5.0 MPI running rough and idle fluctuation

daddyox

daddyox

Member
Apr 1, 2007
244
Connecticut River
Boat Info
Sundancer 320 2004
Engines
260 5.0 B III's
I started my port motor for the first time this season. It started difficult, only when giving it some throttle, and rpm's fluctuated between about 450-550, whereas they are normally 600. When I throttle, its "chugs" a bit and it just doesn't sound right. Water, Oil, etc. all seem correct. Temp was actually lower than starboard- which ran fine. The only strange thing I noticed was that the voltage dropped as I was starting it. I tried a new IAC and that didn't do anything.
Ideas? I think I would start with fuel filter which wasn't switched out yet...but has anyone else run into what I have noted?
thanks!
 
You changed the IAC but did you change the IAC Muffler? That could be causing a restriction. If you don't know where it is, check post#8 in this thread for a diagram. The muffler is Item #6 in the diagram. Also, the intake could need to be cleaned or maybe possible bad gas depending on how long it sat? Was it winterized? If someone fogged it maybe could cause it to run rough for a bit? Not sure on that maybe someone else can chime in there. I don't winterize.

http://clubsearay.com/showthread.php/36072-Impeller-IAC
 
Also, you also mentioned a voltage drop. Could it be a bad battery in that bank causing the injectors not to fire correctly? I have heard of all kinds of weird things happening on fuel injected/ECM controlled engines when a battery is low or bad.
 
Thanks Jason
I actually have never seen the "IAC muffler" before this thread. it looks like its buried in there; is it relatively easy to get to? I have gone through 2 IAC's on each engine (after 380 hours) so yes its probably not good. the voltage drop may be a key as you note; someone (marina...?) left me unplugged and the batteries drained, they've seen been fully charged but I wouldn't be surprised if I have a problem there.
 
daddyox said:
Thanks Jason
I actually have never seen the "IAC muffler" before this thread. it looks like its buried in there; is it relatively easy to get to? I have gone through 2 IAC's on each engine (after 380 hours) so yes its probably not good. the voltage drop may be a key as you note; someone (marina...?) left me unplugged and the batteries drained, they've seen been fully charged but I wouldn't be surprised if I have a problem there.
Click to expand...

Yes the muffler is easy to get to, just take off the flame arrestor and it is tucked right in there. The diagram in the other thread should help you pinpoint it as I don't have a picture. You will need tweezers to get it out. Just be careful not to get anything in the intake. Some people try to clean and reinstall them but they are about $1 each so I just replace them. Inspect them regularly as keeping them clean will prolong the life of the IAC.
 
jason78 said:
Yes the muffler is easy to get to, just take off the flame arrestor and it is tucked right in there. The diagram in the other thread should help you pinpoint it as I don't have a picture. You will need tweezers to get it out. Just be careful not to get anything in the intake. Some people try to clean and reinstall them but they are about $1 each so I just replace them. Inspect them regularly as keeping them clean will prolong the life of the IAC.
Click to expand...

thanks- I actually got in there and it was black and I removed it. I've ordered a spare (two), but I also cleaned the aircleaner and also ran it without the muffler-- and the same problem exists. It chugs when I accelerate and I also noted the volts falling. I'm hoping its just the battery. The other thing I noticed is that the temp in the "bad" engine, even when I rev the RPMs, was quite a bit less than the good engine, which didn't make sense to me. Would a thermostat problem cause the performance/ rpm's issue? thank you!!
 
[QUOTE Would a thermostat problem cause the performance/ rpm's issue? thank you!![/QUOTE]

No, but a couple of cylinders not firing would. Last tune up, filters, ???
 
daddyox said:
No, but a couple of cylinders not firing would. Last tune up, filters, ???
Click to expand...
[/QUOTE]

tuned up fully two years ago...filters all changed annually but fuel filter from May, 2012 still in need of change (about 30 hours).
 
daddyox said:
Jason - sorry for my basic question- what's the "cap"?
Click to expand...

Sorry, Distributor Cap. Changing the cap and rotor is a relatively easy thing to do. Corrosion can build up in the cap and cause these types of issues as well. I believe it would normally be replaced when a tune up is done. The kit is about $50.
 
well, the marina changed the cap and rotor, and fuel filter. Still not running right and doing the same thing. RPM's low, stalled, smoking a bit, noticed voltage dropping after starting. I'm at a loss. Only thing new was....my starboard engine wouldn't start unless I crossed over the batteries? I'm thinking totally unrelated or would some sort of an electrical problem cause a hard start and rough run?
 
Daddyox

My port engine is doing the same thing. What was your final fix? I have replaced plugs,rotor,cap,IAC,IAC muffler.
 
sightunseen said:
Daddyox

My port engine is doing the same thing. What was your final fix? I have replaced plugs,rotor,cap,IAC,IAC muffler.
Click to expand...

It ended up being a cracked head. I believe it wasn't winterized properly. That was beyond my knowledge and capabilities, and the marina took it from there.Sorry that's all I got!

Jon
 
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