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I have an intermittent fault on my starboard Mercruiser 496…

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I have an intermittent fault...
I have an intermittent fault on my starboard Mercruiser 496 MAG engine. Engine will start after a rest period (20 mins or more) run for a couple of minutes then cuts out and will not restart (cranks over OK).
I've had a Mercruiser service engineer to the boat with diagnostics plugged in, no fault code.
Fuel supply all OK, engine not... Read full answer
I have an intermittent fault on my starboard Mercruiser 496 MAG engine. Engine will start after a rest period (20 mins or more) run for a couple of minutes then cuts out and will not restart (cranks over OK).
I've had a Mercruiser service engineer to the boat with diagnostics plugged in, no fault code.
Fuel supply all OK, engine not starting due to no spark on any cylinder. No obvious problems with wiring looms. Any suggestions - the service engineer seems to be struggling and my bill is getting bigger!
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Answered in 2 hours by:
8/22/2009
mercmastertech
mercmastertech, Service Manager, Master technician
Category: Boat
Satisfied Customers: 249
Experience: Service Manager, Master Mercruiser, Master OMC Cobra and Certified Volvo Penta Technician
Verified

Hello,


Welcome to JustAnswer.com and thank you for your question.


An intermittant running/no start issue that you are experiencing on your Mercruiser 496 magnum engine will be near impossible to diagnose over the internet so I can only ask a few questions and offer some suggestions to hopefully assist your service technician resolve your issue.


Has the technician isolated the boats wiring harness from the starboard engine? Meaning, has he disconnected the boats wiring harness from the engine and provided his own harness for troubleshooting (this is a very common diagnostic procedure that will eliminate the possibility of the no start condition being caused by a shorted or grounded wire in the boats wiring loom or gauges as well as a faulty emergency stop switch). This procedure will also require the technician to unplug and inspect the cannon plug on the engine (round plug where the boats wiring harness connects to the engines wiring harness - common spot for a break in the wiring due to corrosion or loose connection).


Has the technician performed a wiggle test on the engine harness and sensor connections with the ignition on or when the engine is running to attempt to locate a possible loose connection within the engine harness or at the sensor connections? The wiggle test may also locate a possible broken wire inside the loom that is intermittantly making and breaking contact as the harness/engine warms up.


Has the technician swapped PCM's from your port engine to the starboard engine to check for a possible issue with the starboard engines PCM after warming up?


Is your technician using the PCM555 diagnostic manual and troubleshooting charts when performing the diagnostics on your engine? If not, he may be relying solely on the EFI scan tool or CDS (laptop) to tell him why the engine will not start, this is normally not a very successful way to diagnose a problem on an EFI engine (especially a fairly complicated engine such as your 496 mag). There are sensors and instances that will not produce a fault code in the ECM (ie: bad crank position sensor, loose/corroded connection in the harness).


The PCM555 diagnostic manual (Mercruiser part number 90-863757002) is an absolute must when troubleshooting engines that use the PCM555 for the EFI system such as your 496 mag, the manual will have full array of diagnostic charts and wiring diagrams to assist a technician in diagnostics. The PCM555 diagnostic manual will have the diagnostic chart for a no start situation that you are experiencing on your engine as well as numerous wiring diagrams for each sensor and harness on your engine to assist the technician in his diagnostic procedures. If the technician is not using this manual, I would strongly suggest that he acquire the PCM555 diagnostic manual in order to expedite the diagnostics and repair on your engine.


Normally, a no start condition such as your circumstance is much easier to diagnose then say... an intermittant engine miss or hesitation. However, the technician must use all availble resources and service manuals to accurately diagnose the problem in an expedited manner.


I apologize that I could not provide more specific information to pinpoint what may be causing your no start condition on your starboard engine but I hope that you or your technician can find some of this information useful. Thanks again for your question.


BTW, If I were taking a wild shot in the dark, I would be looking closely at...

 

1. The crank position sensor (and harness connection at the crank position sensor).
2. Loose/broken connection within the engine harness or at the sensor and PCM connections.
3. Corroded or loose ground wire(s).

4. Possible PCM issue as the engine warms up (hopefully this is not the cause in your case).

 

mercmastertech
mercmastertech, Service Manager, Master technician
Category: Boat
Satisfied Customers: 249
Experience: Service Manager, Master Mercruiser, Master OMC Cobra and Certified Volvo Penta Technician
Verified
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Customer reply replied 9 years ago

Thanks for the tips. I will pass on to the local Mercruiser mechanic whom seems to be struggling (and my bill racking up!). I do have a problem with the starboard trim sensor signal (indicator jumps around erraticaly) so am wondering if this may be related (ground fault?). I've 'accepted' answer and left positive feedback. I may be back in touch depending on how things progress.

Thanks again, Guy

Hello Guy,

 

You're welcome! I hope that you get your running issue on your starboard engine resolved quickly.

 

The trim sender that sends a variable ground signal to the trim gauge is a fairly high failure rate item, it's possible that it may be related to your engine issues but unlikely. The trim sender will either fill up with water and start rusting internally or the sender wire will get pinched behind the bell housing at the outside of the transom which will result in an erratic or a non responsive trim gauge. If the sender has water in it, a trim sender rotor kit can be used to repair the sender as long as the rust isn't extensive. If the rust is extensive or if the sender wires are pinched, then the sender must be replaced.

 

Thanks again for your question and for the "accept".

 

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Customer reply replied 9 years ago

Thanks once again for your help.

For your information the fault proved to be the crankshaft position sensor which was one of your suggestions.

The problem I'm having now is locating a replacement - local dealer in Singapore telling me 4-6 weeks. Trying to source one on the internet.

 

Hello,

 

That's great news! I'm glad the intermittant cause of the no start condition has been found.

 

Wow, 4-6 weeks for a CPS from Mercruiser to Singapore? I wonder why the long delay, I just checked and Mercruiser has the CPS's in stock at all of their warehouses in the US, does your dealer order from an international warehouse? If you cant find one on the internet to have it delivered quicker, you may consider having your technician look for the CPS at a GM automotive parts store if you have one available, I'm told that the automotive 496 CPS is the same (but it would be best to have the old CPS in hand to compare if you choose to go this route, I do suspect that the parts are the same but I have not confirmed it).

 

I hope you get your engine up and running again soon, thanks again!

 

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