Mercruiser 7.3 MPI rpm fluctuation

Good morning. I'm new to this forum and hopeing someone out there has heard of or has experienced the same problem I'm going to describe. I have a 1997 SeaRay with twin Mercruiser 7.3 MPI's and Brovo One outdrives. The engines have 550 hours +/- and run well except for the problem I will describe for the starboard engine. After the engines are all warmed up and I've been cruising ( at 3700 to 3800 rpm, typical ) down the lake for 20 to 30 minutes, the starboard engine kicks up the rpm to the 4100 / 4200 range, stays there for aproximatly 30 seconds and drops back to the correct rpm set by the throttle control. This same thing will occure every 10 to 20 minuites thereafter until I reach my destination. When this first started to occure aproximatly two years ago, I initally thought I was having a cavitation problem and when the rpm would increase, I would immediatly trim the outdrive down and decrease the throtle and it would settle down until the next occurance. I made the assumption that the stainless prop needed to be tured up, which I had done several times to no avail. While it took me longer then it should have, I decided to let the rpm increase and just watch it and see what happens if I do nothing. I noticed not only was the rpm increasing but so was the gps speed, by aproximatly 2 to 3 mph. I've also tried cruising at a higher rpm with no effect on the continuing problem. My current assumption is the the problem is comming from a computer or sending unit but thats where my diagnostic abilities end. I pluged a Mercruiser diagnostic computer into the starboard engine and no codes came up when I first pluged it in nor when I was under way and the rpm increased, several times. I'm at a loss for where to go next. The boat is in Lake Powell Utah and the mechanics there leave alot to be desired. I've quised the Mercruiser guys in Salt Lake City, which is where I take it when I need work done that is beyond my abillity. (six hour plus pull with wide load permits over very steep terrain, something I don't do frquently) Anybody got any ideas???

Going down the path that what you have are 7.4L MPI gasoline engines, get the scan tool back out and look at the sensor inputs and note any change(s) when the RPM shift occurs....especially the MAP sensor.

This should provide some insightas to what is going on...could be as simple as an intermittent connector on one of the sensors causing the injector pulses to widen up a bit. ECT and TPS are sensors to watch, too.

The issue as you describe it is not likely to set a trouble code as nothing appears to be failing but just shifting its "output" a bit (but still within range)...a "bad" sensor has many failure modes...

makomark
Thanks very much for your imput. I have put the boat away for the year. (Utah winter is here) I will borrow the scan tool from my local Mercruiser repair man and re-test as suggested. This is a great forum and we back yard mechanics find tremendious vale in the coments we get from the those in the know like yourself. Thanks again.

Rob

Rob:

thanks for the kind words...

Be sure to stop back in the spring and share the "rest of the story"...that's what makes this site beneficial.