1999 Mercruiser 350MAG Issues

"This is a problem I've ha

"This is a problem I've had since I bought the boat. Some of you may remember my problem. It's with my starboard engine. I'll try to summarize.

I've been using ScannerPro to monitor engines. I know they will never be exactly the same, but I've seen notable differences between port and starboard. I've attach two side by side comparisons. The first is WOT where the starboard engine didn't spool up. The second, it did. Notice the GPH differences. Also, why is the IAC so different?

Symptoms:
- Engine, at times, won't go above ~3300 RPMs
- When RPMs do come up to normal speed, they may stay for duration of trip or may not...it's not consistent. When RPMs do come up, it still requires more throttle to sync RPMs and shows 5 or 6 GPH more than port engine.
- When the problem occurs, if I back the throttle back to where it naturally runs at ~3200 RPMs and match RPM with port engine, starboard engine requires more throttle and shows higher GPH on computer.
- Oil pressure gauge seems to have a "tick" but pressure is fine. Pressure doesn't fluxuate...its more like when you tap a gauge and the needle shakes. But it is constant.
- Engine runs fine (but still requires more throttle than port) at all RPMs below the ~3300 RPMs
- Engine RPMs were not right at delivery but was assumed to be related to steaming (which they fixed so they bailed on me with poorly running engine).

History:
- Dealer: Exhaust manifolds replaced to fix coolant leak into exhaust
- Dealer: Heads re-manufactured
- Dealer: Fuel pressure regulator changed
- Dealer: 3 injectors replaced (damaged during head work)
- Dealer: Cap, rotor, wires, and plugs replaced (and gapped)
- Dealer: Computer swapped with port engine with no change to either engine
- Dealer: TPS swapped with port engine with no change to either engine
- Dealer: MAP replaced
- Me: Fuel pressure on rail is fine (equal to port engine) at all RPMs
- Me: Timing verified in base timing mode to be 8 degrees BTDC
- Me: Throttle position verified by side by side comparison with port engine
- Me: Swapped oil pressure sensor
- Me: Not a fuel quality problem verified by running off of a portable tank with fresh fuel
- Me: Coil swapped with port engine with initially RPMs coming up but problem returned later
- Me: Flame arrestors cleaned
- Me: Transmission fluid verified. No audible difference. No vibrations.
- Me: Voltage seems ok. The meter shoots up when RPMs come up.
- Me: Oil changed and checked. No foaming or discoloration.
- My Marina: Props computer calibrated and reconditioned

Thoughts and Questions:
- The only thing at this point that I take note of is the variability of the problem and the oil pressure gauge having a "tick".
- The variability doesn't seem to have any relationship to anything like temperature or humidity.
- Can fuel injectors be ruled out since it does at times spool up (even though when it does spool up, GPH is notably different)
- Does this scream electrical? If so, how do you track it down? What electrical problem would account for the variability and the GPH difference when it does spool up? When it does spool up, RPMs are smooth.
- The engine does spool up at times so does that rule out mechanical?
- Every sensor has been either replaced or swapped
- I know the IAC shouldn't have anything to do with it...but why in my attached examples were they different?

I very much appreciate you taking the time to read all this and sharing your thoughts.

Take care.

Jon



"

"Jon:

Have you done a compr

"Jon:

Have you done a compression check on the engines for comparison?

"...oil pressure gauge having a "tick"." It's an electrical spike from an electro/mech. device. I don't know the parameters that the computer is set to accept as good or bad info. It may be part of the problem.

"I know the IAC shouldn't have anything to do with it...but why in my attached examples were they different?" Nearly a 100 RPM difference?

"Does this scream electrical? If so, how do you track it down?" The engine's operation seems to dictate an electrical "Gremlin". Since the operating data is based upon voltages less than 1.0 volts, a few millivolts can make the difference in operation of the engine.

I would get a good magnifying glass that has a built in light and take a look at the computer harnesses and all related connectors. What you are looking for are fine cracks in the wires; you'll need to wiggle and tug on each wire.

Also look for a subtle difference in the color and position of one or more contactors in the harness connector. A slightly darker contactor could mean a poor connection.

Also look at the solder joints of the wires on each contactor; you are looking for a cold solder joint which will look dull or an incomplete solder joint where the solder will surround the wire and not spread out over the connector surface. Also look for discoloration or fine cracks on the computer board contact surfaces. The surfaces can be polished with an ink eraser or stiff pencil eraser.

I have found connector pins on a harness that were about 1/32" shorter than the others. The "locking barb" part of the pin was compressed and would not allow the pin to mate.

What electrical problem would account for the variability and the GPH difference when it does spool up?" A small amount of dirt film on the throttle plate and the throat of the body can make a difference in the amount of air entering the engine. A good cleaning of the TB can't hurt the air delivery; use only "throttle body" cleaner and not carb cleaner. The IAC cavity should be spotless.

Finally, contact Merc.'s customer service and talk to one of the engineers. Maybe they can make a determination by interpreting your data.

Hang in there because it can be solved.
."

"Thanks again Guy for the resp

"Thanks again Guy for the response. I apologize for leaving it out but a compression check was done. A leak-down test, however, was not.

I guess I need to just put some music on and climb into the engine compartment and start tracing wires. My concern was with the bundles. But to your point, I can check all the connections.

Thanks very much. Hopefully I can get some offspring free time this weekend. I don't want to wait until mid-season to start looking at it this year.

Jon"