RPMs suddenly going higher, 1994 horizon 190

Mesimpson · **Posted:** Sun Sep 13, 2015 8:56 am

Been a while since I've posted. Boat has been working well (knock on wood).
Except this past week:

Boat has a 5.0L, OMC/Volvo SX. In the middle of the day on the lake, the engine temp climbs suddenly from the normal 159-160 degrees to max (240) on the gauge. I shutdown and let things cool off. Get in the water, brush off the water intake, looks fine. Try to limp back to the ramp and suddenly temp goes back to normal. I assume it's debris. Rest of the day it's normal.

Couple days later in Plymouth Harbor (gasp! salt!), all is fine.

Back to the lake yesterday and now the temp remains fine but the engine which used to top around 4000rpm is now turning 5700 rpm WOT, and is running higher rpm on all throttle settings.

I mention the temp issue only as a weird coincidence? I'm running the same 4 blade prop that I've had for 3 years. I haven't hit anything. Per the manual the boat should be around 4000 rpm WOT and not 5700. If anything the engine sounds a shade quieter.

Shop has a 4 week backlog. I barely have 4 weeks left of boating before I'm reminded how much I'd prefer to live somewhere warmer if only I could convince my other half.

Big problem, small problem? Am I doing harm by using the boat just keeping rpm below 4000 at least until winterization and then tax returns?

tymorrissette · **Joined:** Tue May 31, 2011 10:03 am **Posts:** 225

Are you sure the engine is really going that high? Does it sound like it is Rev'ing THAT much higher? These gages are notorious for being off. You coulld easily check the actual RPMs using an RPM gage.

As for the temps...did the engine feel that hot? Could you not keep you hands on the thermostat hoses or manifolds for a few seconds?

GottWhat · **Posted:** Sun Sep 13, 2015 9:44 am

I agree, I don't think the engine will even rev to 5700 RPM. Check the gauge and wire connections.

Mesimpson · **Posted:** Sun Sep 13, 2015 9:50 am

The engine sounds very slightly like its revving higher but not super noticeable. It was one of those moments tooling around at about 3/4 throttle and I commented to my sister how smooth it's been running since the tuneup in the spring, looked down and was reading 4500 rpm at 3/4 throttle. I didn't notice a higher rev then look at the tach. The tach reading was a surprise.

Pardon my ignorance (give me an air compressor, cut saw, speed square, nail gun and I can build a house, engines....not so much), but I don't have an rpm gauge and if I did wouldn't know where to point it to use it (the prop?). The gauges on my boat are generally crap. Normally, speedo, tach, temp and fuel gauge work: volts, oil pressure, drive position, depth finder have never worked since I bought it 5 years back.

In the temp side, the engine was hot and the hoses too hot to handle, now that's fine. I'm hoping its unrelated and that 5-10 minutes of it running upwards of 240 degrees didn't cause some problem.

LouC · **Posted:** Sun Sep 13, 2015 11:59 am

You may have more of an electrical problem than anything else. Get an IR temp gun and use it to check on your gauge readings. At normal temps the dash gauge should read between 160-175 (the latter after coming off plane). Take a reading with the IR gun on the intake manifold right under the Thermo housing (I've found this spot is closest to the dash gauge usually it's about 10-15 degrees lower than the gauge). Take readings of your exhaust risers; at idle they should be between 95-105 or so; the highest I see is about 135 after coming off plane.
You can check on rpms by hooking up an engine analyzer tach to the ignition coil. If your engine really is revving higher it could be a slipping prop hub.
BTW salt water will not kill your boat; it is fresh water getting into the transom and deck wood coring that kills boats. Fresh water can harbor the bacteria that rots wood; not salt. I'd rather replace a set of manifolds/risers that replace a deck any day!!!

whiplash · **Joined:** Thu Mar 05, 2015 4:45 pm **Posts:** 4

I had a similar issue with my boat, only my RPMs would fluctuate higher than normal sometimes and lower than they should be sometimes and be correct most of the time. The engine sounded fine.

It turned out to be that the nut on the back of the RPM gauge was loose and the wire was not on tightly on the stud. I tightened up the nut and the RPMs are reading correctly now.

rpengr · **Posted:** Wed Sep 16, 2015 7:45 am

There are some free cell phone apps that can display an RPM based on the sound (pitch) of the engine. They just need to know 2-stroke vs 4-stroke and how many cylinders. They can be pretty accurate. When they are off, they are usually off by a factor of two (like double or half).

The engine analyzer/tach is a better solution, but not as cheap

zims1993 · **Posted:** Sat Sep 19, 2015 12:45 pm

Do you have individual round white face gauges or the cluster? I think you have the round gauges I have in my 1993, and I just went thru replacing them as they had become unreliable and inaccurate. It sounds like you have a gauge problem, like Lou mentioned check the temp with an IR gun, and you can verify RPM by using a timing light that has a built in tach.

www.iFourWinns.com

RPMs suddenly going higher, 1994 horizon 190

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