350 Mag MPI WOT bog??
 

I have a Donzi 28 ZX with two 350 Mag MPI's that I have just replaced with Michigan Motorz GM base marine engines. The engines I bought the boat with had not been winterized correctly and both had cracked blocks. The new engines run very well with one exception. The last 2 inches of throttle travel on the sticks are causing the engines to bog slightly slowing the boat down. I have fuel pressure guages on the dash and they are reading 40lbs (measured at the end of the fuel rail) when the bog takes place so I don't think I have a fuel issue. I am using aeromotive adjustable fuel pressure regulators instead of the stock ones. The boat does have colder thermostats in it (120 degrees) but I wouldn't think that would cause the bogging I'm experiencing. According to the Donzi guys this boat should run in the high 60's or low 70's with this set up. I'm only getting 61mph from it right now. Is there a sensor that could cause this, bad TPS maybe?

that is a cold azz thermostat. m 350mpi's have 160* stats. verify the thermostats you have.

What RPM does the bog happen?

When the throttle is at idle are the throttle body blades closed and conversely do you get WOT with the throttle advanced all the way? In other words is the rigging set correctly?

The boat has 120 degree thermostats from Procharger. The boat is geting WOT with the throttle body.

Are the regulators vac operated?

So you have Prochargers on these motors?

If you have ProChargers that changes the entire equation. That has to be answered first.

No the engines are not Procharged currently and the regulators are hooked up to manifold vacuum.

What rpm does the bog happen at?

It could be the TPS sensor. You can check the voltage off the sensor and it should run from basically 0 to 5 Volts as the throttle is swept.

If this boat had ProChargers at one time were the ECU's reprogrammed for the boosted application? You may still be running a SC-Tune with 2-bar MAP sensors. IF these are the only factors a re-flash will fix you up.

It did have prochargers and the ECM's were remapped. I have the mefiburn software and have reloaded them with the E930 mercruiser 350 mag mpi map. I'm not sure about the MAP sensors though. How would I know if 2 bar sensors were in there instead of the factory ones? The bog happens in the last 1/4 of the throttle travel or about 4500-4600 rpm.

The ECM's were tuned for prochargers. I have reprogrammed them with the MEFI burn software package and the now have E930 (stock) mercruiser maps loaded. I don't know if they have 2 bar MAP sensors. What would I look for to figure that out? The bog happens in the last 1/4 throttle travel and around 4500-4600 rpm.

I have essentially the same boat and stock motors... My stat is 170. I would change that 120 deg one if i were you because supposedly the computer runs the engine rich if cold. 2ndly, stock props are Mirage 23P's... With a prior Procharger setup, i would think theres a set of 25P's on your boat.

What'd you do with the pro charger setup???

Both procharger set ups are currently on the shelf. Props are 23P Mirage pluses currently. I can adjust the enrichenment circuit so that the cold choke turns off earlier but I don't think that would cause this bog.

hook up scanner pro and look at the MAP KPA at idle or the voltage.. If it's the correct MAP sensor it should be reading about 60. You can change the bin file by simple checking the 2 BAR MAP variable in the stock tune. It doesn't care what you have on it so long as the program and the sensor type match.

Pulled the MAP sensors tonight and had a look. For some reason both engines have GM 3 Bar map sensors. I ordered two 1 bar sensors. I presume this could cause the issue with a stock 350 Mag Mpi base map. I really hope that I haven't hurt the engines....

Made a mistake on those MAP sensors. They were indeed 1 bar MAPs so I'm still searching for the problem. Should I take it to the dealer and have them check the fuel injectors? I'm running out of ideas.