5.0 MPI Mercruiser over revving

[Captain's Cove Maxum]

Just returned from a short cruise on the river. 1900 SR3 with 5.0 MPI
All gauges reading normally except tach.
Normally idles at just under 1000rpm. Today idled at 1200rpm. Normal cruise is 30mph at 3200rpm. Today same speed and 4500rpm.

To be honest, my ear tells me the engine is actually idling at the normal rev level.

Could the tach have suddenly lost its calibration or is something else wrong?

I'll have it checked by my mechanic but I'm wondering if anyone else has experience with this.

Thanks!

[ss3964spd]

Yup. Actually, your 5.0 should idle closer to 6, maybe 7, hundred RPM - which is where my 5.7 MPI always idles.

Although my indicated idle has remained the same, I have started seeing strange readings, small fluctuations, on the tach at cruise speeds in the last month or so. Sometimes it will read many hundred RPM's higher than the engine is actually running. Everything else is dead normal and there is no change in engine note to correspond with the tach changes. I don't think it's a tach calibration issue, more likely a wiring/connection or feed from the ignition system that is wonky.

Since the engine is running completely normal I've not worried about it thus far, but will eventually get around to figuring out what it is.

Dan

[Captain's Cove Maxum]

Thanks Dan

Is there anyway to trouble shoot this w/o special diagnostic equipment?

[seapuppy]

CCM
sometimes the tach connections at the dash get a bit of corrosion on them...the tach will all of a sudden gain a few hundred rpms while the engine is running great....just tap on the tach and see if it comes back down...if it does..then it's a loose or corrroded connection...
mine has done this several times over the yrs...just tap it a few times and it should return to normal...


SP

[Captain's Cove Maxum]

Thanks SP
Giving it a tap is quite a bit less expensive than calling a mechanic!
I'll give it a try this a.m. Gives me an excuse to visit the marina!

[Nimh]

My tach has recently started doing this too, especially in rougher seas. Bounce hard and it's off by 3-500 rpm's, take another good jolt and everything is back in check.

I've read it's not unusual, and it may be as simple as disconnecting the leads, clean them up and re-install. I've also heard there may be a small dial on the back of the tach for calibrating to different power plants that just needs to be cycled and set to the original increment. Again, corrosion making it behave poorly. I just haven't yet opened mine up and done it yet.

[Captain's Cove Maxum]

Thanks again SP!

Just gave the tach a rap with my knuckles and we're back to normal.

Saved me a whack of money .... I donated to the site as a result!

[seapuppy]

haha....greatly appreciate the donation....although I don't see it...but hey!!...keeps us in beer and women...hahaha..glad to help..



SP