496 mag ho runs terrible after recall work

"Looking for help. I have a 20

"Looking for help. I have a 2003 merc 496ho mag in my wellcraft excalibur, never started untill 2006. Ran like a top untill this spring with 80 hours on it. It had the aluminum water rails and they corroded off in that short of time. I found this was a service or recall issue and merc. shipped new manifolds and they were installed by a local merc. service marina. The problem is every since they did the work my boat runs terrible with missfires, backfiring and no power. I took it back to the marina that did the work, they put new platinum tipped plugs as required for the boat and labor at 600.00, boy did i get ripped. The boat ran fine for about 4 hours of run time and is now back to running terrible again. I have added yamaha ring away to the fuel to unclog any carbon, new fuel and seperator, had a mechaninc check all 8 coils, wires and plugs which all check out ok. On a compression test one cylinder is about 20psi less than the others. Totally stumpped here have a nice boat but cant use it and cannot diagnose the problem. Any help would be greatly appreciated

"

Sounds like a manifold may be

Sounds like a manifold may be leaking water into a cylinder

That is one question I did hav

That is one question I did have on my mind. This boat ran just great prior to the new exhast manifolds being installed. Where would you start? look for water in the oil or pull the manifold?

Check to see which plug is get

Check to see which plug is getting wet. Do a compression test on the wet cylinder to prove it isn't a head gasket. Then pull the manifold and test for leak.

"The mystery so far on this is

"The mystery so far on this is that none of the plugs are wet. When using computer to drop a cylinder when running, dropping cyl #5 seemed to not show any change in engine sound so I am thinking that is the cylinder in question, but the plug is dry and looks like a normal tan burn.THis cylinder is also about 25psi lower on a compression test than the others. Others are around 155-160 this cyl is at 130."

I suspect a valve issue so try

I suspect a valve issue so try doing a leakdown test with the cylinder at TDC inject some air and listen to the intake and exaust for hissing.

"Bt doc is correct... A leak t

"Bt doc is correct... A leak to the exhaust will cause a power loss, while a leak to the intake will wreak havoc with air/fuel distribution in the intake manifold."

"Thanks for all the input..I a

"Thanks for all the input..I am going to take the boat back to the repair shop next week, having sea tow come get it from my dock. It seems the only left is the water geting into a cylinder somewhere. I never had any issues untill the boat shop replaced the manifolds. One thing I do already expect is if it is something they did...they are not going to admit it and I am going to pay for it anyway...but as of now I just want my boat to run. Thanks agian everyone"

Did they use GLM or OEM manifo

Did they use GLM or OEM manifolds? GLM are problematic with porosity (leak) problems and poorly machined surfaces.

Why would you be willing to go

Why would you be willing to go back to the same shop? Al W.