Performance loss - 8.1 HO

[Sindbad]

I have reviewed some of the other threads with similar descriptions of the problem I am experiencing, and looking for advice from the group. My boat is a 2005 390 DA with 8.1 HO. At purchase, July 2014, sea trial results were 34 mph at WOT of 4,800 RPM, and 24 mph at 3,800. At beginning of this season, I noticed degraded performance with port tach reading 500-600 RPM less than Stbd. Props were removed and tuned at beginning of last season. Mechanic suggested running both engines to WOT and making a note of RPM readings which I did a few days ago. Port WOT read 3,700, Stbd 4,300. He believes port must have hit something, which I do not recall hitting anything. At purchase, both engines were tuned up. Something has clearly affected the performance of the boat and I don't know if it is the prop, or something else. The engines sound fine. Rather than waiting until end of season to pull the props, I plan to do now, but I am not sure if that will solve the problem based on other comments I have seen on similar threads. Looking for any and all input from the group......very frustrating.....never had this problem with my 2006 340 DA which had 8.1s. Thanks guys.

 

[sbw1]

Bad spark plug wires can easily cause that kind of rpm difference. Did the tuneup include new wires?

 

[Crushin It]

Did the tune-up include new plugs?

 

[Sindbad]

sbw1, villain style -- thanks for the responses. I pulled the invoice from July 2014 when I had all filters, belts, etc. changed; I don't see spark plugs or wires listed so I will check with the marina service shop. The engine isn't missing so I didn't suspect any problem with plugs or wires.

 

[Sindbad]

Mark - checked another invoice; spark plugs replaced Aug 2014 with double platinum plugs; but it doesn't look like the wires were replaced. Could that cause the problem?

Frank

 

[copb8tx]

I had a very similar experience. I replaced all plugs AND wires and they both ran like tops again. Pretty easy to do and even if it doesn't fix the issue you'll be one step ahead on your maintenance.

 

[Crushin It]

Hi Frank,

Sometimes the computer will compensate for rough running engine and will enable engine to run smooth. That's how mine felt. When I replaced plugs with Champion 13-RS14YC6
and new wires my performance was back. If you have to do new wires you are not going to like the price of them..... Also, a second time this happened with same symptoms, a tech put their scanner on and saw a cylinder was mis firing. Sometime after putting new wires on, the wire was close to a manifold and burned thru it. You couldn't see it from looking at it but once the scanner told him which cylinder wasn't firing he just felt the wire and found that it was broken. So that was a quick fix except for the time for the Tech to diagnose......

 

[Sindbad]

Mark - Thanks for the information; it is starting to make sense given what is happening. At WOT, I can push the port throttle to full open and rpm's don't change. If computer is smoothing for misfire, etc. that could explain. This boat spent its entire life in VA (brackish water/chesapeake periodically); my environment is fresh water. I will check the records to see if prior owner ever had marina (prince william) replace the wires; if not, then these wires are 11 yrs old. Now to replace the wires and do a test run. Thanks.

 

[Sous]

Replace the wires, easy swap and should be done anyway. If that doesn't solve the issue check your fuel pressure to see what it looks like. That would be the first step in seeing if you have the paint delamination issue in the cool fuel module.

 

[Sindbad]

Sous - Thanks for your reply. I checked the prior owner records; wires never replaced, so 11 yrs. old. I will also check the fuel pressure as you suggested. I saw one other post re: cool fuel module. Is the paint issue common to a particular manufacturer, or in general?

Frank

 

[spikedaddy99]

Different motors, but same situation. Port was maxed at 3800. Apparently I knocked off #5 wire. Motor sounded fine, just wouldn't max out. Cheapest repair I've ever had! Check the wires.
Mike

 

[Sindbad]

Thanks Mike. Will definitely check.

 

[Tdady]

Sinbad....a good friend just bought the identical boat as yours and had the same issue. Turns out it was the cool fuel system. This is a VERY well known issue and one you should check immediately if you haven't all ready. The outcome of the delamination can cause you major problems and a lot of down time. FYI do yourself a favor and take it to an AUTHORIZED Merc dealer for repairs and insist that the parts, at a minimum, be covered under warranty.

 

[Sous]

Sous - Thanks for your reply. I checked the prior owner records; wires never replaced, so 11 yrs. old. I will also check the fuel pressure as you suggested. I saw one other post re: cool fuel module. Is the paint issue common to a particular manufacturer, or in general?

Frank

Yes the cool fuel module problem is very common. So much so that I don't think its a matter of if it happens but when.

 

[scoflaw]

Yes the cool fuel module problem is very common. So much so that I don't think its a matter of if it happens but when.

If your handy there's some cheap mods that can be done to the CF3 to make it last a very long time

 

[Sindbad]

Tdady - Thanks for the info. I have the service mechanic coming to the boat this week. How did your friend remedy the problem?

 

[Sindbad]

Scoflaw - I would be interested to know the mods you refer to.

Thanks.

 

[scoflaw]

Don't want to hijack this thread. Send me a pm or start another thread, Simple stuff once you pull it off the engine

 

[Sindbad]

will do PM. thanks.

scoflaw - tried to send pm but mailbox is full. I will start separate thread "CF3 modification".

 

[Tdady]

He had the entire system changed out under warranty. Merc also said they would send him an entire new set of injectors however they're currently on back order. That being the case they are now offering to pay to have them rebuilt, which is the path I believe he's going to take to get him back up and running.