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View Full Version : 496 HO Mag Mercruiser - can ANYONE help?



ParkerRiverRat
05-01-2006, 06:40 AM
This post has be x-posted in other discussions so I'm sorry if you've read this already!
ok, our 2003 Cat is having problems... it's been in he shop for 8 months now and they STILL can't figure out what's wrong with it. So, if ANYONE has ever heard of this problem and has any solution, please let me know.
Here's the problem...
I have a 2003, 496 HO Mag Mercruiser Engine. The Engine stalls when run for approximately 10 minutes. Restarts but continues to stall. When the engine Analyzer is plugged in to ECM, engine will not stall but the screen of the analyzer will go blank when engine would normally stall. The screen will restart wen engine is shut off and restarted.
We've already paid $3500 because it was supposidly a head problem, but that wasn't it...
Also, we got a new computer because the Mercruiser people told us that might be the problem... and that didn't fix it either.
If anyone can think of anything, or knows someone we could call who knows the problem it would help greatly! We've got a trip planned every weekend in June, so we really need it fixed by then.
We're taking the boat to a new shop next week, but after talking to them on the phone they're not sure what's wrong either...
Everyone's telling us they've never heard of this problem, but we're hoping in an engine that's 3 years old someone else has had the problem and can let us know how they fixed it!
Thanks!

Riverjet502
05-01-2006, 06:49 AM
Just a thought here is all? How about a phone call to "Merc" and see if they can give you some info or tests for your dealer to do to that would be more specific to your problem. A dollar to a doughnut says your not the first or only person to have this problem. I've done this with cummins and it sure helped me.
Let us know what you find...

Beer-30
05-01-2006, 06:49 AM
Has anyone checked the power cables yet? I have the same motor, but 2001. I haven't had cable issues on it, but on my dually, I would be driving along and it would die. It would restart, but some time later, die. Had it towed to the dealership. They tore the dash apart and found that the ground the stereo shop had used for the Clifford alarm was loose. The screw holding it to the metal had loosened. When I would hit a bump or turn just right, it would open the ignition circuit.
I would look at every ground on the motor/batteries. Then, if all looked good, I would tap into the main power into the ECM and tie in a volt meter. Run as normal and see if the power goes away when it dies. That would confirm if the power was still constant to the ECM.
Also power supply to the coils, maybe? It would have to be something that would kill all of them.
Oh, ya know, it could be as simple as an ignition switch. Might start there with the volt meter and see if it is going open when it warms up from some current flow.

Beer-30
05-01-2006, 06:50 AM
Just a thought here is all? How about a phone call to "Merc" and see if they can give you some info or tests for your dealer to do to that would be more specific to your problem. A dollar to a doughnut says your not the first or only person to have this problem. I've done this with cummins and it sure helped me.
Let us know what you find...
They don't do that. They tell a person to go to a dealer first; then they say they will step in if the authorized shop can't find it.

phebus
05-01-2006, 06:59 AM
When I was talking with Bart at Alco Marine the other day, he told me of a problem with a boat stalling, but when hooked up to the scan tool, it would never stall. The shop had the boat for repair of the stalling problem, but everytime they worked on the boat, they had it hoked up to diagnostics, and they couldn't get it to stall, and recreate the problem. Finally the boat was run without the scanner attached, and the problem was discovered.
I can't recall what the problem was, but call Alco, talk to Bart, and possibly your issue will be the same

ParkerRiverRat
05-01-2006, 08:15 AM
Just a thought here is all? How about a phone call to "Merc" and see if they can give you some info or tests for your dealer to do to that would be more specific to your problem. A dollar to a doughnut says your not the first or only person to have this problem. I've done this with cummins and it sure helped me.
Let us know what you find...
We've actually talked to the Merc people a few times and so has the person who was been working on the boat the past few months... they're the ones who gave us a new computer thinking that was the problem, they gave us the computer free but that didn't fix it. They've also been claiming they've never heard of this problem.

phebus
05-01-2006, 08:31 AM
I assume they have hooked up a pressure gauge to the fuel rail to rule out a vapor lock problem?

