Poor Mercruiser 4.3 performance, won't reach RPM's at WOT

[tlanders]

Hi,

I have a 2006 Tahoe 215 I/O with a 4.3L Mercruiser. For the last few weeks its performance has been horrible (although it was never as good as I expected) and I'm looking for some advice on what to try next.

For example, a few weeks ago with a load of 4 adults and 6 kids it maxed out at about 10 mph and 3200 RPM and obviously wouldn't plane out. At WOT this engine should hit about 4800 RPM and at least I think I should be able to hit 30 mph with a load like this.

So far I've had it looked at twice and the following things have been done:
- cap, rotor, and spark plugs replaced
- timing, mixture, and idle set
- mechanic replaced an oil pressure switch that was causing the engine to run rich
- mechanic said that the prop had way too much pitch for my engine so he replaced it with a used one (don't have the details though) that is better for it

The boat runs a little better now, but it is still performing way below where it should. The mechanic said it hit 30 mph and 4200 RPM with only him in it (he said this is still pretty poor though), but it only got to 3900 RPM and 10-15 mph with my earlier load of 10. The mechanic said that the boat has a dirty bottom (it's been in the water all summer), but it can't make that much difference, can it?

I've only had this boat for a year so I don't have a lot of history on how it used to run. Last year I could get it to plane out with a decent load of people, but I can't remember where the RPM's maxed out at. I do remember that I always thought that the boat was sluggish because even back then it took quite a while for it to get out of the water and plane out.

The only other negative that the mechanic mentioned is that the engine has more blowby than normal.

So, what do you all think should get looked at next? Are my expectations too high for this boat and engine or is something still off?

Thanks!
Troy

 

[Todd157k]

Re: Poor Mercruiser 4.3 performance, won't reach RPM's at WOT

Blow-by.. compression check time. Leak down test too, which will tell you which part is to blame.

 

[Eric 3.7LX Alpha 1]

Re: Poor Mercruiser 4.3 performance, won't reach RPM's at WOT

See# 13 in the link below:

http://forums.iboats.com/showthread.php?t=288430

"Adults Only" Section at the top of the forum

 

[chomamma]

Re: Poor Mercruiser 4.3 performance, won't reach RPM's at WOT

I'd check compression as well. What prop size are you running? Has it only spooled up to 4200 rpms?

 

[aguy07]

Re: Poor Mercruiser 4.3 performance, won't reach RPM's at WOT

hello, had the same happen with my 3.0l, frist thing i did was use a can of SeaFoam, got the RPM back up at 4200 still not 4800 but try that!

 

[Adirondack]

Re: Poor Mercruiser 4.3 performance, won't reach RPM's at WOT

The only other negative that the mechanic mentioned is that the engine has more blowby than normal.

Troy

Food for thought. Exceive blowby can also be a symptom of an engine that is constantly straining, and not necessarily a cause. I think he was in the right direction with the lower pitch prop. Just didn't go far enough for the load you routinely carry. It adds up. The load you describe after throwing in gear, lunch, and a big cooler of beverages can be 1500 pounds as opposed to the 180 he had in it for testing.

 

[MPII]

Re: Poor Mercruiser 4.3 performance, won't reach RPM's at WOT

I've got a 4.3 Merc and I had way too much pitch (I was testing props). I put a 23 on it and could only turn around 4300 rpms. With a 19, I get around 4900/5000. I settled on a 21 and get 47-4800 which I am told is the right RPM's for that motor at WOT.

 

[burp]

Re: Poor Mercruiser 4.3 performance, won't reach RPM's at WOT

The mechanic said that the boat has a dirty bottom (it's been in the water all summer), but it can't make that much difference, can it?

Thanks!
Troy

A hull covered with algae growth will have a big impact on the boats performance (reduced engine RPM and speed). Pull the boat out of the water, clean the hull and then see how it performs.

 

[tlanders]

Re: Poor Mercruiser 4.3 performance, won't reach RPM's at WOT

Thanks for all the advice!

I'm going to clean off the bottom and see what happens. I'm also going to see what size prop the mechanic put on. I think he said the original was a 23 which sounds like it was way too much. He put something with less pitch on it, but I don't know what.

Troy