8.1 Mercruiser Problems

Tacks can be problematic to starting and I'm not sure how to take it out of the equation with a multi coil engine.
You have no spark correct?

Mitch might see this thread, he's a smart guy, he might have an idea because I'm out of them for the moment.

Have you swapped ECM's yet?
 
yes , swapped ecm last week in this saga even though I hate doing that with resultant possible frying of a good ecm...but I'm getting desperate

correct...no spark

I dug this up and will try tonight after dark if it doesn't storm...

Re: Mercruiser 496 HO - Difficult Diagnosis

This one almost kicked my butt. Since last June (off and on) I have been working on a 39’ performance cruiser with twin Mercruiser 496HO / PCM555 engines. The starboard engine had an intermittent problem with momentarily losing power. The engine would “lurch” (bog for a second and then return to normal) and the alarm would sound as though you very quickly turned the key off and back on. Sometimes it would completely die. It would do it at idle or while cruising, seemed to be totally random. <br /><br />Connecting a scan tool was first. There were no active or stored codes that helped any. I wanted to watch what happened on the scan tool when the power loss occurred, but every time the engine would lurch the scan tool would instantly lose communication. The fact that the scan tool would lose communication suggested that the ECM was losing power.<br /><br />I began a long process of checking the entire electrical system from the battery to the key switch and back to the engine. It came down to the ECM appeared to be shutting down and/or disengaging the main power relay for no reason. I thought it must be the connections at the relay and/or ECM, but they all were good. I thought it HAD to be the ECM. Swapped it with the other engine thinking the problem would switch engines – it did not.<br /><br />What now? I have checked everything I could think of, tried every troubleshooting procedure in the PCM555 manual, tested and/or swapped out anything that I thought even might be causing the problem, and no change at all! <br /><br />I finally did find the problem by pure dumb luck. It was something I never even would have guessed. Anybody think they have an idea?<br /><br />Dave





Final Diagnosis: Bad plug wire <br /><br />I know, sounds ridiculous. The 496 is a DIS (Distributorless Ignition System) engine, meaning that it has a coil for each cylinder. Four on each side of the engine, mounted on the valve cover.<br /><br />In this case, the #7 cylinder spark plug wire was leaking secondary ignition voltage at the coil tower, arcing to the coil frame. The coil frame and entire valve cover became part of the secondary ignition system. Due to a rubber gasket and appearently pretty decent paint job, the valve cover was not grounded and the ignition voltage found the easiest path to ground (intermittently) was arcing across a gap from a bare spot (chip) in the paint on the valve cover next to a mounting bolt, to a bare spot on the upper part of that mounting bolt. Just so happens that mounting bolt was directly under the ECM . The 40,000 volt discharge (about 2.5 inches away) was creating enough EMI (Electro Magnetic Interference) to momentarily shut down the ECM.<br /><br />I only found the problem because I happen to be at just the right place to actually see the spark under a wire bundle. You could not hear it due to normal engine noise. I pulled the wires away a little and I could hardly believe it, a nice fat blue spark from the valve cover to the mounting bolt. Directly under the ECM . The engine lurch happened at the exactly same times as the spark occurred. I put a jumper wire from the valve cover to a good ground. Problem went away. Took it off problem came back. Still unsure what cylinder it was, I put it on the scope. Found an intermittent misfire on #7. Visual inspection confirmed arcing from the coil tower (under the plug wire boot) to the coil frame. Changed #7 coil, wire and plug. No more problem. <br /><br />Thanks for your responses. Maybe someday this will help someone with a similar problem.

 

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interesting, you could remove all the plug wires from the coils and then check for voltage at the coil
 
I know I'm grasping at straws. While in the bilge and having a helper crank the engine my elbow was resting against the engine somewhere. I thought I felt a tingle. Imagination? Had him crank it again and felt it again. Low intensity , but there. I know what it's like to get zapped by plug wire and this wasn't that. Is there a short somewhere migrating to the pcm and shutting it down? Hmmmm...
 
