Running 2002 Baja Outlaw 29 with 2 Mercruiser MX 6.2 MPI's. Believe first edition (1999 era), serial numbers in the 0M09xxx range.

Just did COMPLETE rebuild on both motors at the same time.
Both motors sounded good in the driveway. Being new motors, I didn't really get into the power. Brought up slowly, sounded good, so took out for a test. Disaster!

Sounded good for the 15 min troll in no wake zone. Soon after I started adding power, port motor died and starboard had no power. More I pushed throttle, more it slowed down.
Turned boat around praying I got close to shore. Tried restarting port, wouldn't even crank. Starboard ran consistently around 1200rpm, so didn't touch a thing. Just headed straight for ramp.

Upon getting close to the ramp, the first interesting thing happened. As I pulled back the throttle, engine picked up speed. Had enough time to test this theory, giving a little more throttle, slowing down, picking up, ect.
Upon getting to the ramp, I realized Port motor was my stupidity. In my rush to get back, didn't think to put it in N before trying to crank. When I got home, put fresh water on, Put in N, motor started. Both motors acted similar, like timing problems. Port motor was exactly that. Either I was asleep when I set the timing, or I never did. Was way off. Set timing, motor sounds much better even when I punch the throttle. Hopefully problem solved.

Other motor is different story. Timing was VERY close. Made it perfect, but didn't help anything.
Some observations...
Sometimes (mainly when cold), need to give a little throttle to start, then can back to closed and motor sounds good at idle.
If I move throttle slowly, RPM comes up and motor sounds good. If I punch it from idle, motor boggs and RPM doesn't go over 2000 without it coughing and backfiring.
If I bring throttle up to 1800 and then punch it, I can cause a backfire very consistent around 2000-2200 RPM.
When I set the timing, on good motor, I put in base timing mode, RPM came up as it should. On this motor, I put in base timing mode, RPM remained the same, maybe even dropped a little. Brought up manually to set timing.

What I've tested.
Compression - perfect and consistent 120psi +/- 3psi
Cap / Rotor / Spark Plugs / Plug Wires - Brand new. Same as good running motor.
Fuel Pressure. 23-25psi at idle, 30psi as soon as RPMs start coming up. Exactly what book calls for. Never drops when motor starts backfiring.
Distributor Cap, Check for arcing / leaks - At about 1800-2000 RPM in pitch black, sprayed with soapy water looking for arching. Nothing observed.
Fuel Filter - New
Fuel - Couple months old. Would suspect, except other motor runs great on same fuel.
IAC Valve - Haven't tested and originally was suspect, but don't think should cause backfire at 2000 RPM. Thoughts?

Running out of things to test. I do have Rhinda software package, but don't see anything out of the normal compared to the good motor. No codes present.

Any thoughts?

Cam Degree is wrong.. Does it smell like raw gas close to the Exhaust?

Chris

Yes this would be the 1 st gen of the 6.2 MPI

This app has an IAT in the manifold and has a fuel pressure sensor in the fuel rail. The sensor is very important. It has a MEFI 3 ECM so for setting timing did you put it in base timing mode beforehand ? What did you set your timing at ?

Did you have your fuel injectors flow tested & cleaned for your rebuilds ?

What service manual are using ?

Is your apps stock OEM even after rebuilds ?

Are you still using the MEFI 3 ECM stock OEM tune ?

Did any of the internals and or intake manifold, exhaust change after your rebuilds & still using the stock ECM tune ?

If the IAT was bad or the Fuel Pressure Sensor was bad, would that not throw a code?

I did not have the fuel injectors cleaned or tested before rebuild.

Service Manual 24 with Supplement 31.

Apps? Not sure what apps is, but its probably still OEM. Only thing that really changed was bored .30 over, obviously new pistons, but both motors are exactly the same.

ECMs are stock. I even have a 3rd ECM I swapped out to verify was not the problem.

I had a 3rd motor, same serial number range, that I did swap some parts to combine 3 into 2 good ones. Can't guarantee Intake is the same as one that was running, however it is the same part number.

I did replace a lot of internal parts, but nothing that should affect anything, and both motors got all the same new parts.

I have the same basic setup. just a few things that will help. These parts are a third of the OEM price. Mine was a 2001 29Scarab twin 350MPI 300Hp engines, MEFI 3,,, Is now 383's and use the same sensors. These little grimlins gave me a fit. after replacing one here one there another here, I wised up and just replaced all ,, results are NO MORE Grimlins,, they made the engines do stupid stuff

Map Sensor: Mercrusier #861249A1 - Borg Warner#EC1636
IAC Mercrusier #803149 - Advanced Auto#31023
TPS Mercrusier #803148 - Advanced Auto#EC1033

And get them probably for 2/3rd to 1/2 of Advanced prices at rockauto.com

On the subject: anyone find a water pressure sensor substitute? I'm with Scarab, I'm slaying gremlins and think I may just replace it all. Cheaper than mechanic trips and being pissed off.

Ran more tests today.

Swapped Coils and coil wire. No change.

At 1000rpm, pulled 1 wire off distributor at a time. Not scientific, but noted a similar audible change for each cylinder.

Going to try a different distributor. When that doesn't make a difference, at least I can rule out anything to do with the spark or timing side.

Do a tps sweep , quick and easy , maybe try artificial fuel enrichment with propane.voltage drop test power supply circut to injectors .. prob wont be a hard fix once you track it down .

I found the problem! Well, one of them. Now I have more.

I did change the IAC motor - no help.

I decided to change the distributor since it was the last thing in the spark category and I had another one laying around. I noticed something interesting on the new one (actually very old and didn't look in great shape one), it had a ground wire the one I was replacing didn't have. Looked at the other motor and that one had the same ground wire. Upon removal of the one in the boat, its clear there was never a spot for a ground wire. It was built different and didn't have the same bolt a ground wire could attach to.

Swapped distributors (hooked up ground wire) and everything worked as it should. Put in base timing mode, RPM came up, timing was perfect (got lucky) and no more backfire.

In the end, I'm not sure if it was a bad distributor, or if it was the ground wire, because I know the distributor that didn't have the ground wire was successfully running on an engine before I started this project. But then I've never seen a Halifax sensor only go partially bad.

Unfortunately, the motor still wasn't running great, seemed to be smoking a lot and my happiness was short lived because the motor died and wouldn't restart. Don't know what happened yet, but have a gut feeling something internal failed. Don't know if it was my screw up in assembly, or if something got damaged from all the backfiring. Going to do a compression test later, but think this motor is going back to the bench.