ParkerRiverRat
05-01-2006, 08:36 AM
I assume they have hooked up a pressure gauge to the fuel rail to rule out a vapor lock problem?
We're not sure that the guy who's working on it did anything "for sure" but he claims he's been over everything and can't figure out what it is. We'll add this to the list though! Thanks!

Jbb
05-01-2006, 08:36 AM
I have seen a problem similar to this .....and it was something in the fuel tank partially covering the pickup...

ParkerRiverRat
05-01-2006, 08:38 AM
When I was talking with Bart at Alco Marine the other day, he told me of a problem with a boat stalling, but when hooked up to the scan tool, it would never stall. The shop had the boat for repair of the stalling problem, but everytime they worked on the boat, they had it hoked up to diagnostics, and they couldn't get it to stall, and recreate the problem. Finally the boat was run without the scanner attached, and the problem was discovered.
I can't recall what the problem was, but call Alco, talk to Bart, and possibly your issue will be the same
Alco is actually the new place we're taking it too... So, hopefully they'll be able to fix it then! We'll give them a call though too! But we're supposed to drop it off over there in the next week! Thanks!!! That gives us hope that it might actually be fixed this time!!!

ParkerRiverRat
05-01-2006, 08:40 AM
Has anyone checked the power cables yet? I have the same motor, but 2001. I haven't had cable issues on it, but on my dually, I would be driving along and it would die. It would restart, but some time later, die. Had it towed to the dealership. They tore the dash apart and found that the ground the stereo shop had used for the Clifford alarm was loose. The screw holding it to the metal had loosened. When I would hit a bump or turn just right, it would open the ignition circuit.
I would look at every ground on the motor/batteries. Then, if all looked good, I would tap into the main power into the ECM and tie in a volt meter. Run as normal and see if the power goes away when it dies. That would confirm if the power was still constant to the ECM.
Also power supply to the coils, maybe? It would have to be something that would kill all of them.
Oh, ya know, it could be as simple as an ignition switch. Might start there with the volt meter and see if it is going open when it warms up from some current flow.
The person working on it claims things have been checked.. but we've suspected it may be some type of electrical problem since we've had issues with the stereo because the person we bought the boat from got a good system but not the best wiring job, so if we open the electric hatch the stereo shuts off, or other random things that use the electrical. So that may also be an issue. Thanks!

phebus
05-01-2006, 08:47 AM
I would remove the stereo wiring from the system, and see if the problem was solved. If it was, I would then take it to a good stereo shop, and have it rewired.
It sucks having a boat in and out of the shop while they are chasing demons. If you could do a little trouble shooting on your own, the process might go a little smoother. Removing the stereo from the wiring system would be real easy, and might just solve your problem. Remember though, when trouble shooting, never do more than one thing at a time before testing. You don't want to end up chasing your tail.

ANXIETY ATTACK
05-01-2006, 08:51 AM
check the engine main harness at the cannon plug, sometimes the pins have corosion or a pushed in pin, or lack of a tight fit between pin and female pin

PHX ATC
05-01-2006, 09:31 AM
PINS! Check the entire wiring harness at all the connection pins. Make sure all the pins are straight and not bent, laying sideways or broken.
Camshaft sensor and crankshaft sensor are especially vulnerable to this.
I had this exact problem (I got an entirely new engine from Merc for it) and it was the crankshaft sensor pins weren't connecting but about 1/2 the time, which caused the computer to freak out and shut it down.

Just Tool'n
05-01-2006, 10:26 AM
I was going to suggest looking at the cam & crank senors also.
If they can not get it to reproduce on scan tool it is time to start back probing senors using a lab scope, along with fuel guage to check flow & delivery pressure.
A lab scope will show voltage & signal waveform going back to the computer.
A voltage signal will dropout so the computer reads zero or low volts from that sensor. Also a ground is supplied to most sensors', plus a 5v refrence voltage to each sensor. These are all done via the ECM ( Computer ).
This is what is used in the automotive industry to find drivability problems. not a lot of people using in the marine side from what I have seen.
Just a lot of shop throw parts at what they think is the problem.
Hope this better helps you understand the working side of how electronic controls work.

