2002 496 MAG port engine lags at WOT
 

Hello all, new to this forum. Lots of good info here. So, we have a 2002 Cobalt 282 with twin Mercruiser 496 Mags, non HO. Mercruiser Bravo 3s swinging 30p props. Both engines have around 360 hours on them and have been well maintained. I had asked the Marina to sync my Throttles so at the second detent, we are sync'ed at 2300rpm which puts us at right about 25mph. The tech came back and said the cables were stretched and needed replacement, but that there was something not right with the port engine. At WOT the starboard engine runs up to 4800rpm, but the port engine lags at about 4200 to 4300 rpm. We replaced the cables and resynced the controls so at WOT the TPSs are both reading the same on two computers, but still rpms are lagging on the pot engine. The tech now wants to do a compression test and leakdown test to determine if we have a weak or "dead hole". We just spent $990.00 for the diagnosis and cable change, now he wants another $700.00 for the two tests. He seems to think I may have a tulip'd valve or bad rings or something. The boat idles fine. No alarms, no back fires, no dying out, no guardian mode, temp stays at 175 on both. Tech said the fuel pressure on the rails is good. He also indicated that when he is listening to the engines running at WOT, with the captains call open and another tech driving, he said it sounds different from the starboard engine, but not a miss, more like a vibration. I am reading here about the paint in the Fuel pumps ruining the pumps and clogging injectors, the CPS on the older 496s, etc. I just changed the two fuel filters on each engine last summer, haven't used the boat too much. We have only but 45 hours on it since then. My problem is my boat is dry stacked at this marina and I am not allowed to do any mechanical work on it myself.
Any info, comments, questions or insights would be greatly appreciated. I am contemplating moving the boat to a more mechanically permissible marina where I can do my own work.
My engine serial # is OM348383

Thank you in advance.
Kevin

Could be a bad plug, plug wire, coil or exhaust leaking water into the engine. Pulling the plugs you would probably see rust or a fouled plug..

That's a start.

Did they check fp under wot?

The tech told me he did, but he did not tell me the readings. Also, do any of the scanners read spark profile and injector performance on these older PCMs? That would be good info.

Well, here is the rest of the story.
I discussed all the possible suggestions with the tech at the Marina. Since I can't do the compression and leakdown because the boat is rack stored, I agreed to have the marina do it. The compression and leakdown showed 150-160psi on compression and no leaks. What he did find was bad plugs. Replaced the plugs and on sea trial, we are back up to 66.3mph with both engines WOT matching at 4800rpm.
So, I need to investigate what was wrong with the plugs and why did they fail in less than a year. These were new last year.

I can’t stand the .060” gap of the stock plugs. Eats ignition parts.

But yeh, year is too early. Coils may be a little weak ? Or wires ? However, smaller gaps help everything.

.060” is what gm determined for emissions for trucks.

Somebody cracked a plug putting them in.
which cylinder had the bad plug?