Misfire, erratic tach

S

Signature 99

New Member
Jun 28, 2022
13
Boat Info
Searay Signature 230
Engines
350 mercruiser
Hello, I have a ‘99 signature 230, 5.7 merc., 400 hours. Recently tach jumped all over and engine misfiring. Tach erratic with no change in engine as well.Only does this after 1 1/2 -2 hours running time. Idles perfect. Runs perfect for short excursions.
This spring had I new distributor, rotor, ignition wires and plugs done as preventative maintenance. Other than that, only gimbal besring and water pump replaced. Boat has been basically flawless for 20 years.
Same marina has serviced it since new. Hard part is recreating the problem for tech. Any suggestions. ?
Thank you!
 
Last edited:
The best clues are that it misfires, not stalls, and only under load when hot. Based on your description, it seems you ruled out tune, carb, fuel contamination. Only thing left is spark.
 
Thanks for the responses, will be out this weekend and will report back.
 
Ignition work by a new owner is likely cause. My 2003 5L parts are identical to the auto equivalent except the threads are different. I recall noticing and not trying to tighten.
 
Unhooked tach and hoping it’s that simple. I’m the original owner btw.
 
If you did not personally install Mercruiser parts, see if the rotor is tight or can be rattled around on mismatched screws.
 
Have ran it twice now with tach unhooked for at least 90 minutes. No misfires, runs smooth. Only odd thing is a couple of brief instances where boat doesn’t slow but there is a “dragging” sensation. Engine changes sound briefly but doesn’t slow? Sorry, that’s the only way I can describe it.:(
 
Check all the wiring plugs and connections for any sign of corrosion. Is your engine a carb w/ Thunderbolt Ignition or FI?
 
To be clear, I was talking about the wiring harness plugs (like to the ECM and sensors), not ignition wires
 
Update. After 12 hours of running without tach all was good, which consisted of 30-90 minute rides. Go out today, starts misfiring after 2 hours of run time?
 
I bought an automotive rotor as it was available and cheaper. However, there was abnormal resistance as I screwed the new rotor on. Lacking my usual impulsiveness, I backed out the new screws and laid the threads up against the old. They did not match. The car rotor screws were now metric or something. It would be easy enough to screw the new screws in till they seemed tight enough but the rotor would not be firmly seated on the shaft. Erratic operation could result.
 
Update 2.
Hooked tach backup while having new ignition coil installed. 90 minutes into ride, misfires again. Up to that point perfect as always, after misfire will start and idle perfect all day, then Once it reaches 2000 rpm starts misfiring.
 
I apologize for belaboring this point. Please remove the distributor cap and see if the rotor is firmly seated or wiggles on its mounting screws. It is not supposed to wiggle. Some girlfriend!
 
Original ca
scoflaw said:
Did you replace dist. parts? ign. module , pickup coil

You said it's not fuel?
Click to expand...

Yes, all those parts were replaced. Original cap had quite a bit of corrosion. Oddly enough the new one was corroded a bit after just 10 hrs run time. That one was replaced this week when new coil was installed.
As for fuel ,topped off with fresh fuel in spring 35 gallons and have since added 50 gallons probably. Again runs great for 90 minutes then acts up but still runs great under 2000 rpm. Doesn’t stall, idles smooth.
 
Arminius said:
I apologize for belaboring this point. Please remove the distributor cap and see if the rotor is firmly seated or wiggles on its mounting screws. It is not supposed to wiggle. Some girlfriend!
Click to expand...

I’ll double check with my mechanic. Thanks
 
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