I've heard of some that wait until its dark, then have someone crank the engine while you look for blue sparks in the dark engine room. Worth a try.

My last boat had 8.1's and I had a ground issue that drove me nuts for months. Sometimes the engine would fail to even try to start. No beeps, no fuel pump running, and no starter cranking. Turned out the main ground on the engine stud had developed some corrosion that was not visible on the outside, but once taken off and cleaned it totally solved the problem for the 14yrs I had the boat.

Also, I do have a Rinda Tech Mate Pro Scanner, but it has an attachment for my current boat's 4.3 V6 engines. Not sure if the 8.1's need a different plug adaptor if that would help. I checked the website and think my adaptor works on my PCM555 and the 8.1's ECM555 so it should work. Let me know.
 
Jimmy Buoy...thanks for the info and inserting yourself into this thread. I did take the grounds off to make sure there were no questionable connectors. They were not broken and there was no corrosion. It is getting dark now and we are about to look for any arcing.
It would be great if we could put the Rinda on and see what it spits out. I am at Bridgeview Marina.
cheers...
 
Update
Looked for any arcing in the pitch black. NO arcing observed.
As to the rpm on cranking thing...I selected the digital readout to provide more accurate reading. Upon commencement of cranking it switches to "station 1" whatever that means.
I tried it on the starboard engine and it does the same thing although the starboard engine starts of course.
 
Jimmy Buoy...thanks for the info and inserting yourself into this thread. I did take the grounds off to make sure there were no questionable connectors. They were not broken and there was no corrosion. It is getting dark now and we are about to look for any arcing.
It would be great if we could put the Rinda on and see what it spits out. I am at Bridgeview Marina.
cheers...

If you're going to be working on it tomorrow I may be able to drop by with the Rinda. You can PM your phone number to me if you like so we can text info back and forth.
 
I know it's spark related but just to confirm, you do hear the fuel pump click when the key is cycled, correct?

I would still lean towards something with the cam or crank sensors. Double check the harness or the connectors closely. Broken wires? Corrosion? Pins not making contact? Damaged or not the same length? Swap your good sensors over (both) from the running engine. Crank longer than normal. Maybe they have to sync somehow during cranking??
 
yes , swapped ecm last week in this saga even though I hate doing that with resultant possible frying of a good ecm...but I'm getting desperate

correct...no spark

I dug this up and will try tonight after dark if it doesn't storm...

Re: Mercruiser 496 HO - Difficult Diagnosis

This one almost kicked my butt. Since last June (off and on) I have been working on a 39’ performance cruiser with twin Mercruiser 496HO / PCM555 engines. The starboard engine had an intermittent problem with momentarily losing power. The engine would “lurch” (bog for a second and then return to normal) and the alarm would sound as though you very quickly turned the key off and back on. Sometimes it would completely die. It would do it at idle or while cruising, seemed to be totally random. <br /><br />Connecting a scan tool was first. There were no active or stored codes that helped any. I wanted to watch what happened on the scan tool when the power loss occurred, but every time the engine would lurch the scan tool would instantly lose communication. The fact that the scan tool would lose communication suggested that the ECM was losing power.<br /><br />I began a long process of checking the entire electrical system from the battery to the key switch and back to the engine. It came down to the ECM appeared to be shutting down and/or disengaging the main power relay for no reason. I thought it must be the connections at the relay and/or ECM, but they all were good. I thought it HAD to be the ECM. Swapped it with the other engine thinking the problem would switch engines – it did not.<br /><br />What now? I have checked everything I could think of, tried every troubleshooting procedure in the PCM555 manual, tested and/or swapped out anything that I thought even might be causing the problem, and no change at all! <br /><br />I finally did find the problem by pure dumb luck. It was something I never even would have guessed. Anybody think they have an idea?<br /><br />Dave





