454 Mercruiser intermitent power lose

[derekmc1]

I have 1987 welcraft with a 454 Merc 7.4L with Bravo 1 drive.

while out this weekend the boat would to lose power for a second while i was motoring below 2000 RPM. It would only drop 200-400 RPM for a second then recover, I could feel the power just decrease then come back. This was pretty consistent all weekend.
It also did it a few times while I was on plane. A few times I was planing at 3200rpm then all of a sudden the baot would gain RPM and Speed up like i punched the throttle down, but i didnt touch it.

Oil pressure was normal (30-40 while on plane). Water temp normal (140). I pulled a plug it looked good. checked my filter/water seperator no water in it. all my plug / coil wires seemed tight and were connected.

Can anyone help me figure the problem out?
Thanks,

 

[26aftcab454]

Re: 454 Mercruiser intermitent power lose

mine had the same symptoms and was a cracked distributor cap- put on a new merc replacement and runs great.- do not buy a cheaper imitation

 

[derekmc1]

Re: 454 Mercruiser intermitent power lose

thanks Aftcab,

i was thinking it must be electrical or fuel related. i will try that out and re-post.

 

[derekmc1]

Re: 454 Mercruiser intermitent power lose

Ok i went to the boat tonight to check things out did not get any new parts yet and i checked all my plugs again and one of my plugs looked wet and dirty almost like small peices of rust maybe. all the others looked fine. i am guessing i missed this plug when I checked them the other day. my oil looks ok on the dip stick, but there is soem white foamiy water coming through the vent hoses into the spark arestor.

could this be a riser gasket leaking? the plug with water was the rear port side plug.

any help much apreciated.

 

[derekmc1]

Re: 454 Mercruiser intermitent power lose

my risers was not leaking i think my plug was just fouled. i put on a new distributor cap and button. went out last night and the boat worked great no issues. but i went out today and it started doing the same thing again dropping revs after about an hour of driving?

Any thoughts?

thanks,

 

[Lyle29464]

Re: 454 Mercruiser intermitent power lose

sounds like you are getting water into the oil. The oil in a 454 runs hot enought to steam off a small amount It will show up in the vent hoses.
If you look inside of your valve covers you might find them wet. My guess is a riser leak. Did you winterize it propertly if needed? That could give you the symptoms your experiencing.

 

[derekmc1]

Re: 454 Mercruiser intermitent power lose

* the boat was winterized by a local boat mechanci who does a few other boater i know and came with great recomendations i think it was done properly yes*

When I found the fouled plug I thought I might have had a riser leak on that side. I pulled the riser off and the riser and gasket were not leaking, they actually looked pretty good. When I look inside the valve covers through the vent hose or oil fill cap I can see what I think is the same oily/water stuff.

But I dont see anything wrong with my oil on the dipstick.

Do you think the oily/water mix comig from my vent hoses is what is causing my intermitent power loss? Is the water getting in a cylinderonce a while and resulting in a bad fire?

Thanks,

 

[Lyle29464]

Re: 454 Mercruiser intermitent power lose

It sure sounds like the engine is getting water into the chamber.
I think I would try a compression test. I have seen many times a 454 running a bit lean and it eats through the manifold or riser. you can test this by putting two long bolts inplace of the short ones that hold the manifold. Make sure the others are not froze. then start the engine and run it about 20 minutes. shut off leave your water supply running and quickly remove all the bolts except the two long ones. Then just slide the manifold away from the engine till it hits the heads of the bolts. You need to do it fast and you may very well see water run out the back port if its leaking there. Of course if the compression test was low you will need to look at head gasket or a crack.