Mercruiser 5.0 Question

Serenity

I own a 2005 320 with twin Mercruiser 5.0 and Bravo 3 outdrives. The motors run perfect. Buttttttttt. Lets say I am running on plane and slow down to idle speed when entering my canal. The port motor will drop down to 600rpm good-- always have done that and I am good. Now the starboard motor when coming off plane will idle down to 900 rpm. What I have to do is while coming in at idle speed just put the starboard motor in neutral and then back in gear-now both are 600rpm. I tried adjusting the throttle cable by turning the black peg a turn at a time but it has not done anything. I also do not have any issues with shifting and they shift in and out of gear very smooth. The motors have 225 hours each. Top speed and max rpms are perfect. Very puzzled. Please help.

212rowboat

Carb or throttle body?  With engines off, get in there with a can of silicone spray and pull throttle plates through full range while spraying linkage exterior of throats. 

Serenity

They are throttle body. I did that with silicone first. Thank you though. There are also new cables installed by a certified mercruiser mechanic in the Spring.

Serenity

I did not see one. There is an IAC which is new.

JoeStang

Mine does that every so often. I have to just barely tap the throttle backwards and it'll settle down to 600 rpm. Not anywhere close to shifting into neutral, but just a degree or two towards it. It seems to happen more often in cooler weather, so I figured probably either the cable or idle speed sensor. 

212rowboat

It's gotta be in the linkage either at the throttle body or at the helm... Because dropping out of gear and back in matches it, I'd lean that direction, anyway... I tell ya what, though, the difference in cable length on that speed difference is about the width of a grommet sleeve, if you can track that... I'd look to make sure the grommet holding the cable end isn't split or broken.

frodo13056

What are you using to verify the RPM? The Faria tachs are famous for being inaccurate. If you do a search on the forum for Faria tachs, you'll find that most owners have to do the tach tap tap tap every so often to get the tachs to "settle out". I know I have to do that on my 08 320. More often than not, there is a few hundred RPM difference between port and starboard. I have added the Mercruiser NMEA 2000 gateway and display RPM and a bunch of other data on my Lowrance display so now I don't really care that the tachs don't match all the time.

JoeStang

Knock on (fiberglass covered) wood, my Faria gauges have been 100% fault-free in almost 2 years of use.

I had the dealer take me out for a ride when they had the laptop hooked up and the tach was dead-on all the way to 4k rpm.

Dream_Inn

I'm guessing something with throttle cable.  I've got a 342 buddy that has something similar going on.  idle is good. While getting on plane, throttles are positioned differently. On plane, back to matching.  hadn't figured it out yet.

Serenity

Thank you for the suggestions. It is not the gauges. During the past winter I tossed all the Faria gauges-tired of the inaccuracy and having sent back twice. I replaced with all Smartcraft. The Mercruiser mechanic that installed did verify with his gauges that the new ones are now spot on. Last night I checked all the linkages both on the throttle body and at the shifter. All were tight. I guess it has to be a minor adjustment. Thank you all for your suggestions. Will let you know what I come up with.

aero3113

When you say all linkages were tight, is there enough play to prevent binding?

212rowboat

I still say check throttle body linkage for binding.... The cable could release pressure on the arm but the arm not release when it's that close to closed, anyway... A bur, a debris, a slightly bent throttle arm could be causing this. 

Black_Diamond

I get a similar problem, like JoeStang though I just give the throttle(s) a little tap and they settle right down to 600rpm/idle.  Been that way for 11 years, I just deal with it with no concern.

I did have the issue of the throttles being 'off' to each other while getting on plane, I replaced both PCV's and both air temp/pressure sending units (both in one sensor on my 350 MAG MPI)...been great all season and the problem gone.

rinker312

I have to do the tap tap thing too.  when I pull them down off plane, they never seem to settle out until I tap both throttles, then things tend to even out.  One of these days I would love to change my throttle cables and see if I can get easier shifting.  I did the lower cables a couple years ago and it didn't really help much

Serenity

Sorry when I said tight--I meant snug. I will have to investigate deeper. I will look at it binding--maybe that is it or the arm is bent a slight bit. I do not feel anything in the throttle but you never know. I was just thinking-when in the slip and I advance the throttle only--it always drops down to 600 when moved back. That is what is weird about the whole thing. Thank you everyone.

Dream_Inn

BD, air temp/pressure sending unit?  Is this the MAP sensor?

I will definitely let my buddy know to try that one out!  he has the 350 MAG MPI as well.

Black_Diamond

Unlike the manual claims, on the 2003 350 MAG MPI there is one sensor for both the air temp and pressure. Mounts right behind the throttle body into the intake manifold. I got mine on ebay.