Mercruiser 5.0L WOT RPM 1000rpm higher than normal

[ctzchris]

I purchased a used boat 1995 Celebrity 180 with a 5.0L Mercruiser and Alpha 1 outdrive at the beginning of the season. The boat has been flawless all season. I noticed my temp gauge stopped working, so I replaced the sender, and got it working again (also noticed the fuel line was leaking because it was loose, fixed that too). While I was at the lake testing it out, I noticed that at WOT, the RPM would get to 5500. WOT RPM used to always be at 4500 RPM, so I backed it off to 4500, engine sounded alot slower that in usually does at that RPM and the speed also dropped. Used to be 45mph at 4500, now it's about 30. I can still get it to 45mph at WOT, but the RPM is way too high, 5500. What would cause this? Bad tach? Can the drive be slipping? The lake is at 7000' elevation, prop pitch is 21. The only thing I did was add some drive oil before I went out. It was just under the full line in the reservoir, so I just put a tiny bit more in there to get it to the line.

 

[futz]

Re: Mercruiser 5.0L WOT RPM 1000rpm higher than normal

you mess around under the dash bye chance?

Lay down on your back and locate the back of the tach and see a switch on it for 4-6-8 cylinders? Cycle the switch back and forth and set it to the amount of cylinders you have. While under there all the wires good not grounding out?

 

[ctzchris]

Re: Mercruiser 5.0L WOT RPM 1000rpm higher than normal

I did mess around under there when I was testing the temp gauge, just grounded out the sender terminal to test the gauge. It's possibe I knocked it over to 6. I run and check. My friend has a snap on timing light with a tach readout. I'm going to take that with me next time. I also noticed the idle seemed about 500 rpm high, but I couldn't tell be listening if it was running faster than normal.

 

[cr2k]

Re: Mercruiser 5.0L WOT RPM 1000rpm higher than normal

Those numbers sound about right for a tach set to 6 cyls on a V-8.

 

[ctzchris]

Re: Mercruiser 5.0L WOT RPM 1000rpm higher than normal

The switch on the back of the tach has a few positions, 2p,3p,4p, 5p, and 6p. 2 is marked for 4cyl, 3 is for 6c, and 4 is for 8cyl. It's on 4, but 5p and 6p dont say what cylinder range its for. The legend on the decal indicates that "P" is for "Pulse". I don't think it moved.

 

[ctzchris]

Re: Mercruiser 5.0L WOT RPM 1000rpm higher than normal

Hey futz, I think you hit the nail on the head. I got under there again, and wiggled the selector with a screwdriver, fired it up with the earmuffs, and the tach showed it was idling at 300rpm lower than it did at the lake. The tach also has a calibration screw. I'm going to get the timing light and recalibrate it. I'm pretty sure that 5500rpm reading was wrong.

 

[futz]

Re: Mercruiser 5.0L WOT RPM 1000rpm higher than normal

Good luck glad it was a simple fix, and the best part it was free.