mercruiser 350 mag mpi blowing fuse

[ditch-digger]

Re: mercruiser 350 mag mpi blowing fuse

Chris, it is the "A" fuse. With the link Don sent me, I traced the solid pink wire that comes out of the fuse to what I am assuming is the fuel pump relay. Incidentally, while I was sitting dead on the lake when the fuse blew, I immediately put in another fuse and simply turned on the key without cranking motor over. The fuse immediately blew. I then unhooked fuel pump wiring(red/black) and put in another fuse and turned key without cranking motor and blew immediately also. Like I posted earlier, after sitting for approx. 30-45 minutes, I could put new fuse in and get it to hold as long as I kept boat at idle. If I throw the power to it, it will blow within 20-30 seconds. To answer your question, It is definitely fuse "A". Thx again for your advice!!!

 

[achris]

Re: mercruiser 350 mag mpi blowing fuse

That fuse is also power for the IAC valve (Idle Air Control)... And explains why you can't give it more than idle without popping....

I say, replace IAC, should be problem solved...

To prove it... Next time the fuse pops, disconnect the IAC connector and see if the fuse still pops...

Chris.....

EDIT: Confused.... Red/blk fuel pump wire? Are you talking about the wire at the main power relay or at the fuse holder?

 

[Fun Times]

Re: mercruiser 350 mag mpi blowing fuse

That fuse is also power for the IAC valve (Idle Air Control)... And explains why you can't give it more than idle without popping....

I say, replace IAC, should be problem solved...

To prove it... Next time the fuse pops, disconnect the IAC connector and see if the fuse still pops...

Chris.....

EDIT: Confused.... Red/blk fuel pump wire? Are you talking about the wire at the main power relay or at the fuse holder?

Good call Chris, IAC's have been know to be heat sensitive at times. The merc 496's have been known to get so hot they have melted down.

 

[ditch-digger]

Re: mercruiser 350 mag mpi blowing fuse

Chris, thanks for the help. I ordered an IAC and it will be in, in the morning. I was bored today and decided to hook my boat up to some water and see if I could get it to fail in the driveway. While I haven't ever got it to fail in the driveway (and it didn't today either), the IAC was extremely hot to the touch and could only keep my fingers on it for a couple seconds. Is that consistent with a part that is failing? Also, the red and black wires I was referring to were the wires coming out of the electric fuel pump.

Thanks!

 

[achris]

Re: mercruiser 350 mag mpi blowing fuse

If it was that hot... She dead mate!!!

You disconnected the wires at the fuel pump and it still blow fuse 'A'.... Dead IAC valve....

Please, let us know how it goes when you get the new one in...

Cheers,

Chris.........

 

[muc]

Re: mercruiser 350 mag mpi blowing fuse

Might want to order an IAC muffler at the same time. When they start plugging up it causes the IAC to work harder.

 

[ditch-digger]

Re: mercruiser 350 mag mpi blowing fuse

Thanks for the info guys. I installed a new IAC valve and ran it like I did yesterday and while it did get warm, (normal???) it didn't get nearly as hot as yesterday. I will put it into the water tomorrow and keep my fingers crossed. When I was at the boat shop and asked about an IAC muffler as was suggested, he looked at me like a deer lost in the headlights. I guess I may need to be a bit more educated. Is there another name for it and where is it located. Thanks much for the assistance!

 

[Don S]

Re: mercruiser 350 mag mpi blowing fuse

Item 6 in the picture below is the muffler.

View attachment IAC Muffler.pdf

 

[ditch-digger]

Re: mercruiser 350 mag mpi blowing fuse

Hey guys, first of all thanks for the help. Today was test day on the water. After changing, the fuel pump relay and the IAC valve, I was sure that baby sounded better. I got out on the lake, warmed it up and then put the hammer down. Without a doubt the boat was running better, both in acceleration and top speed. I went approximately 10 miles, had my smoking hot wife with me, and had a smile on my face from ear to ear. Then it happened. Same fuse popped. I came armed with extra fuses and threw one in right away. It instantly popped. I unhooked the new IAC (it was still hotter than a pistol, could only touch it for a couple seconds) and it popped again. Not sure what to, remembered the IAC muffler in the throttle body and thought it could be making the IAC work to hard. Ran boat again at full throttle for another 10 minutes and it popped again. I fiddled with the IAC again and unhooked and ran boat in with it unhooked. Didn't make one bit of difference. It runs exactly the same either way. (my boat idles at approx 600 rpm if that matters) Anybody have any ideas? Any would be much appreciated. Thanks again.

 

[ditch-digger]

Re: mercruiser 350 mag mpi blowing fuse

My wife just read my post and wanted me to be clear that when I ran it in with the IAC unhooked that it was only at probably 2500-3000 rpm. It was getting dark, and didn't want to take a chance of getting stuck so I didn't run at full throttle in that scenario. If she wasn't so dang hot, i would tell her to mind her own business. Thanks again

 

[achris]

Re: mercruiser 350 mag mpi blowing fuse

Oh boy!!! That really sucks! I'm at a loss now... Give me a day or so to think about it... It's only the fuel pump relay and the IAC on that fuse... The problem can ONLY be one of those or the wiring associated with them... The fact that the new IAC is getting just as hot and the fuse still blow with the IAC unplugged....

Start looking for chaffed wires running to the IAC and fuel pump... If there's a chaffed wire (that's resistive to ground, not a dead short) and some current is leaking off, then the voltage available to the IAC will be lower, it will draw more current to do the same job, it will get 'smoking hot'...

I wish we'd solved the problem with the new valve...

Chris......

 

[ditch-digger]

Re: mercruiser 350 mag mpi blowing fuse

I've checked the wires all the way to the big bundle in the wiring harness. Ive also inspected the bundle itself and it looks fine. So, Ive considered bypassing the pink wire from the fuse to the relay, and also to the IAC. I know cutting wires isn't a great idea, It would simply eliminate the wiring being an issue if I did. Also, there is another pink wire that runs into the ECM on pin #23. I don't see it going there from the wiring diagram, but the explanation is "MPR Output (to ECM). Are the pink wires related? Once again, any thoughts on this appreciated.

 

[achris]

Re: mercruiser 350 mag mpi blowing fuse

The Pink is 'Switched 12v, 20A fused' .. and yes, it runs to pin 23 on the 'A' connector on the ECM (it's basically telling the ECM that power is available).. The only things on that pink are the coil for the fuel pump relay, pin 23 and the IAC... They all come together in splice 106.

I've been through the circuit a thousand times, and I'm at a total loss.. If it's not the IAC (you replaced it) or the fuel pump relay (you replaced that too), then it can only be either, the ECM (if that was drawing enough to blow a 20A fuse, I think you'd have more problems than that), or a chaffed wire.

Unfortunately it looks like it's going to have to be fixed the really hard way....
Next time you're out (I know that's not looking like a pleasant experience, and I feel for you)... When the fuse blows, disconnect connector A from the ECM and replace the fuse. Turn the key to ON... Nothing should happen.... But then use a jumper wire and connect the wire on pin 22 (Pink/dark green) of that connector to ground (key still ON). That should close the main power relay. If the fuse still blows, then there is a wiring problem. If it doesn't blow, then connect pin 19 (dark green) to ground as well. That will close the fuel pump relay. The fuel pump should start running. If the fuse still hasn't popped, then the problem looks like the ECM.

Also have a really good look at that connector... Splice 106 (Switched 12v, 20A fused) and splice 104 (engine ground) are on adjacent pins, 23 and 24...

Chris........