Final Diagnosis: Bad plug wire <br /><br />I know, sounds ridiculous. The 496 is a DIS (Distributorless Ignition System) engine, meaning that it has a coil for each cylinder. Four on each side of the engine, mounted on the valve cover.<br /><br />In this case, the #7 cylinder spark plug wire was leaking secondary ignition voltage at the coil tower, arcing to the coil frame. The coil frame and entire valve cover became part of the secondary ignition system. Due to a rubber gasket and appearently pretty decent paint job, the valve cover was not grounded and the ignition voltage found the easiest path to ground (intermittently) was arcing across a gap from a bare spot (chip) in the paint on the valve cover next to a mounting bolt, to a bare spot on the upper part of that mounting bolt. Just so happens that mounting bolt was directly under the ECM . The 40,000 volt discharge (about 2.5 inches away) was creating enough EMI (Electro Magnetic Interference) to momentarily shut down the ECM.<br /><br />I only found the problem because I happen to be at just the right place to actually see the spark under a wire bundle. You could not hear it due to normal engine noise. I pulled the wires away a little and I could hardly believe it, a nice fat blue spark from the valve cover to the mounting bolt. Directly under the ECM . The engine lurch happened at the exactly same times as the spark occurred. I put a jumper wire from the valve cover to a good ground. Problem went away. Took it off problem came back. Still unsure what cylinder it was, I put it on the scope. Found an intermittent misfire on #7. Visual inspection confirmed arcing from the coil tower (under the plug wire boot) to the coil frame. Changed #7 coil, wire and plug. No more problem. <br /><br />Thanks for your responses. Maybe someday this will help someone with a similar problem.
I just started following this thread and have the same engines as you but my starboard motor has the same issue as the info that you posted above. It is intermittent and happens exactly as stated above and my mechanics (I don’t do my own engine maintenance) have not been able to find the cause. It only happens on the one engine and also could be at cruise or idle but rarely. It might be totally coincidental but I had the spark plugs changed this spring and have not experienced the issue but if it does I’ll have them check for the sparking and report back.
 
douglee25...totally agree. Cam and crank sensors and associated wiring and connectors were high on my priority list. I have switched them out and examined carefully. No go.
 
tripsdad...thanks for the input. The plugs only have about 20-30 hours on them so I'm doubtful. Besides , I am not getting a spark to the plug.
 
Thinking broken wire in the harness. I'd pin out the pcm plug especially the grounds.
 
Thinking broken wire in the harness. I'd pin out the pcm plug especially the grounds.

I think that's where I'd go next as well.

I had a problem like this before and it turned out that one female pin that went into the ecm wasn't making contact from the harness plug. The only way I found it was that all the random codes that it was generating were tied to sensors that had a common wire in the wiring diagram. I went back to the ecm and you could physically see that the female side of the harness was more open than the rest. I crimped it down slightly and it started right up.
 
I think that's where I'd go next as well.

I had a problem like this before and it turned out that one female pin that went into the ecm wasn't making contact from the harness plug. The only way I found it was that all the random codes that it was generating were tied to sensors that had a common wire in the wiring diagram. I went back to the ecm and you could physically see that the female side of the harness was more open than the rest. I crimped it down slightly and it started right up.
I had a friend with a 2005 350x same issue, took a while to solve it being the main wiring harness
 
Unless someone here has it you could call merc support tomorrow and they will email it to you. 920-929-5040
 
thank you scoflaw
Jimmy Buoy is coming this afternoon with his rinda. Knowing what you know about this situation , any guidance going forward? What to look for with the scanner?
 
By the way , I spent the morning swapping out the starters in another Hail Mary. The port starter showed more wear than starboard (probably due to all the cranking) but no change.
 
It will go through a long self test and probably won't throw a code but who knows. It will fire all injectors individually and I believe it will do the same with the coils but not 100% sure with coils.
 

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