Raylar
05-01-2006, 06:53 PM
It sounds like this problem is definitly a power problem. the only way a scan tool will go blank after its plugged in and powered on is for the power to the scan tool being interrupted. This is probably a power to the motor problem in the harness, power leads or connections. I would have them hook up a voltmeter to the power terminal on the motor harness and watch what it does when this situation occurs. If the voltage (power) is even interrrupted for a second it will drop to zero and it will kill the motor and the scan tool will have to be restarted when this happens. This needs a good wiring troubleshooting job to find the "gremlin", its there just hiding. This repair should not take any good shop months to fix this problem.
Ray @ Raylar

Beer-30
05-01-2006, 07:07 PM
Exactly. Start with the basics.

TCHB
05-01-2006, 07:12 PM
I had the same problem with our Howard. It was vapor locking. We had to install a new low pressure fuel pump.

Family Jewell
05-01-2006, 07:15 PM
The person working on it claims things have been checked.. but we've suspected it may be some type of electrical problem since we've had issues with the stereo because the person we bought the boat from got a good system but not the best wiring job, so if we open the electric hatch the stereo shuts off, or other random things that use the electrical. So that may also be an issue. Thanks!
Here is a question! Did the previous owner have the same problem with the boat after the stero was installed/worked on /or changed in any way?

boater012
05-01-2006, 07:56 PM
I also have a question?? Was the boat worked on at all PRIOR to this happening?????????? I had a boat with a 350 mag ho that had been worked on by one of the owners employees. What it was doing was exactly what you describe. it would run fine for ten minutes then start running like crap and dying almost within a minute or two of it running poorly. other times it wouldn't even run poor it would just die all of a sudden. What ended up happening was, The guy had had the thermostat replaced and his mechanic left out the holder for the thermostat and just dropped in a new thermostat into the intake and bolted it back together. Well what was happening is that the water in block would not circulate that way. The water would enter the t-stat housing and go right into the exhaust manifolds and go right over board. The water in the block was staying there and not circulating at all. The motor would over heat in the block and the computer would shut it down. The temp sender for the guage and for the ecm were in the top of the thermostat housing where the water was flowing good not causing the scan tool or the gauge to show an overheating problem at all. I actually figured it out by unhooking the wire for the knock sensor and the crankshaft sensor to hook up to the lab scope ( previously described invaluable diagnostic tool) When I unhooked the crankshaft sensor the timing cover felt really warm. Well when I reached under the manifolds to unhook the knock sensor wire the knock sensor burned my fingers badly. I used a non contact thermometer. (another invaluable tool) to check the block temp. It was like 225 degrees if I remember correctly. And the thermostat temp and the exhaust manifold temp was like 130 degrees. The fix was easy but a complicated diagnostic procedure none the less. Im not saying that that is the problem but ive had two other bpoats come in with similar probs and the plastic ring in the t-stat housing was broken into pieces and were clogging the water flow in the block. The boat went to 4 other shops before I even saw it. When I finally found it, the owner kept the boat instead of selling it. (which was the original plan). I was leaning toward ecm failure or a bad sensor but I wanted to check everything before I just threw parts at it until it was better. (not a good way to fix anything btw) My .02

Marty Gras
05-01-2006, 09:31 PM
If you have the 'DDT' going dead, then you are losing the 5 volt (can one voltage) The 'DDT' uses 12 volts from the battery, but it's signal comes thru the 5 volt reference line. I would first look at the three fuses that are on top of the motor, replace each one. Then with that out of the way go to the relays. #1 fuel pump relay, won't effect the 'DDT' (fault code) #2 injector power relay, won't effect the 'DDT' (fault code) #3 The main power relay (read that ECM POWER) WILL EFFECT the 'DDT' and how the motor runs. Few wire harnesses go bad, but relays and connectors that fail are common practice. Look at the power circuit to the ECM, that includes GROUNDS too.