8.1 Merc Momentary dies*Fixed*

HappyKamper

HappyKamper

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2005 496 Merc(8.1)
At idle, cruise, acceleration at random times the engine will die for a moment. At cruise it will go down 200-300 rpm's then go back to normal. At the same time I will get a 1 second long alarm. The rpm read out, oil pressure read out, engine temp read out all go blank. Engine runs fine except at random times does this.
I wiggled all wiring harness connections. I checked contact on ignition key and tach. All battery terminals are clean.
Could it be a dirty flame arrestor?
IAC sensor?
Crank position Sensor
Fuel filter was just changed
Fuel is fresh
Any thoughts?
 
Sure sounds like a loose/corroded wiring connection. Most of the MerC engines have a main electrical connector that's a prime suspect. As you check "connections" be sure to look at Ground connections as well as the Hot side.
 
When one of my blue MPI I/B's did that on rare occasions for almost 2 years, I couldn't find the problem for the life of me . Nothing ever showed on the Rinda diagnostic scans and the Wiggle Test never changed anything because they were always done when the problem did not manifest.

.....Until after a test run with the installer mechanic that again did not show any hesitation underway, I went to start the engine at the dock and for the 1st time it would crank but not fire. No sound of fuel pump priming that engine which was a good clue. THEN with key ON , the mechanic wiggled the harness wires to the fuel pumps and fuel pump relay and the pump started to run. It was a very slightly bent pin in the relay connector. It didn't even look bent to me but the experienced mechanic saw it right off , carefully straightened the pin and reconnected. No problem with that connection or the hesitation ever since so you might care to more carefully inspect that area.

Also suggest you look carefully at high and low pressure fuel pump wiring connections . They have a tendency to corrode just enough too cause fuel delivery problems that can be intermittent at 1st.

But,... none of the above will cause gauges to go blank , so as j-d suggests check that main harness connector and battery and battery switch and ignition switch & + supply, ignition fuse holder contact, and engine ground connections (maybe lanyard switch too) for loose, corroded, (bent?) or grounded/chafe-shorting connections. Even a momentary internally-shorting gauge can cause your symptoms.

The quick alarm was likely due to the resultant loss of oil pressure until rpms popped back up with renewed combustion activity.

Boy, all that sure narrows it down, eh?
 
My first thought was the main connector. Second was battery ground to block. I'm just copying JD
 
Today I checked the main connector and battery to block connections.
Seem fine.
Last week when I was wiggling wires while it was running I got
zapped by an ignition wire. I thought that should not be, so I replaced
those wires today.
Now I need a test run.
I will keep you posted
 
quote:
Originally posted by jtybt15

My first thought was the main connector. Second was battery ground to block. I'm just copying JD

Click to expand...


Exactly. Had the same problem and it turned out the battery cable attach lug to the solenoid was loose. Always do a complete condition check, starting at the battery and check each and every connection....then check the ground cable.
 
Could it be a limp home mode cuz ECM not getting the right signal thru faulty connection.
 
I changed the spark plug wires and all is good. I did a run to Shelter
Island for the holiday and the boat ran like a champ.
All I can figure is a wire would short to the motor momentarily.
Possibly triggering a knock sensor?
 
Good find and fix.
Probably just momentarily cutting off ignition pulse to a cylinder, though on an 8 cyl engine at, say, cruise of 3200(?) rpm dropping one cylinder should show a loss of about 400 RPM (1/8th) until the shorting to ground stops.

I doubt the knock sensor was involved and retarding timing, as I believe that sensor is tuned to sense a very specific frequency which just dropping a cylinder should not produce. But who knows...?
 
Some knock sensors can trigger a drop in couple cylinders as well as some retarding timing.
 
Glad to hear it was a learning experience and a reasonably cheap fix. Thanks for posting the results of your troubles. It helps all.
 
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