Mercruiser 496 won't stay running

[Aguyindallas]

Hey all,

I'm still having issues with my port engine. A couple of weeks ago, I started it up. It ran for 15 or do minutes and died. At that time I was able to restart it but it would immediately die again. Eventually it wouldn't restart at all.

Today I replaced the IAC since much of my research pointed that way. Engine started right up and this time ran for 5 minutes and died. It would not restart.

As an aside (maybe), there is a 5V DC CKT LO indicator on my Smart Craft.....but in all fairness, that error has been there for at least 2 years.

Any ideas on where to look next? I reset several sensor connections just for good measure.




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[romanservices]

First place I would start would be checking for trouble codes. If that's clear then check that you are getting fuel, check fuel pressure, fuel filter ect...

 

[Espos4]

5volts is the reference voltage used by the sending units and control modules. If you are not getting the full 5 volts supplied to the components, than the information reported back to the ecm will not be accurate.

For example, the ecm sends 5v to the throttle position sensor. The sensor is a variable resistor. So the voltage returned to the ecm varies with throttle position. If you are sending less than 5v to the sensor, the voltage returned for a given throttle position will be lower than it should be. The result is that the ecm doesn't have accurate info on the throttle position.

I inderstand that you've had this code for 2 years, but maybe the reference voltage has dropped so low that it has finally become a problem.

Break out a volt meter meter and see what your reference voltage is. Pick any 3 wire sensor and you should be able to find the reference voltage on one of those wires.

 

[Bt Doctur]

start at the battery connection and all the ground lead connections located on various parts of the motor

 

[aerobat77]

so for verification : it cranks over but does not fire up or does it not even crank ?

 

[Aguyindallas]

so for verification : it cranks over but does not fire up or does it not even crank ?

Correct. I arrived at the boat, stone cold. Starts and runs for a few minutes and died and will not restart. Plenty of crank, not starting.

From stone cold, it starts (and as always, displays the 5V D.C. Lo on the Smart Craft.

It runs for a while 5-15 mins and shuts off.

Makes me think either a temp sensor or a pressure sensor (which electronically is variable) is being pushed to a failure point (from partial short to full short) and it's shutting it down.


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[Aguyindallas]

start at the battery connection and all the ground lead connections located on various parts of the motor

I haven't verified elsewhere, but this season, I replaced all of my batteries so I checked cables etc. No corrosion and great connection. And....the issue was before the battery swap.


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[Espos4]

When it warms up and doesn't start, you need to check for spark, if you have spark, then check fuel pressure. Let us know what you find.


Heat could also be a problem. On a car engine, a common trouble spot for a warm engine that stalls is the crankshaft position sensor.
A Faulty sensor will shut down when it gets hot. Once it cools off, the crankshaft position sensor will function again.

I'm not saying that this is definitely your problem, but just something to consider. One advantage that you have, an identical engine where you can swap components from to test parts.

 

[paulswagelock]

Have your cool fuel modules been replaced or rebuilt yet?
Get a fuel pressure gauge and connect it to the Schrader valve. I think you might find the fuel pressure is near zero when it won't restart.
Your symptoms are similar to the ones I had at the beginning of the end of my CF module.

 

[CliffA]

Correct. I arrived at the boat, stone cold. Starts and runs for a few minutes and died and will not restart. Plenty of crank, not starting.

From stone cold, it starts (and as always, displays the 5V D.C. Lo on the Smart Craft.

It runs for a while 5-15 mins and shuts off.

Makes me think either a temp sensor or a pressure sensor (which electronically is variable) is being pushed to a failure point (from partial short to full short) and it's shutting it down.


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this may indicate a problem with a sensor.....when the engine is cold the ECM ignores the input signals from the sensors....this is called 'open loop mode'......once the engine reaches operating temp the ECM starts making adjustments based on sensor input....this is called 'closed loop'....it sounds like once your engine warms up and goes into closed loop a sensor is throwing bad signals and the ECM can't compensate enough to keep the engine running......one sensor that is considered to be the 'master sensor' is the coolant temperature sensor....if this sensor is not sending correct signals to the ECM then all other sensor data can be messed up......you might want to verify that sensor is acting properly as a start....

cliff

 

[Henry Boyd]

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Makes me think either a temp sensor or a pressure sensor (which electronically is variable) is being pushed to a failure point (from partial short to full short) and it's shutting it down.


