BradmanVA

07 Donzi 28zxo with twin 6.2's

Port engine kicks out at around 4500-4600 most of the time and its like a light switch. Its off then drops back 500-700 rpm then comes back to life then once back to 4500 rpm back off then back to life and so on and so forth. If I back throttle down by two hundred RPMs to say 4300 the motor runs great.

Starboard motor runs and pulls non stop as hard as it can.

I can have the port flat out sometimes and its fine but its rare and short lived. I have installed all new plugs, wires, cap, rotor, swapped distributors, new fuel filters, checked for anti siphon valves, pulled fuel lines to inspect, and replaced the map sensor due to a random code. I have fuel pressure around 38-40 with key cycled on, with engine running its around 35-36. Up to speed it drops down to in the lower 30's say 33psi. I have read that the pressure is not to exceed the 40psi mark so I do not see that.

Also, I seem to sometimes get backfires through the throttle body at lower cruise speeds, when motor is under more strain, coming up on plane, but not up and running. At least I think, its hard to hear at 60+. Oil pressures are great, cooling is great, and you would never know that a thing is wrong other than occasional backfire and the top end issue. I always run good gas, use either quicksilver fuel treatment or star tron. It hasnt stopped us from using the boat, but its nice to own a Ferrari, but not with the valet key.

Anyone have any ideas........ I was thinking of swapping the fuel pressure regulators from the engines next to see if this makes any difference, but figured I would shout out to see if anyone knows anything more.

Thanks for your help if you have any to give

Expensive Date

I had the exact same thing happen to me a few years back. Ended up being the injector harness at the #7 cylinder. With the boat running pull push move every wire, start at the front and work you way back.

Someone else had a similar problem and I think it was a loose wire on the starter solenoid, do a search there is a lot of info on here about the 6.2s

BradmanVA

I also have checked and moved everything on the port side of the engine. Checked all powers, grounds, and at one point I could change the way it ran at the main harness plug at rear, but I took it apart cleaned it and re-installed. Now it runs and idles fine and cannot change with any movement of wiring. I checked the injector plugs, unplugged all sensors, and examined with no problems found.

Also, this motor was replaced due to a spun main bearing since the previous rich owner didn't care to keep up with his maintenance, care, etc.. We replaced the engine with a Jasper re-manufactured motor at the beginning of the season after going through the whole boat, transom, drives etc. and cleaning everything to get it back in A+ shape. Absolutely love the boat, but want it fixed right at least once before the end of the season.

Unsolved mystery continues.........

tunertech

Your issue sounds like a lean out condition, back firing, runs good to 4200 or so, runs good after backing out of throttles....

you should check the injectors for resistance. They should all be within 1-2% of each other.

FP is low WO Pressure should be closer to 37-38, try disconnecting the vacuum reference line and test.

how do the plugs look

BradmanVA

Plugs looked good when removed and installed new ones since they were out and wanted to rule them out. I plan on swapping the Fuel Pressure Regs this week, and retest with fuel pressure gauge this weekend. Going to have someone record the gauge throughout power range to see if it spikes or loses pressure at WOT. Im going to test both engines so I can get a good comparison. I will check the the injectors and are you refering to the vac line that runs to the Reg?

BradmanVA

Swapped regulators today from the fuel rail, and cycled both engines and pressure primed to 42PSI then dropped back to 39-40. Let sit and pressure dropped little over time to 35psi. Ran both engines on hose, started and idled fine, warmed them up, and had good throttle response from both engines one at a time. Both fuel pressure reading mimicked the other. No difference in either while idle or snap of throttle. Also removed the flame arrest-er, inspected the TB, butterfly, and removed the dirty foam muffler from the port since the starboard was already MIA. So now they both hiss like a pissed off pit viper. I also checked the Crank sensor since I read a post about that, it seems to be fine, the plug is in good shape, snaps in tight, wires look good.

Will do test run in water tomorrow to see what the fuel pressures are at WOT. I'd be willing to pay to get it fixed since obviously I am not having any luck, but so far nobody can nail this one down. I've spent so much money this year on this boat, and it would be awesome to get this fixed and have it run right at least once.

Expensive Date

Talked to a friend who is a GM tech this week, was told 30 PSI at wide open throttle is not enough to cycle those injectors properly.

This sounds so much like mine a few years back, keep in mind we are just throwing ideas out but one guy on hear had a broken pickup in the fuel tank. If it is a fuel problem that could be it.

BUP

iTrader: (9)

fuel pressure at 1800 rpms should be 43 PSI.

BradmanVA

OK, filled up and ran star tron treatment.

Upon initial start up and run at the docks both motors ran well. Fuel pressure gauge was hooked up to port motor since I switched the regulators. Pressure primed to 42 then back to 38 running. Left the docks and through the no wake zone with no issues either motor. Went to get up on plane and heard the same popping through TB, but once up and running seemed to be running fine. Started out cruising out of the creek at around 35 then cleared the creek and started to run the boat up to speed. Seemed to run all the way up to full RPM's for a brief 2 minute until the initial problem started where port started kicking out. Fuel pressure was steady throughout even when high rpm problem started acting up. Then all of a sudden the starboard motor sounded a guardian alarm of two beeps every minute and I brought the boat to a stop in the water. When cutting back on throttles the boat settled in the water and the starboard engine stalled. This was puzzling, but I restarted it and had to give throttle to keep it running. I tired to back it down to idle again only to have it stall again. So, I figure that the fuel pressure regulator that I swapped had to have caused this new issue on the starboard side motor since I have had no problem with this at all except cold starts. The thing that does not make since is that I still have the same port issue, but have now created another issue on the starboard also.

After a brief stop, went underway again with guardian alarm still beeping twice every minute till reaching our destination 10 minutes away. We anchored up and I raised engine hatch and switched the fuel pressure gauge to the starboard motor to get a comparable fuel pressure reading on the return trip. We enjoyed the day, and loaded up to leave.

Upon the return trip the starboard motor would not run above 3000 rpm without the guardian alarm sounding at constant beep. If i kept the boat on plane with the port motor just long enough to drop the starboard motor to around 1K rpm the constant alarm would go off and I could resume power on it as long as I did not use more than 3K. Starboard motor was in Guardian protection and port was even dying out at lower RPM due to the strain on motor causing the cutout at even lower than 4500. Now it was dropping out below 4K. Fuel pressures seemed to maintain on starboard motor as well but due to guardian I could not get a WOT reading.

So Port still has the cutting out issue, and now Starboard has its own problem since the swap of the fuel pressure regulator. I guess the fuel pressure regulator is bad, but obviously is not the fix for the ports entire issue. I flushed the motors at home and when I removed the vac line off regulator the starboard motor would actually idle. CRAZY.....

I do not want to touch anything till I can scan the computer to see what the codes/failures are. I am waiting to hear from my boat tech today so hopefully I will have more news later.

Im so pissed off and frustrated........

Expensive Date

IAC valve is bad, possibly a totally separate problem.