problems found.

I tested the boat in the water (as opposed to on the muffs) with a mechanical gauge. I had no electronic sender installed (I had one plugged in to keep the PCM happy, but it was not installed in the cooler), so I could only get a reading of 5-6 lbs at about 2500 RPM before Guardian started dropping cylinders. Based on this, I assumed that I would see more pressure at higher RPMs. I was speaking with my new mechanic (see sender #3) on the phone, and he recommended that I buy one more sender from a different store since he had personally bought 2 bad ones in a row once. Here's a sender recap:

  • Original sender - reads 0 KO/EO - max pressure 2.5psi at any RPM - throws engine into Guardian mode at 3100 RPM
  • 2nd sender - read 29.5 KO/EO - returned
  • 3rd sender - read 30 psi KO/EO - had mechanic on board for a couple of hours. By the time he left, this sender read 43.4 psi KO/EO, which is the same reading I get with NO sender plugged in. (And with this, my former mechanic has officially broken more things on my boat than he has fixed. I'm done with him.).
Once again, I got 30 psi with key on / engne off. With the engine running, it gets to 36.5 psi at about 1800 RPM and stays put up to 4K. Knowing that the PCM wouldn't go into Guardian with 36 psi sea water pressure, I decided to take an on water test. I took it out for about an hour and a half. She runs fine at all speeds. The temperature gauge locked right in at its normal 165-167 degrees for the duration of the cruise. Conclusion - PCM problem (I have already metered all 3 wires in the harness from the PCM connector to the sender plug. All have less than 1/2 ohm resistance).

When I got back, I started talking with the owner of the marina about this. He agreed that this is an electrical issue. He started telling me about a lightning strike in front of the marina last fall. He said that it blew out all of the electronics in his house, and a couple of the security cameras in the marina bar. My boat sits on the racks, which are located between his house and the bar.

Looks like the water pressure section of the PCM, and the sender got blown out. :smt101
 
Wow sorry to hear that you have electrical Gremlins. It's good that you found a mechanic that's willing to help sort it out. It seems like you're being forced to solve this yourself. I've been down this road and wish you much luck. Did you check the big harness plug? I helped a friend who had issues with a new boat and we found the pins in the big harness plug were damaged from the factory. The boat had been back to the dealer three times and they still couldn't figure it out.
Rob
 
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