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Don't limit yourself, the entire engine, ECM and its support systems function on 5 volts. Keep in mind the voltage source is 12ish. "Something" turns 12 into 5 for the ECM system, if that isn't functioning correctly, or if it is but 5vdc is not getting to the ECM, nothing will function correctly.

If it were me, I'd call it a day on diagnosing this myself and get a Merc technician on the case. You can quickly make matters worse. A couple of years ago one of our members had a problem with one of his 8.1s. He thought he had a bad ECM (he did). To test his theory he swapped the good ECM from his other engine, and cooked that as well because the ECM wasn't the cause, but the result of a bad IAC.

That also makes wonder about the IAC you replaced. Was it a Merc part, or an "equivalent" from an auto parts source? I've read of compatibility issues on IAC replacements, even OEM grade auto parts.

Henry
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[scoflaw]

I would get the scan tool on this motor, it will show you what the ecm is seeing for out puts from your sensors. Like mentioned , make sure your ecm is seeing that the engine temperature is high enough. Even though your temp gauge says 160, doesn't mean the ecm is seeing that. That will effect the whole show.

If that checks out, I'd start hunting down the low 5v reference signal to each sensor. While your in there you can clear that code that pops up, see if it returns and will probably point you in the right direction. That tool will be quite useful for this problem of yours

 

[Aguyindallas]

I would get the scan tool on this motor, it will show you what the ecm is seeing for out puts from your sensors. Like mentioned , make sure your ecm is seeing that the engine temperature is high enough. Even though your temp gauge says 160, doesn't mean the ecm is seeing that. That will effect the whole show.

If that checks out, I'd start hunting down the low 5v reference signal to each sensor. While your in there you can clear that code that pops up, see if it returns and will probably point you in the right direction. That tool will be quite useful for this problem of yours

Can you point me to a proper scan tool?


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[Aguyindallas]

I had planned on unplugging one sensor at a time (with a stone cold engine) to see if it will eliminate the 5VLo error and hoping that points to the culprit. Good idea or bad?


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[scoflaw]

Rinda scan tool for the 555 ignition. Unplugging sensors won't work. You could back probe the sensors with a DVM to see if your getting the 5v , but I would get the tool on it first.

 

[Aguyindallas]

Interesting turn....

Went to the boat tonight, one by one I unplugged sensors. No results.

This time, even stone cold, it wouldn't start. Plenty of crank, no fire. Gauges and Smart Craft all operating as normal (still have the 5V DC LO error). Disconnected a spark plug and tried to test Spark......no Spark.

Any ideas?


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[Aguyindallas]

Went out this evening and tested a theory.

When you crank the starboard (working) engine, it starts as expected, and you can look at the Smart Craft display and see the RPM's climb from zero and then of course, it fires up.

On the non-working port engine as I'm cranking, there is nothing displayed for the RPM's on the Smart Craft.

This leads me to think that whatever sensor provides that output to the ECM and then to the Smart Craft is not working. I "think" it's the Cam position sensor that is located at about the 1 o'clock position above the crankshaft...correct?



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[Espos4]

Re-read post #10.

A picture would help too. That could be the crankshaft position sensor.

If it is easy to remove, swap it with the other motor.

 

[Aguyindallas]

Re-read post #10.

A picture would help too. That could be the crankshaft position sensor.

If it is easy to remove, swap it with the other motor.

Well, I went back out and swapped the sensors from one engine to the other. Still no correction. Boat will not start at all and no Spark. Admittedly I haven't checked any voltage on that sensors harness so I suppose the problem could be there. I've all but given up.



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[PlayDate]

Well it is a guess ......I would disconnect and clean the contacts on the engine connector. We just had one that behaved like yours which lost 12 volts moments after starting up. It was difficult to diagnose because it looks like several problems. In the end, we found it by watching the 12 volts die on an engine analyzer just before the engine quit. Then, it would not restart because the 12volt wire did not have power due to a bad connection. It might also solve your Smartcraft error message.

It was similar to your story because it would start cold and die within a few minutes.