Mercruiser 7.4 MPI rpm / acceleration fluctuation problem

robslc

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Good afternoon. I'm new to this forum and hoping someone out there has heard of or has experienced the problem I'm going to describe. I have a 1997 SeaRay 330 with twin Mercruiser 7.4 MPI engines and Brovo One outdrives. The engines have 550 hours +/- and run very well except for a problem I will describe affecting the starboard engine. After the engines are warmed up and I have been crusing down the lake ( at 3700 to 3800 rpm, typically ) for 20 to 30 minutes, the starboard engine rpm increases, on its own, to the 4100 to 4200 range, stays there for 10 to 15 seconds and then settles back to the rpm previously set by the throttle control. This same thing will occure every 10 to 20 minutes 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 because I had hit a rock earlier in the year. It was realitively minor but I did have the stainless prop trued up. When the rpm would increase, I would immediatly trim the starboard outdrive down and decreased the throttle and it would settle down until the next occurance. When the problem continued, I had the prop checked out by other prop shops but to no avail and the problem continued.
While it took me longer then it should have, about a month ago I decided to let the rpm increase and just watch it and see what happens if I do nothing. Thats when I noticed not only was the rpm increasing, but so was the speed ( measured by GPS ) by aproximatly 2 to 3 mph! So much for cavitation.
Since then I've tried crusing at a higher and lower rpm but it has had not effect on the continuing problem. I have also recently plugged in a Mercruiser diagnostic computer into the starboard engine and no codes came up when I plugged in nor when I was under way and experiencing the problem.
My current assumption is that there is a bad sending unit or something else electrical that manages fuel distribution to the starboard engine but that is where my diagnostic abilities end. Any thoughts are appreciated.
 

MarkSee

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Re: Mercruiser 7.4 MPI rpm / acceleration fluctuation problem

Welcome to IBOATS.

I have a 270 'dancer with a single 7.4 mpi and Bravo 3 and I have noticed what you have described but I pretty much always found that it was the slight changes in the sea condition that was causing this.
I have already found the trim gauge up about 1/4 is optimal rmp/speed then use the trim tabs to adjust as needed.

Don't know if you are also in the ocean but we'll see what the experts think about if this might really be an issue but since you're saying the other engine does not exhibit this behavior, I'm at a loss.

Mark
 
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Fun Times

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Re: Mercruiser 7.4 MPI rpm / acceleration fluctuation problem

I have also recently plugged in a Mercruiser diagnostic computer into the starboard engine and no codes came up when I plugged in nor when I was under way and experiencing the problem.
My current assumption is that there is a bad sending unit or something else electrical that manages fuel distribution to the starboard engine but that is where my diagnostic abilities end. Any thoughts are appreciated.
Hi there, if it was a bad sensor you would not necessarily get a code as long as the sensor voltage stayed within operating range.

Did you by chance keep an eye on the live data numbers of the scan tool while cruising down the lake? The one I would be looking at first for a starting point would be the throttle position sensor to see if it's changing values while you are not touching the controls. If no change in the TPS then move through the rest of the data numbers to see if and how much they have changed from before the problem starts.
 

robslc

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Re: Mercruiser 7.4 MPI rpm / acceleration fluctuation problem

Thank you Mark see and fun times for your input. I have winterized and put the boat away for the year, it's now winter in Utah. I will definatly learn how to use the scan tool better over the winter and borrow it again in the spring from my local Mercruiser guy and re-test as suggested. The boat is at Lake Powell and as you may know, it can get windy with blowing dust / sand that could very possibly find it's way into one of the sensors. Thanks again for the suggestions. This is a great forum, enabling us backyard mechanics the possibility of gaining knowledge from the experts like yourself.
 

agallant80

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Re: Mercruiser 7.4 MPI rpm / acceleration fluctuation problem

Its not fun but you could start moving components from one engine to the other one at a time to figure out what may be wrong. If there are no codes and it happens every now and then it will be hard to chase down.
 

Walt T

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Re: Mercruiser 7.4 MPI rpm / acceleration fluctuation problem

The throttles are fly by wire I assume so I would start there, then move to the TPS on the engine.
 

robslc

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Re: Mercruiser 7.4 MPI rpm / acceleration fluctuation problem

Hey Walt T, not to sound too stupid, but what does "throttles are fly by wire" mean. In the spring, I'm prepared to replace the TPS sensor and possibly the MAP sensor on the starboard engine, as sugested by another. Does that have anything to do with either of those sensors?
 

Fun Times

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Re: Mercruiser 7.4 MPI rpm / acceleration fluctuation problem

Hi there Rob, I did see your private message (Thanks) and also your other thread on the other forum and that was good advice as well.:)

I also read that you plan on borrowing a marine scan tool form the service tech up in the lake Powell area. Since it seems that your going to try and figure this out on your own for the most part plus the fact that you run on an almost deserted lake at times with miles of no help. I would suggest to you that maybe instead of borrowing a scan tool, you could just buy yourself a present so that you always have one on board either for you or a fellow boater out on the water with his hatch open.;)

You can either buy direct from the company or find them online someplace like ebay normally for around $400.00

Since you have MPI's, its the only way to go.

Rinda Technologies - MerCruiser Scan Tool Spec.

Having the tool hooked up before, during and after changing out parts would be a big help. Also it's best to use the live data readings when setting in a new TPS. You'll be watching both the voltage and the throttle position "percentage" value so that when you install the TPS, you'll be sure to set it at 0% vs 1 or 2 + %. Take note of the original TPS voltage reading with the throttle body at rest. Also having the "engine off" 'key on' for safety reasons is fine. The voltage should end up around 0.48 - 0.54 volt range if I recall correctly.

You could also try diagnosing the parameters of the live data by carefully wiggling the wires, sensors around to see if you get any unusual movement. Also you could try slowly moving the throttle body from open to close to see if you get an even voltage reading vs erratic voltage reading.

By the way, you're throttle body is controlled by cables still, the only fly by wire Mercruiser uses was developed in I believe 2005 called Mercruiser digital throttle and shift (DTS) still used today on some engine models.

Just a thought, Good luck.:)
 

robslc

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Re: Mercruiser 7.4 MPI rpm / acceleration fluctuation problem

Thanks again to everyone for their interest and responses to my engine problem. I'll be back to you in the spring (April / May) with the latest feeback after I pursue a solution to the problem.

Rob
 

MarkSee

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Re: Mercruiser 7.4 MPI rpm / acceleration fluctuation problem

...I'll be back to you in the spring (April / May) with the latest feeback after I pursue a solution to the problem.

As I mentioned above, I've got that same engine as you and may have even noticed your symptom a couple of times so I look forward to reading what you find out as I may need to do the same.

Mark
 
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