350 MAG MPI high idle and suction noisie

[stillcantfly]

Yea I can put another one in I’ve bought 3 of them now

 

[alldodge]

Being above 1000 rpm the IAC will have little effect, I would suggest looking at what Kenny suggest in post 15

 

[stillcantfly]

Got the boat back together today and dry dock started it after replacing the fuel injectors. Still ideling high 1000rpm and has the suction noise. Let it get warm taped off the PVC hole and the oil cap hole. There is suction coming from the throttle body where the PVC tube would plug in does this mean cracked intake manifold?

 

[alldodge]

There is suction coming from the throttle body where the PVC tube would plug in does this mean cracked intake manifold?

No, that spot should have suction. Kenny was saying if you block that port, and the valve cover ports, and there is suction at the valve cover port, then the intake may be cracked on the under side

 

[stillcantfly]

So I’m an idiot not quite understanding I’m sorry I under Basic engine stuff I fly planes for a living. Sorry again. When I covered all three ports. Oil,PVC valve both on the valve cover and the one at the throttle body it dies.

 

[stillcantfly]

Also when the pvc valve on the valve cover was blocked the rpm was at 1000-1100 and the suction noise was gone just idling high. IAC I can hear moving and adjusting to what I was doing.

 

[alldodge]

When I covered all three ports. Oil,PVC valve both on the valve cover and the one at the throttle body it dies.

This has me thinking the IAC does not have a air supply path. With all the ports closed off, the IAC should keep it running, but then again there is a PVC valve and it will supply a certain amount of air. Closing it off would have some kind of an impact but not thinking it would die.

A note for me, and maybe others:
Looking at the breakdown of your motor parts, I am unable to see where the IAC is located. There is no IAC shown on the throttle body or on the manifold. I am assuming its part of the throttle body.

Also when the pvc valve on the valve cover was blocked the rpm was at 1000-1100 and the suction noise was gone just idling high. IAC I can hear moving and adjusting to what I was doing.

You were able to close the PVC off and it continued to run at 1000 rpm, but when the other 2 valve covers where closed off it died. This might mean an intake issue, but still wonder about why the IAC wouldn't keep it running

 

[alldodge]

Has the PVC been replaced?

Edit: The manual shows the IAC is part of the TB, so it should get its air from the TB and not PVC line.

Can someone find the part number for the IAC so it can be used to see if it was replaced with the correct one????

 

[alldodge]

Found IAC, its not shown in the pic but part number is 803149

Is this the part that was used or another?

 

[stillcantfly]

Sorry for the late response but yes replaced it with the 803149 IAC. The IAC is attached to the throttle body if looking at the back of the boat where the motor is it’s on the left side with the TPS. As far as the test I covered the PVC port on the valve cover and the oil cap hole. Boat ran around 1000 rpm still when I plugged the nipple the pvc hose slips onto on the throttle body it dies. But when just the valve cover ports are covered and the air is being sucked through the PVC nipple the suction noise is gone. when I cover the inlet inside the throttle body just enough to drop the rpm down to 600 sounds amazing no issues at all idles just fine minus the fact my finger is controlling the amount of air. Also I do have a new PVC valve on it. It is getting to much something as the exhaust is really bad under mercruiserparts.com serial number 0L671154 you’ll see all the diagrams. Been chasing every wire every fuse beyond stumped ready to burn the boat.

 

[stillcantfly]

One last thing with the oil hole and the pvc hole plunged air sucking through the pvc hose nipple on TB the inlet hole inside throttle body has very little suction don’t know if this means anything.

 

[alldodge]

Have been using mercruiserparts.com and it doesn't show the IAC, just list the part number, and the reason for the question.

Boat ran around 1000 rpm still when I plugged the nipple the pvc hose slips onto on the throttle body it dies

One last thing with the oil hole and the pvc hole plunged air sucking through the pvc hose nipple on TB the inlet hole inside throttle body has very little suction don’t know if this means anything.

Its running rich and high rpm
Reducing the amount of air coming in (closing off PVC) makes it more rich and would kill the motor

Remove the TB and then the IAC and check to see how clean the idle port passage is. While off move the throttle plate and see how it seats against the body.

 

[stillcantfly]

took the TB off the butterfly looks to be perfect no issues ports all look clean no build up what so ever.

 

[alldodge]

How is the IAC idle passage?

 

[stillcantfly]

the inlet to the IAC looks fine I'm trying to understand how this thing works..... there are three holes under the throttle body but the gasket covers the center but has a tiny hole in a circle shape it doesn't appear that it goes any where the other two you can see go into the TB below the plate. took the boat out tonight and nothing new.

 

[achris]

I'm going to ask the obvious silly question (only silly question is the one not asked ). You have the vent and PCV valve hoses plumbed the right way? PCV valve in the port valve cover and its hose going to the fitting on the throttle body, and the vent hose going to the flame arrestor?

Chris.....

 

[stillcantfly]

Yes pvc valve connecting through the hose to the TB nipple the flame arrestor connecting to the vent hose but mine is reversed from the picture.... meaning if I’m standing inside the boat looking backwards at the engine the hoses to the vent and fuel regulator are on the left side and the pvc hose goes to the right when standing inside the boat looking aft at the engine

 

[stillcantfly]

Also ran a vacuum test was getting 14-15” of suction from the throttle body nipple that the pcv would have plunged into

 

[alldodge]

there are three holes under the throttle body but the gasket covers the center but has a tiny hole in a circle shape it doesn't appear that it goes any where the other two you can see go into the TB below the plate. took the boat out tonight and nothing new.

The IAC should have a clear port to outside air, and with other TB's there is a IAC muffler inside the bore.

If the IAC does not have a clear or next to clear port to outside air (inside bore of TB) then this needs to be investigated

Also ran a vacuum test was getting 14-15” of suction from the throttle body nipple that the pcv would have plunged into

2 comments

14-15 on vacuum is good, but should have that with the PVC still working. Taking vacuum from the PVC port increases the vacuum the gauge will see because no leaking from the valve

The motor dies when the PVC is plugged, the gauge plugs the line, why didn't the motor die?

 

[stillcantfly]

When I plug the pcv the hissing/suction gets louder I don’t know if it dies I’ll have to check... I can hook a T hose up to connect to the pcv line and the nipple on TB and see what the vacuum is then. I’ll also snap some pictures of the bottom of the throttle body tomorrow.