Mercruiser engine revs from 3200 rmp to 3400 rpm and back

[Fixing mercrui]

Hello all, Ive been running my 1998 5.7 ltr mercruiser EFI throttle body corb with Bravo 3 drive. It runs smoothly, no backfires. I feel it used to go much faster. I noticed I was running at 3200 RPM and at times it would go up to 3400 RPM's without touching the trottle, and then back down to 3200. At first I thought it was cavitation or a spun hub but checked and it is not..
Its not a constant issue, but it seems like its not running on all 8 all the time....only sometimes for a moment.
At full throttle i'm only getting 4500 rpm, and it used to get up to approx 4800-5000 if I remember correctly.
Any ideas where to start checking as Im usually solo so hard to test at cruising speed.
Replaced cap rotor and plugs.... dont know why marina didnt replace wires but they didnt...

 

[alldodge]

Need to check fuel pressure. Depending on your serial number it will take a Tee fitting or other to check

Tee fitting 91-806901

 

[QBhoy]

Check your temperature sensor isn’t making it run rich perhaps. Likely has an IAC too.
But I’d be more inclined to think it’s a fuel delivery or quality issue first off.

 

[Fixing mercrui]

Any chance you feel its the ignition module or ignition coil?
They are probably original 22 years old, and it seems like it may not always be firing all 8 cylinders....just a loss of power.....

 

[Bondo]

Any chance you feel its the ignition module or ignition coil?
They are probably original 22 years old, and it seems like it may not always be firing all 8 cylinders....just a loss of power.....

Ayuh,..... Welcome Aboard,...... Those either work, or they don't,......

I suggest you pull the fuel filter, 'n examine it's contents for anything but clean fresh gasoline,....

 

[Scott Danforth]

my guess is you have water in the fuel system and it doesnt run well on water, hence the drop in the RPM. as Bondo stated, check the contents of your fuel filter.