W.O.T. and High RPM (indicated)

[gengiant]

Hi all,

I am running a 454 EFI engine in my '94 270 DA (BR III outdrive). Over the winter I had the engine and outdrive pulled to have all the seals and bellows at the stern as well as the shift cables replaced. Shortly after the service I did a short test run and everything seemed to check out ok. Yesterday we spent the day on the lake )only the second time out since the major service). At full throttle the engine is humming along nicely, doing just under 45 mph (GPS). To my amazement the tach read 6,800 rpm!! :smt043 I know - not possible. Prior to the service the W.O.T. was at just under 4,500 rpm, right were it is supposed to be (I am running slightly oversized props. Not only that, but if the engine would truly be turning 6,800 RPM under full throttle with a full load on the props I'd be doing well in excess of 55 mph! :grin: So, I figure that there's something wrong somewhere. Back on shore I check the props. They seem to be tight on the shaft. At this point I am pretty clueless as to what might be causing the high rpm indication. :smt017 Btw., as I mentioned above, at W.O.t. the engine was humming along VERY nicely. No unusually high whining, no unusual vibrations, no excessive engine heat - nothing physical that I could tell to indicate that there was a problem (other than the tach indication). Does anyone have any ideas as to what may be going on?

 

[MLauman]

Gotta be a tach problem - your engine would have come apart at 6800. My first guess would be the gauge - did you try tapping on it?

 

[gengiant]

Gotta be a tach problem - your engine would have come apart at 6800. My first guess would be the gauge - did you try tapping on it?

That was my first thought. Tapping on it didn't help, though. I spoke to my local MM service manager. He suggested that the tachs they use on these boats are "universal" for 4-, 6, and 8-cylinder engines and that there is a little screw on the back of it where one can adjust the "engine setting". According to him, vibration may have caused this setting to have gotten changed. I am not looking forward to pulling the dash (again), but unless anyone has a better idea as in "before you pull the dash, try doing this...." I may not have any other options.

 

[MLauman]

I thought about that setting, but wouldn't the RPMs display as lower rather than higher if it was set at 6cyl? I'm thinking it works by counting coil pulses, but I may be wrong.

 

[gengiant]

I thought about that setting, but wouldn't the RPMs display as lower rather than higher if it was set at 6cyl? I'm thinking it works by counting coil pulses, but I may be wrong.

But wouldn't smaller engines rev higher?

 

[MLauman]

But wouldn't smaller engines rev higher?

No, revs (RPMs) would be similar. My theory (which may be wrong) is that the coil has to fire 8 times per revolution on a V8 versus 6 times per revolution on a V6.

 

[Dave M.]

So, if it is in the V8 setting, it takes 8 pulses before it registers two rotations. If in the V6 position, it will register two rotations with only 6 pulses. So it racks up rotations quicker in the V6 position, and therefore displays a higher RPM. No?

But if you take 6/8ths of 6800, you get 5100, still pretty fast.

 

[gengiant]

From Wikipedia.com: In older vehicles, the tachometer is driven by the pulses from the low tension (LT) side of the ignition coil, whilst on others (and nearly all diesel engines, which have no ignition system) engine speed is determined by the frequency from the alternator tachometer output (a special circuit inside the alternator to convert from rectified sine wave to square wave) , which is directly proportional to engine speed. Tachometers driven by a rotating cable from a drive unit fitted to the engine (usually on the camshaft) also exist- usually on simple diesel-engined machinery with basic or no electrical systems. With modern engine management systems found in present day vehicles, the tachometer is driven directly from the engine management ECU.

With a newer EFI engine (like mine), wouldn't the tach be computer controlled?

 

[boatrboy]

I went thru this last year. I had to tap the tach a few times and watched it return to the correct rpm. I bought the newer digital version and just installed it. While I am not in the water yet, it seems ok now.