General MPI Question

[Thumbtack]

I have heard this noise on a few different MPI engines (Merc, Cruisader) and am wondering what it might be as I have one now that is getting very loud. From the throttle body with the engine in gear and under a load there is a loud noise almost like metal vibrating but I know it has something to do with the air flow. The engine runs great other than that noise. Any idea what that noise is? I'm sure it is a common thing as I've seen it a number of times already I just never looked into it.

 

[Rat Capri]

Re: General MPI Question

injectors firing or pcv valve

 

[Thumbtack]

Re: General MPI Question

What is the pvc valve? I haven't done much on mpi engines as they usually run fine. Is there a way to test the injectors or the pvc valve before I start replkacing parts? I do have the computer diagnostic tool.

 

[Rat Capri]

Re: General MPI Question

positive crankcase ventilation valve....it's a ticking sound you're talking about right ?

 

[Zeeter]

Re: General MPI Question

Look for a fairly large hose going from one of the valve covers to the intake manifold. On the valve cover end will be the PCV valve. They can rattle or set up a buzzing sound.

 

[Thumbtack]

Re: General MPI Question

Thanks, I'll take a look for that valve. As far as describing the noise, it is not a sharp distinct tick it is more like a rattle of small metal parts. It only happens with the boat in gear and under a load, I guess because of the greater vaccuum in the intake area. It kinda sounds like if the engine is pinging (where you hear that rattle noise coming from everywhere) only in this case it is isolated to the throttle body.
I also noticed the noise gets better without the flame arrestor on. I did clean that but it didn't change anything.

 

[Thumbtack]

Re: General MPI Question

Also, what about the IAC valve? Could that cause a noise if it goes bad?

 

[Thumbtack]

Re: General MPI Question

I just noticed there is a IAC muffler in the pictures in the book but I didn't notice one on the boat. Looks like it might be some form of foam.

 

[Rat Capri]

Re: General MPI Question

Also, what about the IAC valve? Could that cause a noise if it goes bad?

only controls idle air, if it goes bad you'll have low/rough or out of spec idle speed condition

 

[Rat Capri]

Re: General MPI Question

I just noticed there is a IAC muffler in the pictures in the book but I didn't notice one on the boat. Looks like it might be some form of foam.

never heard of or saw an IAC "muffler"...sure it's not a breather filter ?

 

[Thumbtack]

Re: General MPI Question

As far as that "muffler" I'm looking at Mercruiser manual #31 Part #90-864260001. Page 5B-17 and 5B-18. Engine serial number OM300000 and above (MPI).
If you look down into the throttle body you will see two big ports on the side of it where I guess the idle air comes into. The pictures shows a big piece of foam in one of the ports. That is where I'm getting that idea from.
I don't have any other bad running conditions though, no rough idle ect...

 

[Rat Capri]

Re: General MPI Question

As far as that "muffler" I'm looking at Mercruiser manual #31 Part #90-864260001. Page 5B-17 and 5B-18. Engine serial number OM300000 and above (MPI).
If you look down into the throttle body you will see two big ports on the side of it where I guess the idle air comes into. The pictures shows a big piece of foam in one of the ports. That is where I'm getting that idea from.
I don't have any other bad running conditions though, no rough idle ect...

learn something new everyday, I've never heard of such a thing....1 of those ports is an idle air port for the IAC

 

[Thumbtack]

Re: General MPI Question

Yeah, who woulda thought that would be there. I guess I'll start with that and see what happens as there is none there. I guess they make it out of foam or something incase it gets sucked into the engine it will just get destroyed.